Re: [PSES] Spark Gap PCB Layout on AC Mains
In message 1378784340.50598.yahoomail...@web160404.mail.bf1.yahoo.com, dated Mon, 9 Sep 2013, Bill Owsley wdows...@yahoo.com writes: Indeed... the protection scheme that passes Safety, causes the system to 'fail safe' which does not meet the EMC immunity requirements Plain EMC requirements do not consider fault conditions. Functional EMC requirements do. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk If dictionaries were correct, we would only need one, because they would all give the same information. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
Hello Brian, I've employed spark gaps, like you, not because you 'have' to but because it seemed good practice. It involved a control installation with cables strung externally. My advice is to use propriatory discharge tubes. They're cheap and their performance is more predictable than engineering your own air gap across PCB tracks or using pointy pins and are much less influenced physical and environmental conditions such as temperature and humidity, etc. And should they ever be needed, the consequences can be much less messy. I found a good selection available and looked at PCB mounting tubes with breakdown voltages of between 3 and 12 kV. I finally used 4kV, 5kA/10kA (10/1 discharges) devices having been influenced by what professional LAN GPS installers were using which largely ranged between 3 and 6 kV. Just my thoughts. T - Original Message - From: Kunde, Brian Sent: 09/06/13 04:56 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian - *LECO Corporation Notice:* This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to LT; emc-p...@ieee.org GT; All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas LT; emcp...@radiusnorth.net GT; Mike Cantwell LT; mcantw...@ieee.org GT; For policy questions, send mail to: Jim Bacher LT; j.bac...@ieee.org GT; David Heald LT; dhe...@gmail.com GT; - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
Some years ago I was working at in a wireline telecomms equipment maker. Robust protection was the order of the day; the outside physical plant was just *waiting* for lightning. We protected each circuit to a level it could withstand, and worked back to the line inputs, where (IIRC) we had to withstand 2 KV Oc or 500 Amp short circuit lightning transients. It helped that everything in the Central Office had a common-point ground, even if it was 150 feet below, in the basement, as this made backdoor entry less likely. In a later incarnation, I found aviation customers who wanted to test transient protection without opening the equipment, which is another story -- and on these, we had to inject transients on the CASE. I suggested some commercially available modules for AC power protection here because one does NOT want to try to protect the AC power network, only what he builds, and just throwing in spark gaps, gas tubes, Tranzorbs(tm) or MOV's might be asking for trouble. Know the threat, and protect against THAT. Cortland Richmond On 9/9/2013 1130, Kunde, Brian wrote: What are the safety considerations using gas tubes on the AC mains? Do you have to fuse them or are they not likely to fail shorted? Can you use them between line and PE? Do you have to use multiple parts in series? I often see them in series with MOVs in a “T” configuration to protect against line to line and line to PE surges. A few year back we had a product that had several surge suppression circuits located on different PC boards within (some assemblies were very expensive and we wanted to protect them). Well, at our customer site they experienced some kind of huge surge, transient or overvoltage (we do not know what exactly happened). Of all the equipment that was on-site including many of our competitors equipment, only our instrument was damaged. Our surge suppressors were blown up, charred, and/or vaporized. The warranty repair cost was $10,000US but the hit to our reputation was probably worst. We believed that our equipment probably protected all the other equipment on-site but it is hard to get your customers to believe you. So now we want to better control our surge protection and if we see a huge surge we hope it to destroy something much less expensive to replace or at least minimize the damage. What we are currently thinking is to use over the counter Surge Suppressor modules, but they are only good to about 3KV – 4KV. Then we thought we would add a spark-gap in the board that would only kick in if our surge suppressors failed. Maybe we can add some very high voltage Gas Tubes also or instead of the spark-gap. I’m not sure what more we can do. Many of the circuits/assemblies we are trying to protect are buy/sell components where we do not control spacings. Any comments? Thanks to all. The Other Brian - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
What are the safety considerations using gas tubes on the AC mains? Do you have to fuse them or are they not likely to fail shorted? Can you use them between line and PE? Do you have to use multiple parts in series? I often see them in series with MOVs in a “T” configuration to protect against line to line and line to PE surges. A few year back we had a product that had several surge suppression circuits located on different PC boards within (some assemblies were very expensive and we wanted to protect them). Well, at our customer site they experienced some kind of huge surge, transient or overvoltage (we do not know what exactly happened). Of all the equipment that was on-site including many of our competitors equipment, only our instrument was damaged. Our surge suppressors were blown up, charred, and/or vaporized. The warranty repair cost was $10,000US but the hit to our reputation was probably worst. We believed that our equipment probably protected all the other equipment on-site but it is hard to get your customers to believe you. So now we want to better control our surge protection and if we see a huge surge we hope it to destroy something much less expensive to replace or at least minimize the damage. What we are currently thinking is to use over the counter Surge Suppressor modules, but they are only good to about 3KV – 4KV. Then we thought we would add a spark-gap in the board that would only kick in if our surge suppressors failed. Maybe we can add some very high voltage Gas Tubes also or instead of the spark-gap. I’m not sure what more we can do. Many of the circuits/assemblies we are trying to protect are buy/sell components where we do not control spacings. Any comments? Thanks to all. The Other Brian From: Anthony Thomson [mailto:ton...@europe.com] Sent: Monday, September 09, 2013 4:18 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hello Brian, I've employed spark gaps, like you, not because you 'have' to but because it seemed good practice. It involved a control installation with cables strung externally. My advice is to use propriatory discharge tubes. They're cheap and their performance is more predictable than engineering your own air gap across PCB tracks or using pointy pins and are much less influenced physical and environmental conditions such as temperature and humidity, etc. And should they ever be needed, the consequences can be much less messy. I found a good selection available and looked at PCB mounting tubes with breakdown voltages of between 3 and 12 kV. I finally used 4kV, 5kA/10kA (10/1 discharges) devices having been influenced by what professional LAN GPS installers were using which largely ranged between 3 and 6 kV. Just my thoughts. T - Original Message - From: Kunde, Brian Sent: 09/06/13 04:56 PM To: EMC-PSTC@LISTSERV.IEEE.ORGmailto:EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to LT;emc-p...@ieee.orgmailto:emc-p...@ieee.orgGT; All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas LT;emcp...@radiusnorth.netmailto:emcp...@radiusnorth.netGT; Mike Cantwell LT;mcantw...@ieee.orgmailto:mcantw
Re: [PSES] Spark Gap PCB Layout on AC Mains
Yet another example that transistors and IC’s were invented to protect fuses and surge protectors. Gary From: Kunde, Brian [mailto:brian_ku...@lecotc.com] Sent: Monday, September 09, 2013 8:31 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains What are the safety considerations using gas tubes on the AC mains? Do you have to fuse them or are they not likely to fail shorted? Can you use them between line and PE? Do you have to use multiple parts in series? I often see them in series with MOVs in a “T” configuration to protect against line to line and line to PE surges. A few year back we had a product that had several surge suppression circuits located on different PC boards within (some assemblies were very expensive and we wanted to protect them). Well, at our customer site they experienced some kind of huge surge, transient or overvoltage (we do not know what exactly happened). Of all the equipment that was on-site including many of our competitors equipment, only our instrument was damaged. Our surge suppressors were blown up, charred, and/or vaporized. The warranty repair cost was $10,000US but the hit to our reputation was probably worst. We believed that our equipment probably protected all the other equipment on-site but it is hard to get your customers to believe you. So now we want to better control our surge protection and if we see a huge surge we hope it to destroy something much less expensive to replace or at least minimize the damage. What we are currently thinking is to use over the counter Surge Suppressor modules, but they are only good to about 3KV – 4KV. Then we thought we would add a spark-gap in the board that would only kick in if our surge suppressors failed. Maybe we can add some very high voltage Gas Tubes also or instead of the spark-gap. I’m not sure what more we can do. Many of the circuits/assemblies we are trying to protect are buy/sell components where we do not control spacings. Any comments? Thanks to all. The Other Brian From: Anthony Thomson [mailto:ton...@europe.com] Sent: Monday, September 09, 2013 4:18 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hello Brian, I've employed spark gaps, like you, not because you 'have' to but because it seemed good practice. It involved a control installation with cables strung externally. My advice is to use propriatory discharge tubes. They're cheap and their performance is more predictable than engineering your own air gap across PCB tracks or using pointy pins and are much less influenced physical and environmental conditions such as temperature and humidity, etc. And should they ever be needed, the consequences can be much less messy. I found a good selection available and looked at PCB mounting tubes with breakdown voltages of between 3 and 12 kV. I finally used 4kV, 5kA/10kA (10/1 discharges) devices having been influenced by what professional LAN GPS installers were using which largely ranged between 3 and 6 kV. Just my thoughts. T - Original Message - From: Kunde, Brian Sent: 09/06/13 04:56 PM To: EMC-PSTC@LISTSERV.IEEE.ORGmailto:EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to LT;emc-p...@ieee.orgmailto:emc-p...@ieee.orgGT; All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions
Re: [PSES] Spark Gap PCB Layout on AC Mains
Hi Brian, Standard IEC 60950-1 only allows use of MOV or combination MOV + GDT in primary circuit. No other surge protectors are allowed. As you mentioned, manufacturers are mostly using T protection (2 MOVs and one GDT). Best regards, Bostjan On 9. sep. 2013, at 18:00, Cortland Richmond k...@earthlink.netmailto:k...@earthlink.net wrote: Some years ago I was working at in a wireline telecomms equipment maker. Robust protection was the order of the day; the outside physical plant was just *waiting* for lightning. We protected each circuit to a level it could withstand, and worked back to the line inputs, where (IIRC) we had to withstand 2 KV Oc or 500 Amp short circuit lightning transients. It helped that everything in the Central Office had a common-point ground, even if it was 150 feet below, in the basement, as this made backdoor entry less likely. In a later incarnation, I found aviation customers who wanted to test transient protection without opening the equipment, which is another story -- and on these, we had to inject transients on the CASE. I suggested some commercially available modules for AC power protection here because one does NOT want to try to protect the AC power network, only what he builds, and just throwing in spark gaps, gas tubes, Tranzorbs(tm) or MOV's might be asking for trouble. Know the threat, and protect against THAT. Cortland Richmond On 9/9/2013 1130, Kunde, Brian wrote: What are the safety considerations using gas tubes on the AC mains? Do you have to fuse them or are they not likely to fail shorted? Can you use them between line and PE? Do you have to use multiple parts in series? I often see them in series with MOVs in a “T” configuration to protect against line to line and line to PE surges. A few year back we had a product that had several surge suppression circuits located on different PC boards within (some assemblies were very expensive and we wanted to protect them). Well, at our customer site they experienced some kind of huge surge, transient or overvoltage (we do not know what exactly happened). Of all the equipment that was on-site including many of our competitors equipment, only our instrument was damaged. Our surge suppressors were blown up, charred, and/or vaporized. The warranty repair cost was $10,000US but the hit to our reputation was probably worst. We believed that our equipment probably protected all the other equipment on-site but it is hard to get your customers to believe you. So now we want to better control our surge protection and if we see a huge surge we hope it to destroy something much less expensive to replace or at least minimize the damage. What we are currently thinking is to use over the counter Surge Suppressor modules, but they are only good to about 3KV – 4KV. Then we thought we would add a spark-gap in the board that would only kick in if our surge suppressors failed. Maybe we can add some very high voltage Gas Tubes also or instead of the spark-gap. I’m not sure what more we can do. Many of the circuits/assemblies we are trying to protect are buy/sell components where we do not control spacings. Any comments? Thanks to all. The Other Brian - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.orgmailto:emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe)http://www.ieee-pses.org/list.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.netmailto:emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.orgmailto:mcantw...@ieee.org For policy questions, send mail to: Jim Bacher j.bac...@ieee.orgmailto:j.bac...@ieee.org David Heald dhe...@gmail.commailto:dhe...@gmail.com - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send
Re: [PSES] Spark Gap PCB Layout on AC Mains
In message 64D32EE8B9CBDD44963ACB076A5F6ABB0266533F@Mailbox-Tech.lecotech.local, dated Mon, 9 Sep 2013, Kunde, Brian brian_ku...@lecotc.com writes: What are the safety considerations using gas tubes on the AC mains? Do you have to fuse them or are they not likely to fail shorted? Not likely, but in theory anything with metal electrodes can arc over, and the arc has negative resistance. Fuses may not help, because they can arc over as well. Can you use them between line and PE? Do you have to use multiple parts in series? I often see them in series with MOVs in a “T” configuration to protect against line to line and line to PE surges. Opinions tend to differ on this. The text in IEC 60950-1 was quite controversial. A few year back we had a product that had several surge suppression circuits located on different PC boards within (some assemblies were very expensive and we wanted to protect them). Well, at our customer site they experienced some kind of huge surge, transient or overvoltage (we do not know what exactly happened). That's a pity. It might have paid to hire an expert to try to find a cause. Of all the equipment that was on-site including many of our competitors equipment, only our instrument was damaged. Our surge suppressors were blown up, charred, and/or vaporized. Surges can be extremely selective like that. The warranty repair cost was $10,000US but the hit to our reputation was probably worst. We believed that our equipment probably protected all the other equipment on-site but it is hard to get your customers to believe you. So now we want to better control our surge protection and if we see a huge surge we hope it to destroy something much less expensive to replace or at least minimize the damage. Do you know about the collection of science and lore on protection in the ITU-T series K publications? You can get these from: http://www.itu.int/rec/T-REC-K/e and I think they are free, but there may be a limit on how many one person can download. What we are currently thinking is to use over the counter Surge Suppressor modules, but they are only good to about 3KV – 4KV. Then we thought we would add a spark-gap in the board that would only kick in if our surge suppressors failed. Maybe we can add some very high voltage Gas Tubes also or instead of the spark-gap. I’m not sure what more we can do. Many of the circuits/assemblies we are trying to protect are buy/sell components where we do not control spacings. I suggest you look at the K series publications before making any decision. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk If dictionaries were correct, we would only need one, because they would all give the same information. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
Brian, The main safety considerations in using VDR/MOV or MOV and GDT in series in primary circuit (at least per 60950) are outlined in section 1.5.9 and Annex Q One key factor is about to change in EN60950-1 A2 (currently released as IEC60950-1 ed2.2 and draft A2 for EN60950-1). Previous revisions allowed BASIC insulation to be bridged by MOV and GDT if met certain conditions (perm. Connected equipment, Pluggable Type B and perm. Connected earth) and GDT complying with FUNCTIONAL insulation requirements and spacing. New A2 will require GDT bridging BASIC insulation to comply with BASIC insulation requirements and spacing. And it is not allowed to bridge Double, re-enforced or supplementary insulation by VDR. mark From: Kunde, Brian [mailto:brian_ku...@lecotc.com] Sent: Monday, September 09, 2013 8:31 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains What are the safety considerations using gas tubes on the AC mains? Do you have to fuse them or are they not likely to fail shorted? Can you use them between line and PE? Do you have to use multiple parts in series? I often see them in series with MOVs in a “T” configuration to protect against line to line and line to PE surges. A few year back we had a product that had several surge suppression circuits located on different PC boards within (some assemblies were very expensive and we wanted to protect them). Well, at our customer site they experienced some kind of huge surge, transient or overvoltage (we do not know what exactly happened). Of all the equipment that was on-site including many of our competitors equipment, only our instrument was damaged. Our surge suppressors were blown up, charred, and/or vaporized. The warranty repair cost was $10,000US but the hit to our reputation was probably worst. We believed that our equipment probably protected all the other equipment on-site but it is hard to get your customers to believe you. So now we want to better control our surge protection and if we see a huge surge we hope it to destroy something much less expensive to replace or at least minimize the damage. What we are currently thinking is to use over the counter Surge Suppressor modules, but they are only good to about 3KV – 4KV. Then we thought we would add a spark-gap in the board that would only kick in if our surge suppressors failed. Maybe we can add some very high voltage Gas Tubes also or instead of the spark-gap. I’m not sure what more we can do. Many of the circuits/assemblies we are trying to protect are buy/sell components where we do not control spacings. Any comments? Thanks to all. The Other Brian From: Anthony Thomson [mailto:ton...@europe.com] Sent: Monday, September 09, 2013 4:18 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hello Brian, I've employed spark gaps, like you, not because you 'have' to but because it seemed good practice. It involved a control installation with cables strung externally. My advice is to use propriatory discharge tubes. They're cheap and their performance is more predictable than engineering your own air gap across PCB tracks or using pointy pins and are much less influenced physical and environmental conditions such as temperature and humidity, etc. And should they ever be needed, the consequences can be much less messy. I found a good selection available and looked at PCB mounting tubes with breakdown voltages of between 3 and 12 kV. I finally used 4kV, 5kA/10kA (10/1 discharges) devices having been influenced by what professional LAN GPS installers were using which largely ranged between 3 and 6 kV. Just my thoughts. T - Original Message - From: Kunde, Brian Sent: 09/06/13 04:56 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian _ LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received
Re: [PSES] Spark Gap PCB Layout on AC Mains
Hi Gert: Thanks, it's nice to see that gas tubes rated at several thousand volts do exist. I looked at the data sheet and it is unclear whether these gas tubes would reset to the off condition after the surge ends, given that in Brian's application there might be 120/240 VRMS across the gas tube. The so-called follow-on current that the normal 120/240 VRMS supply can deliver might keep the gas tube in the on condition, unless the gas tube extinguishes very rapidly in response to a zero crossing of the 50/60 Hz waveform. This may be one of the reasons why some applications use an MOV in series with the gas tube. The MOV would block the follow-on current, while the gas tube would set the turn-on voltage. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com The First Google hit .. http://www.littelfuse.com/products/gas-discharge-tubes/high-voltage-gdt.aspx (no connections with them) Anyway, the primary circuit always need to be build for high surge currents, and one should always insert induction to limit surge currents. As the impedance of the spark gap is low, only a small amount of induction will reduce the surge current substantially. Regards, Ing. Gert Gremmen, BSc Van: Joe Randolph [mailto:j...@randolph-telecom.com] Verzonden: Friday, September 06, 2013 11:32 PM Aan: EMC-PSTC@LISTSERV.IEEE.ORG Onderwerp: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Gert: Do you know of a source for a small, inexpensive gas tube with a rated breakdown voltage in the range of 6 KV to 10 KV? I don't, but I'd like to know of a suitable source. As I noted, conventional gas tubes are typically rated at a few hundred volts. Putting a 400 volt gas tube across a barrier that breaks down at 6 KV to 10 KV invites surge currents that otherwise would not occur. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com An air gap is a gas tube without the tube And the air properties vary, and so the performance with altitude , humidity and pollutions. So just use a commercial available gas discharge tube .! I have seen and recommended ( in that order ;) mains wire coiled into a 10 cm coil before being connected to a spark device., thus adding a bit of inductance to reduce the current. That fits nice with your observations on high breakdown voltages. Gert Gremmen Van: Joe Randolph [mailto:j...@randolph-telecom.com] Verzonden: vrijdag 6 september 2013 21:06 Aan: EMC-PSTC@LISTSERV.IEEE.ORG Onderwerp: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: You mention that there are companies that make air gaps. Are these devices anything like what I described? What I have in mind is an inexpensive component about the same physical size and cost as a gas tube, with two tungsten electrodes separated by an air gap of 5 to 10 mm to achieve a nominal air breakdown levels in the range of 6 KV to 10 KV. I don't think corona would be an issue in an application where the normal working voltage is just 120/240 VRMS AC main voltages. The intended use would be what I think Brian had in mind, which is to provide a known path for surges that exceed the rating of the insulation. If the size of the air gap is coordinated properly with a good insulation barrier, the gap could be expected to trigger very rarely (if ever) in the product's lifetime. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com Joe, Yes we are in agreement, and you make several valid points regarding gas discharge tubes. As is always the case in any design, there are trade-offs. I have found it beneficial to use a combination of the very good ideas being discussed on this thread. For example, solid insulation barriers in combination with EMI filtering and surge suppression. The solid insulation an be judicially placed with thickness sufficient to prevent punch-thru and sufficient creepage distance to prevent flashover. An EMI filter adds come level of series impedance to high frequencies and surge suppression devices behind this are less likely to fail. Gas discharge tubes have another problem in that a poorly selected voltage breakdown may cause the gas to glow under normal operating conditions and these devices are nothing like the old neon lamps, they will fail as a result. In addition, when they do fire, they are unlike MOVs in that they clamp to near zero volts and the only way to extinguish the plasma within the tube is a zero crossing of the line voltage. MOVs are always suspect because of the leakage current heating problem and catastrophic failure mode where they sputter metal on nearby surfaces. There are companies who make air gaps and these are viable so long as they do not have a problem with corona when the electrodes erode (due to arcing) into ragged edges. Sharp points can lower
Re: [PSES] Spark Gap PCB Layout on AC Mains
Indeed... the protection scheme that passes Safety, causes the system to 'fail safe' which does not meet the EMC immunity requirements. The previous 'remove the protection from the circuit for the hi-pot test' seem to have been removed. Now the IGBT's serve to cause the fuse to go open, which is fine for the Safety guys. From: McInturff, Gary gary.mcintu...@esterline.com To: EMC-PSTC@LISTSERV.IEEE.ORG Sent: Monday, September 9, 2013 11:42 AM Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Yet another example that transistors and IC’s were invented to protect fuses and surge protectors. Gary From:Kunde, Brian [mailto:brian_ku...@lecotc.com] Sent: Monday, September 09, 2013 8:31 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains What are the safety considerations using gas tubes on the AC mains? Do you have to fuse them or are they not likely to fail shorted? Can you use them between line and PE? Do you have to use multiple parts in series? I often see them in series with MOVs in a “T” configuration to protect against line to line and line to PE surges. A few year back we had a product that had several surge suppression circuits located on different PC boards within (some assemblies were very expensive and we wanted to protect them). Well, at our customer site they experienced some kind of huge surge, transient or overvoltage (we do not know what exactly happened). Of all the equipment that was on-site including many of our competitors equipment, only our instrument was damaged. Our surge suppressors were blown up, charred, and/or vaporized. The warranty repair cost was $10,000US but the hit to our reputation was probably worst. We believed that our equipment probably protected all the other equipment on-site but it is hard to get your customers to believe you. So now we want to better control our surge protection and if we see a huge surge we hope it to destroy something much less expensive to replace or at least minimize the damage. What we are currently thinking is to use over the counter Surge Suppressor modules, but they are only good to about 3KV – 4KV. Then we thought we would add a spark-gap in the board that would only kick in if our surge suppressors failed. Maybe we can add some very high voltage Gas Tubes also or instead of the spark-gap. I’m not sure what more we can do. Many of the circuits/assemblies we are trying to protect are buy/sell components where we do not control spacings. Any comments? Thanks to all. The Other Brian From:Anthony Thomson [mailto:ton...@europe.com] Sent: Monday, September 09, 2013 4:18 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hello Brian, I've employed spark gaps, like you, not because you 'have' to but because it seemed good practice. It involved a control installation with cables strung externally. My advice is to use propriatory discharge tubes. They're cheap and their performance is more predictable than engineering your own air gap across PCB tracks or using pointy pins and are much less influenced physical and environmental conditions such as temperature and humidity, etc. And should they ever be needed, the consequences can be much less messy. I found a good selection available and looked at PCB mounting tubes with breakdown voltages of between 3 and 12 kV. I finally used 4kV, 5kA/10kA (10/1 discharges) devices having been influenced by what professional LAN GPS installers were using which largely ranged between 3 and 6 kV. Just my thoughts. T - Original Message - From: Kunde, Brian Sent: 09/06/13 04:56 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian LECO Corporation Notice:This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you
Re: [PSES] Spark Gap PCB Layout on AC Mains
Hi Brian: Your story about the surge failure a few years ago sounds like it *might* have been a case of having a surge protector threshold that was so low that it invited surge currents into the equipment, although the failure could also have been due to other causes. Lightning always seeks the lowest impedance path to ground. So, if you connect ten different pieces of equipment in parallel on the same AC mains line, the one with the lowest breakdown voltage to earth will try to draw all of the surge current. In some cases it can become the sacrificial element that protects all the other elements. These types of scenarios are one of the reasons that I prefer to block surges when possible rather than conduct them, as I described in an earlier posting. You mention that for your current project, many of the circuits/assemblies you are trying to protect are buy/sell products where you don't control spacings. If that is the case, putting big spacings on your AC distribution board may not help much. The surge will just find a weaker spacing farther inside your system, which is precisely what you are trying to avoid. I don't know enough about your system to suggest a solution. As you note, one option is to put your protection right at the AC mains input and then make sure that it holds surge voltages below the breakdown level of the weakest circuit/assembly behind it. Properly done, you might be able to arrange things so that most of the predictable failure modes damage only that front end board. Another option that would be technically preferable but possibly too big and expensive would be to put a high-dielectric isolation transformer right at the AC mains input. That can greatly simplify the protection scheme and make it more robust too. One thing to keep in mind is that in large interconnected systems that have multiple connections to earth ground, a phenomenon called ground potential rise (GPR) can cause surges to come up through one ground connection and go out another. The GPR mechanism is probably the most difficult one for most people to visualize. I included some simplified drawings of this mechanism in the IEEE paper posted on my web site. If your system has multiple paths to earth ground, you should look very carefully at whether GPR presents a risk for your system. If so, you may need to place some requirements on how the system is installed and bonded to earth ground. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com What are the safety considerations using gas tubes on the AC mains? Do you have to fuse them or are they not likely to fail shorted? Can you use them between line and PE? Do you have to use multiple parts in series? I often see them in series with MOVs in a âTâ configuration to protect against line to line and line to PE surges. A few year back we had a product that had several surge suppression circuits located on different PC boards within (some assemblies were very expensive and we wanted to protect them). Well, at our customer site they experienced some kind of huge surge, transient or overvoltage (we do not know what exactly happened). Of all the equipment that was on-site including many of our competitors equipment, only our instrument was damaged. Our surge suppressors were blown up, charred, and/or vaporized. The warranty repair cost was $10,000US but the hit to our reputation was probably worst. We believed that our equipment probably protected all the other equipment on-site but it is hard to get your customers to believe you. So now we want to better control our surge protection and if we see a huge surge we hope it to destroy something much less expensive to replace or at least minimize the damage. What we are currently thinking is to use over the counter Surge Suppressor modules, but they are only good to about 3KV 4KV. Then we thought we would add aa spark-gap in the board that would only kick in if our surge suppressors failed. Maybe we can add some very high voltage Gas Tubes also or instead of the spark-gap. Iâm not sure what more we can do. Many of the circuits/assemblies we are trying to protect are buy/sell components where we do not control spacings. Any comments? Thanks to all. The Other Brian From: Anthony Thomson [mailto:ton...@europe.com] Sent: Monday, September 09, 2013 4:18 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hello Brian, I've employed spark gaps, like you, not because you 'have' to but because it seemed good practice. It involved a control installation with cables strung externally. My advice is to use propriatory discharge tubes. They're cheap and their performance is more predictable than engineering your own air gap across PCB tracks or using pointy pins and are much less influenced physical and environmental conditions such as temperature and humidity, etc. And should they ever be needed
Re: [PSES] Spark Gap PCB Layout on AC Mains
The First Google hit . http://www.littelfuse.com/products/gas-discharge-tubes/high-voltage-gdt.aspx (no connections with them) Anyway, the primary circuit always need to be build for high surge currents, and one should always insert induction to limit surge currents. As the impedance of the spark gap is low, only a small amount of induction will reduce the surge current substantially. Regards, Ing. Gert Gremmen, BSc Van: Joe Randolph [mailto:j...@randolph-telecom.com] Verzonden: Friday, September 06, 2013 11:32 PM Aan: EMC-PSTC@LISTSERV.IEEE.ORG Onderwerp: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Gert: Do you know of a source for a small, inexpensive gas tube with a rated breakdown voltage in the range of 6 KV to 10 KV? I don't, but I'd like to know of a suitable source. As I noted, conventional gas tubes are typically rated at a few hundred volts. Putting a 400 volt gas tube across a barrier that breaks down at 6 KV to 10 KV invites surge currents that otherwise would not occur. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com http://www.randolph-telecom.com/ An air gap is a gas tube without the tube.. And the air properties vary, and so the performance with altitude , humidity and pollutions. So just use a commercial available gas discharge tube ! I have seen and recommended ( in that order ;) mains wire coiled into a 10 cm coil before being connected to a spark device., thus adding a bit of inductance to reduce the current. That fits nice with your observations on high breakdown voltages. Gert Gremmen Van: Joe Randolph [mailto:j...@randolph-telecom.com] Verzonden: vrijdag 6 september 2013 21:06 Aan: EMC-PSTC@LISTSERV.IEEE.ORG Onderwerp: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: You mention that there are companies that make air gaps. Are these devices anything like what I described? What I have in mind is an inexpensive component about the same physical size and cost as a gas tube, with two tungsten electrodes separated by an air gap of 5 to 10 mm to achieve a nominal air breakdown levels in the range of 6 KV to 10 KV. I don't think corona would be an issue in an application where the normal working voltage is just 120/240 VRMS AC main voltages. The intended use would be what I think Brian had in mind, which is to provide a known path for surges that exceed the rating of the insulation. If the size of the air gap is coordinated properly with a good insulation barrier, the gap could be expected to trigger very rarely (if ever) in the product's lifetime. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com http://www.randolph-telecom.com/ Joe, Yes we are in agreement, and you make several valid points regarding gas discharge tubes. As is always the case in any design, there are trade-offs. I have found it beneficial to use a combination of the very good ideas being discussed on this thread. For example, solid insulation barriers in combination with EMI filtering and surge suppression. The solid insulation an be judicially placed with thickness sufficient to prevent punch-thru and sufficient creepage distance to prevent flashover. An EMI filter adds come level of series impedance to high frequencies and surge suppression devices behind this are less likely to fail. Gas discharge tubes have another problem in that a poorly selected voltage breakdown may cause the gas to glow under normal operating conditions and these devices are nothing like the old neon lamps, they will fail as a result. In addition, when they do fire, they are unlike MOVs in that they clamp to near zero volts and the only way to extinguish the plasma within the tube is a zero crossing of the line voltage. MOVs are always suspect because of the leakage current heating problem and catastrophic failure mode where they sputter metal on nearby surfaces. There are companies who make air gaps and these are viable so long as they do not have a problem with corona when the electrodes erode (due to arcing) into ragged edges. Sharp points can lower the breakdown voltage (e-fields) just as the ice pick experiment did back in high school. You would also need a way to replenish the air supply within the gaps since corona can build up and eventually arc over without requiring a surge event. In any design it is useful to test your mitigation attempts in the actual product design. thanks, -doug Douglas E Powell doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Joe Randolph mailto:j...@randolph-telecom.com Sent: Friday, September 06, 2013 11:34 AM To: Doug Powell mailto:doug...@gmail.com ; EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: I think we
Re: [PSES] Spark Gap PCB Layout on AC Mains
In message FCA549BE3ECF9D4CB8CB8576837EA48920A1A4@ZEUS.cetest.local, dated Sat, 7 Sep 2013, ce-test, qualified testing bv - Gert Gremmen g.grem...@cetest.nl writes: Anyway, the primary circuit always need to be build for high surge currents, and one should always insert induction to limit surge currents. As the impedance of the spark gap is low, only a small amount of induction will reduce the surge current substantially Quite true, but the mains supply itself (unless it's from a very nearby private MV/LV transformer, has inductance of the order of a millihenry (0.3 ohms at 50 Hz), so there is not much point in adding only a few microhenrys. IEC TR60725 gives 'reference impedances' for various types of mains supply. While these tend to be higher than the actual impedance at the 'point of common coupling', they are a guide to the impedance at a typical wall-socket. This impedance limits the prospective short-circuit current. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk If dictionaries were correct, we would only need one, because they would all give the same information. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
That depends of course how the transient/surge has been coupled into the mains. If that is due to a ligtning event at a short range, the impedance may be much less. Your analysis is right when the even has been generated far away. Regards, Ing. Gert Gremmen, BSc . -Oorspronkelijk bericht- Van: John Woodgate [mailto:j...@jmwa.demon.co.uk] Verzonden: Saturday, September 07, 2013 11:35 AM Aan: EMC-PSTC@LISTSERV.IEEE.ORG Onderwerp: Re: [PSES] Spark Gap PCB Layout on AC Mains In message FCA549BE3ECF9D4CB8CB8576837EA48920A1A4@ZEUS.cetest.local, dated Sat, 7 Sep 2013, ce-test, qualified testing bv - Gert Gremmen g.grem...@cetest.nl writes: Anyway, the primary circuit always need to be build for high surge currents, and one should always insert induction to limit surge currents. As the impedance of the spark gap is low, only a small amount of induction will reduce the surge current substantially Quite true, but the mains supply itself (unless it's from a very nearby private MV/LV transformer, has inductance of the order of a millihenry (0.3 ohms at 50 Hz), so there is not much point in adding only a few microhenrys. IEC TR60725 gives 'reference impedances' for various types of mains supply. While these tend to be higher than the actual impedance at the 'point of common coupling', they are a guide to the impedance at a typical wall-socket. This impedance limits the prospective short-circuit current. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk If dictionaries were correct, we would only need one, because they would all give the same information. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
I often see spark gaps on the mains side of AC-DC power supplies PCBs. I consider it a best practice. Gary Tornquist Director of Product Safety Microsoft Corp From: Kunde, Brian [mailto:brian_ku...@lecotc.com] Sent: Friday, September 6, 2013 8:56 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.orgmailto:emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.netmailto:emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.orgmailto:mcantw...@ieee.org For policy questions, send mail to: Jim Bacher j.bac...@ieee.orgmailto:j.bac...@ieee.org David Heald dhe...@gmail.commailto:dhe...@gmail.com - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
Brian, I am personally opposed to such an idea. The first time I saw this was circa 1982 on an electronic security system; designed to tie into the public telephone network. The manufacturer wanted additional protection above the outdoor surge protector. The design involved two parallel zig-zag traces, one ground and the other telephone line with alternating points in close proximity. The problem with this is if the gaps you created ever activate, they always leave a permanent carbon track in the surface of the PCB. Given adequate time and humidity exposure, these tracks become conductive and leakage current can begin to increase dramatically. Eventually, this circuits will become permanently shorted. This the main reason for evaluating comparative tracking index (CTI) of insulating materials along with creepage evaluation. Best to simply use a glass or ceramic spark gaps which are inorganic and cannot produce carbon when arced (http://www.globalsources.com/manufacturers/Glass-Switching-Spark-Gap.html). thanks, –doug Douglas E Powell doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Kunde, Brian Sent: Friday, September 06, 2013 9:56 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher j.bac...@ieee.org David Heald dhe...@gmail.com - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
Hi Doug: I think we are in agreement about some of the pitfalls of spark gaps that are simply added in the copper layout. Regarding spark gap components, the devices you cite (also known as gas tubes) typically fire at just a few hundred volts. Adding these to enhance the surge tolerance of an air gap can have unintended consequences. My philosophy for lightning protection is that the designer has only two options: 1) Block the surge current using adequate insulation 2) Direct the surge current to a known, safe path My preference is to use Option 1 whenever possible. As I noted in my earlier posting, the probability of experiencing a given surge drops off dramatically with increasing voltage. So, if a product is designed with spacings that break down at 10 KV, the probability of a surge exceeding that threshold is fairly low. If you then add a 400 volt gas tube across the barrier, the probability of a surge exceeding that threshold is far higher. The result is that the gas tube invites surge current that otherwise would not have flowed. This may be okay *if* the gas tube is reliable and *if* the surge path that it creates is also reliable and robust. The key thing to be aware of is that with 400 volt gas tubes installed across the barrier, surge currents will flow across the barrier on a fairly regular basis. If the system has a 10,000 volt breakdown without the gas tubes installed, surge currents flowing across the barrier will be far less common without the gas tubes (probably by a factor of 1000 or more). Another unintended consequence of using conventional gas tubes is that they create a very steep current rise time when they trigger. This generates an electromagnetic pulse that can propagate through the system and upset sensitive electronics. In the IEEE PSES Telecom TAC we have been discussing this problem for a while now. Interestingly, some Ethernet ports have higher failure rates with gas tubes installed than without the gas tubes installed. For the above reasons, I prefer to use Option 1 (block the surge current) whenever possible. If I had a system that could withstand 10 KV but I wanted to add a spark gap for the (rare) cases where the surge exceeds 10 KV, I would try to set the spark gap breakdown as high as possible, such as 9 KV. I am not aware a conventional gas tube with a 9 KV threshold, but perhaps such devices exist. One alternative that I have often wished someone would make is a simple air gap component with tungsten electrodes that could withstand multiple surges. Such a device could be made quite inexpensively and would be more robust than a simple gap in a copper layout. If the nominal trigger voltage was very high (say, 9 KV), the device could be expected to trigger very rarely, if ever, in actual use. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com Brian, I am personally opposed to such an idea. The first time I saw this was circa 1982 on an electronic security system; designed to tie into the public telephone network. The manufacturer wanted additional protection above the outdoor surge protector. The design involved two parallel zig-zag traces, one ground and the other telephone line with alternating points in close proximity. The problem with this is if the gaps you created ever activate, they always leave a permanent carbon track in the surface of the PCB. Given adequate time and humidity exposure, these tracks become conductive and leakage current can begin to increase dramatically. Eventually, this circuits will become permanently shorted. This the main reason for evaluating comparative tracking index (CTI) of insulating materials along with creepage evaluation. Best to simply use a glass or ceramic spark gaps which are inorganic and cannot produce carbon when arced (http://www.globalsources.com/manufacturers/Glass-Switching-Spark-Gap.html). thanks, doug Douglas E Powell doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Kunde, Brian Sent: Friday, September 06, 2013 9:56 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for
Re: [PSES] Spark Gap PCB Layout on AC Mains
The power supply PCB’s I see have holes or slots cut in them between the pointy conductors. The arc occurs across the air gap. This avoids the carbon tracking on the PCB condition raised below. Gary T. From: Doug Powell [mailto:doug...@gmail.com] Sent: Friday, September 6, 2013 9:33 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Brian, I am personally opposed to such an idea. The first time I saw this was circa 1982 on an electronic security system; designed to tie into the public telephone network. The manufacturer wanted additional protection above the outdoor surge protector. The design involved two parallel zig-zag traces, one ground and the other telephone line with alternating points in close proximity. The problem with this is if the gaps you created ever activate, they always leave a permanent carbon track in the surface of the PCB. Given adequate time and humidity exposure, these tracks become conductive and leakage current can begin to increase dramatically. Eventually, this circuits will become permanently shorted. This the main reason for evaluating comparative tracking index (CTI) of insulating materials along with creepage evaluation. Best to simply use a glass or ceramic spark gaps which are inorganic and cannot produce carbon when arced (http://www.globalsources.com/manufacturers/Glass-Switching-Spark-Gap.html). thanks, –doug Douglas E Powell doug...@gmail.commailto:doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Kunde, Brianmailto:brian_ku...@lecotc.com Sent: Friday, September 06, 2013 9:56 AM To: EMC-PSTC@LISTSERV.IEEE.ORGmailto:EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.orgmailto:emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.netmailto:emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.orgmailto:mcantw...@ieee.org For policy questions, send mail to: Jim Bacher j.bac...@ieee.orgmailto:j.bac...@ieee.org David Heald dhe...@gmail.commailto:dhe...@gmail.com - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.orgmailto:emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.netmailto:emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.orgmailto:mcantw...@ieee.org For policy questions, send mail to: Jim Bacher j.bac...@ieee.orgmailto:j.bac...@ieee.org David Heald dhe...@gmail.commailto:dhe...@gmail.com - This message is from the IEEE Product Safety Engineering
Re: [PSES] Spark Gap PCB Layout on AC Mains
If the PCB material is removed between the spark-gap layout making an air-gap, does this resolve the carbon tracking problem? The spark-gap would not be our primary approach to dealing with surge pulses; but only to better deal with those higher surge pulses than what we design for. The thought is that a really high voltage surge pulse is going to arc somewhere so we would rather it arcs and destroys a $50 PCB rather than a $1000 PCB. Thanks for all the input so far. The Other Brian From: Doug Powell [mailto:doug...@gmail.com] Sent: Friday, September 06, 2013 12:33 PM To: Kunde, Brian; EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Brian, I am personally opposed to such an idea. The first time I saw this was circa 1982 on an electronic security system; designed to tie into the public telephone network. The manufacturer wanted additional protection above the outdoor surge protector. The design involved two parallel zig-zag traces, one ground and the other telephone line with alternating points in close proximity. The problem with this is if the gaps you created ever activate, they always leave a permanent carbon track in the surface of the PCB. Given adequate time and humidity exposure, these tracks become conductive and leakage current can begin to increase dramatically. Eventually, this circuits will become permanently shorted. This the main reason for evaluating comparative tracking index (CTI) of insulating materials along with creepage evaluation. Best to simply use a glass or ceramic spark gaps which are inorganic and cannot produce carbon when arced (http://www.globalsources.com/manufacturers/Glass-Switching-Spark-Gap.html). thanks, –doug Douglas E Powell doug...@gmail.commailto:doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Kunde, Brianmailto:brian_ku...@lecotc.com Sent: Friday, September 06, 2013 9:56 AM To: EMC-PSTC@LISTSERV.IEEE.ORGmailto:EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Spark Gap PCB Layout on AC Mains Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.orgmailto:emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.netmailto:emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.orgmailto:mcantw...@ieee.org For policy questions, send mail to: Jim Bacher j.bac...@ieee.orgmailto:j.bac...@ieee.org David Heald dhe...@gmail.commailto:dhe...@gmail.com LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http
Re: [PSES] Spark Gap PCB Layout on AC Mains
In message 64D32EE8B9CBDD44963ACB076A5F6ABB02664E67@Mailbox-Tech.lecotech.local, dated Fri, 6 Sep 2013, Kunde, Brian brian_ku...@lecotc.com writes: I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? I believe it is done - an air-gap is arranged to break down at 6 kV on 230 V mains. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk If dictionaries were correct, we would only need one, because they would all give the same information. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
Murphy says it will destroy BOTH boards. Ed Price WB6WSN Chula Vista, CA USA From: Kunde, Brian [mailto:brian_ku...@lecotc.com] Sent: Friday, September 06, 2013 10:15 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains The thought is that a really high voltage surge pulse is going to arc somewhere so we would rather it arcs and destroys a $50 PCB rather than a $1000 PCB. The Other Brian - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
PCB gaps (with or without slot) are okay for low-current discharges. For high-current discharges such as occur on mains circuits, the current will vaporize the copper and increase the clearance distance, and thereby increase the breakdown voltage with each overvoltage event. Rich On 9/6/2013 10:08 AM, John Woodgate wrote: In message F5B74858AAFC4C929D1E6F81869E1EA0@ComputerNo11, dated Fri, 6 Sep 2013, Doug Powell doug...@gmail.com writes: Best to simply use a glass or ceramic spark gaps which are inorganic and cannot produce carbon when arced You are right: using PCB tracks is to invite tracking! A slot in the board with electrodes on both sides is better. These were used on CRT base PCBs for many years with no significant problems. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
An air gap is a gas tube without the tube.. And the air properties vary, and so the performance with altitude , humidity and pollutions. So just use a commercial available gas discharge tube ! I have seen and recommended ( in that order ;) mains wire coiled into a 10 cm coil before being connected to a spark device., thus adding a bit of inductance to reduce the current. That fits nice with your observations on high breakdown voltages. Gert Gremmen Van: Joe Randolph [mailto:j...@randolph-telecom.com] Verzonden: vrijdag 6 september 2013 21:06 Aan: EMC-PSTC@LISTSERV.IEEE.ORG Onderwerp: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: You mention that there are companies that make air gaps. Are these devices anything like what I described? What I have in mind is an inexpensive component about the same physical size and cost as a gas tube, with two tungsten electrodes separated by an air gap of 5 to 10 mm to achieve a nominal air breakdown levels in the range of 6 KV to 10 KV. I don't think corona would be an issue in an application where the normal working voltage is just 120/240 VRMS AC main voltages. The intended use would be what I think Brian had in mind, which is to provide a known path for surges that exceed the rating of the insulation. If the size of the air gap is coordinated properly with a good insulation barrier, the gap could be expected to trigger very rarely (if ever) in the product's lifetime. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com http://www.randolph-telecom.com/ Joe, Yes we are in agreement, and you make several valid points regarding gas discharge tubes. As is always the case in any design, there are trade-offs. I have found it beneficial to use a combination of the very good ideas being discussed on this thread. For example, solid insulation barriers in combination with EMI filtering and surge suppression. The solid insulation an be judicially placed with thickness sufficient to prevent punch-thru and sufficient creepage distance to prevent flashover. An EMI filter adds come level of series impedance to high frequencies and surge suppression devices behind this are less likely to fail. Gas discharge tubes have another problem in that a poorly selected voltage breakdown may cause the gas to glow under normal operating conditions and these devices are nothing like the old neon lamps, they will fail as a result. In addition, when they do fire, they are unlike MOVs in that they clamp to near zero volts and the only way to extinguish the plasma within the tube is a zero crossing of the line voltage. MOVs are always suspect because of the leakage current heating problem and catastrophic failure mode where they sputter metal on nearby surfaces. There are companies who make air gaps and these are viable so long as they do not have a problem with corona when the electrodes erode (due to arcing) into ragged edges. Sharp points can lower the breakdown voltage (e-fields) just as the ice pick experiment did back in high school. You would also need a way to replenish the air supply within the gaps since corona can build up and eventually arc over without requiring a surge event. In any design it is useful to test your mitigation attempts in the actual product design. thanks, -doug Douglas E Powell doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Joe Randolph mailto:j...@randolph-telecom.com Sent: Friday, September 06, 2013 11:34 AM To: Doug Powell mailto:doug...@gmail.com ; EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: I think we are in agreement about some of the pitfalls of spark gaps that are simply added in the copper layout. Regarding spark gap components, the devices you cite (also known as gas tubes) typically fire at just a few hundred volts. Adding these to enhance the surge tolerance of an air gap can have unintended consequences. My philosophy for lightning protection is that the designer has only two options: 1) Block the surge current using adequate insulation 2) Direct the surge current to a known, safe path My preference is to use Option 1 whenever possible. As I noted in my earlier posting, the probability of experiencing a given surge drops off dramatically with increasing voltage. So, if a product is designed with spacings that break down at 10 KV, the probability of a surge exceeding that threshold is fairly low. If you then add a 400 volt gas tube across the barrier, the probability of a surge exceeding that threshold is far higher. The result is that the gas tube invites surge current that otherwise would not have flowed. This may be okay *if* the gas tube is reliable and *if* the surge path that it creates is also reliable and robust. The key thing to be aware of is that with 400 volt gas
Re: [PSES] Spark Gap PCB Layout on AC Mains
The devices I had in mind were DIN Rail mounted and rated for 6 KV and up. In addition they have built in arc breakers and half cycle surge current ratings of a few thousand amps.For the life of me I cannot recall the company name right now. When I have an opportunity, I will check through my data sheet archive to locate.Thanks, - dougDouglas Powellhttp://www.linkedin.com/in/dougp01 From: Joe RandolphSent: Friday, September 6, 2013 1:05 PMTo: Doug Powell; EMC-PSTC@LISTSERV.IEEE.ORGSubject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: You mention that there are companies that make air gaps. Are these devices anything like what I described? What I have in mind is an inexpensive component about the same physical size and cost as a gas tube, with two tungsten electrodes separated by an air gap of 5 to 10 mm to achieve a nominal air breakdown levels in the range of 6 KV to 10 KV. I don't think corona would be an issue in an application where the normal working voltage is just 120/240 VRMS AC main voltages. The intended use would be what I think Brian had in mind, which is to provide a known path for surges that exceed the rating of the insulation. If the size of the air gap is coordinated properly with a good insulation barrier, the gap could be expected to trigger very rarely (if ever) in the product's lifetime. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com Joe, Yes we are in agreement, and you make several valid points regarding gas discharge tubes. As is always the case in any design, there are trade-offs. I have found it beneficial to use a combination of the very good ideas being discussed on this thread. For example, solid insulation barriers in combination with EMI filtering and surge suppression. The solid insulation an be judicially placed with thickness sufficient to prevent punch-thru and sufficient creepage distance to prevent flashover. An EMI filter adds come level of series impedance to high frequencies and surge suppression devices behind this are less likely to fail. Gas discharge tubes have another problem in that a poorly selected voltage breakdown may cause the gas to glow under normal operating conditions and these devices are nothing like the old neon lamps, they will fail as a result. In addition, when they do fire, they are unlike MOVs in that they clamp to near zero volts and the only way to extinguish the plasma within the tube is a zero crossing of the line voltage. MOVs are always suspect because of the leakage current heating problem and catastrophic failure mode where they sputter metal on nearby surfaces. There are companies who make air gaps and these are viable so long as they do not have a problem with corona when the electrodes erode (due to arcing) into ragged edges. Sharp points can lower the breakdown voltage (e-fields) just as the ice pick experiment did back in high school. You would also need a way to replenish the air supply within the gaps since corona can build up and eventually arc over without requiring a surge event. In any design it is useful to test your mitigation attempts in the actual product design. thanks, doug Douglas E Powell doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Joe Randolph Sent: Friday, September 06, 2013 11:34 AM To: Doug Powell ; EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: I think we are in agreement about some of the pitfalls of spark gaps that are simply added in the copper layout. Regarding "spark gap" components, the devices you cite (also known as gas tubes) typically fire at just a few hundred volts. Adding these to enhance the surge tolerance of an air gap can have unintended consequences. My philosophy for lightning protection is that the designer has only two options: 1) Block the surge current using adequate insulation 2) Direct the surge current to a known, safe path My preference is to use Option 1 whenever possible. As I noted in my earlier posting, the probability of experiencing a given surge drops off dramatically with increasing voltage. So, if a product is designed with spacings that break down at 10 KV, the probability of a surge exceeding that threshold is fairly low. If you then add a 400 volt gas tube across the barrier, the probability of a surge exceeding that threshold is far higher. The result is that the gas tube "invites" surge current that otherwise would not have flowed. This may be okay *if* the gas tube is reliable and *if* the surge path that it creates is also reliable and robust. The k
Re: [PSES] Spark Gap PCB Layout on AC Mains
Joe, Yes we are in agreement, and you make several valid points regarding gas discharge tubes. As is always the case in any design, there are trade-offs. I have found it beneficial to use a combination of the very good ideas being discussed on this thread. For example, solid insulation barriers in combination with EMI filtering and surge suppression. The solid insulation an be judicially placed with thickness sufficient to prevent punch-thru and sufficient creepage distance to prevent flashover. An EMI filter adds come level of series impedance to high frequencies and surge suppression devices behind this are less likely to fail. Gas discharge tubes have another problem in that a poorly selected voltage breakdown may cause the gas to glow under normal operating conditions and these devices are nothing like the old neon lamps, they will fail as a result. In addition, when they do fire, they are unlike MOVs in that they clamp to near zero volts and the only way to extinguish the plasma within the tube is a zero crossing of the line voltage. MOVs are always suspect because of the leakage current heating problem and catastrophic failure mode where they sputter metal on nearby surfaces. There are companies who make air gaps and these are viable so long as they do not have a problem with corona when the electrodes erode (due to arcing) into ragged edges. Sharp points can lower the breakdown voltage (e-fields) just as the ice pick experiment did back in high school. You would also need a way to replenish the air supply within the gaps since corona can build up and eventually arc over without requiring a surge event. In any design it is useful to test your mitigation attempts in the actual product design. thanks, –doug Douglas E Powell doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Joe Randolph Sent: Friday, September 06, 2013 11:34 AM To: Doug Powell ; EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: I think we are in agreement about some of the pitfalls of spark gaps that are simply added in the copper layout. Regarding spark gap components, the devices you cite (also known as gas tubes) typically fire at just a few hundred volts. Adding these to enhance the surge tolerance of an air gap can have unintended consequences. My philosophy for lightning protection is that the designer has only two options: 1) Block the surge current using adequate insulation 2) Direct the surge current to a known, safe path My preference is to use Option 1 whenever possible. As I noted in my earlier posting, the probability of experiencing a given surge drops off dramatically with increasing voltage. So, if a product is designed with spacings that break down at 10 KV, the probability of a surge exceeding that threshold is fairly low. If you then add a 400 volt gas tube across the barrier, the probability of a surge exceeding that threshold is far higher. The result is that the gas tube invites surge current that otherwise would not have flowed. This may be okay *if* the gas tube is reliable and *if* the surge path that it creates is also reliable and robust. The key thing to be aware of is that with 400 volt gas tubes installed across the barrier, surge currents will flow across the barrier on a fairly regular basis. If the system has a 10,000 volt breakdown without the gas tubes installed, surge currents flowing across the barrier will be far less common without the gas tubes (probably by a factor of 1000 or more). Another unintended consequence of using conventional gas tubes is that they create a very steep current rise time when they trigger. This generates an electromagnetic pulse that can propagate through the system and upset sensitive electronics. In the IEEE PSES Telecom TAC we have been discussing this problem for a while now. Interestingly, some Ethernet ports have higher failure rates with gas tubes installed than without the gas tubes installed. For the above reasons, I prefer to use Option 1 (block the surge current) whenever possible. If I had a system that could withstand 10 KV but I wanted to add a spark gap for the (rare) cases where the surge exceeds 10 KV, I would try to set the spark gap breakdown as high as possible, such as 9 KV. I am not aware a conventional gas tube with a 9 KV threshold, but perhaps such devices exist. One alternative that I have often wished someone would make is a simple air gap component with tungsten electrodes that could withstand multiple surges. Such a device could be made quite inexpensively and would be more robust than a simple gap in a copper layout. If the nominal trigger voltage was very high (say, 9 KV), the device could be expected to trigger very rarely, if ever, in actual use. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
Hi Doug: You mention that there are companies that make air gaps. Are these devices anything like what I described? What I have in mind is an inexpensive component about the same physical size and cost as a gas tube, with two tungsten electrodes separated by an air gap of 5 to 10 mm to achieve a nominal air breakdown levels in the range of 6 KV to 10 KV. I don't think corona would be an issue in an application where the normal working voltage is just 120/240 VRMS AC main voltages. The intended use would be what I think Brian had in mind, which is to provide a known path for surges that exceed the rating of the insulation. If the size of the air gap is coordinated properly with a good insulation barrier, the gap could be expected to trigger very rarely (if ever) in the product's lifetime. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com Joe, Yes we are in agreement, and you make several valid points regarding gas discharge tubes. As is always the case in any design, there are trade-offs. I have found it beneficial to use a combination of the very good ideas being discussed on this thread. For example, solid insulation barriers in combination with EMI filtering and surge suppression. The solid insulation an be judicially placed with thickness sufficient to prevent punch-thru and sufficient creepage distance to prevent flashover. An EMI filter adds come level of series impedance to high frequencies and surge suppression devices behind this are less likely to fail. Gas discharge tubes have another problem in that a poorly selected voltage breakdown may cause the gas to glow under normal operating conditions and these devices are nothing like the old neon lamps, they will fail as a result. In addition, when they do fire, they are unlike MOVs in that they clamp to near zero volts and the only way to extinguish the plasma within the tube is a zero crossing of the line voltage. MOVs are always suspect because of the leakage current heating problem and catastrophic failure mode where they sputter metal on nearby surfaces. There are companies who make air gaps and these are viable so long as they do not have a problem with corona when the electrodes erode (due to arcing) into ragged edges. Sharp points can lower the breakdown voltage (e-fields) just as the ice pick experiment did back in high school. You would also need a way to replenish the air supply within the gaps since corona can build up and eventually arc over without requiring a surge event. In any design it is useful to test your mitigation attempts in the actual product design. thanks, doug Douglas E Powell doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Joe Randolph Sent: Friday, September 06, 2013 11:34 AM To: Doug Powell ; EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: I think we are in agreement about some of the pitfalls of spark gaps that are simply added in the copper layout. Regarding spark gap components, the devices you cite (also known as gas tubes) typically fire at just a few hundred volts. Adding these to enhance the surge tolerance of an air gap can have unintended consequences. My philosophy for lightning protection is that the designer has only two options: 1) Block the surge current using adequate insulation 2) Direct the surge current to a known, safe path My preference is to use Option 1 whenever possible. As I noted in my earlier posting, the probability of experiencing a given surge drops off dramatically with increasing voltage. So, if a product is designed with spacings that break down at 10 KV, the probability of a surge exceeding that threshold is fairly low. If you then add a 400 volt gas tube across the barrier, the probability of a surge exceeding that threshold is far higher. The result is that the gas tube invites surge current that otherwise would not have flowed. This may be okay *if* the gas tube is reliable and *if* the surge path that it creates is also reliable and robust. The key thing to be aware of is that with 400 volt gas tubes installed across the barrier, surge currents will flow across the barrier on a fairly regular basis. If the system has a 10,000 volt breakdown without the gas tubes installed, surge currents flowing across the barrier will be far less common without the gas tubes (probably by a factor of 1000 or more). Another unintended consequence of using conventional gas tubes is that they create a very steep current rise time when they trigger. This generates an electromagnetic pulse that can propagate through the system and upset sensitive electronics. In the IEEE PSES Telecom TAC we have been discussing this problem for a while now. Interestingly, some Ethernet ports have higher failure rates with gas tubes installed than without the gas tubes installed. For the above reasons, I prefer to use Option 1 (block the surge current) whenever
Re: [PSES] Spark Gap PCB Layout on AC Mains
I'm not a power engineer, but I suspect you'll need something more than pointed bus bars. A quick search comes up with this: www.erico.com/public/library/fep/Surge/LT19915.pdf? Good luck! Cortland Richmond On 9/6/2013 1156, Kunde, Brian wrote: Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
In message F5B74858AAFC4C929D1E6F81869E1EA0@ComputerNo11, dated Fri, 6 Sep 2013, Doug Powell doug...@gmail.com writes: Best to simply use a glass or ceramic spark gaps which are inorganic and cannot produce carbon when arced You are right: using PCB tracks is to invite tracking! A slot in the board with electrodes on both sides is better. These were used on CRT base PCBs for many years with no significant problems. -- OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk If dictionaries were correct, we would only need one, because they would all give the same information. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher: j.bac...@ieee.org David Heald: dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
Hi Brian: I think it is a good idea to increase the spacings if you have room available. The statistical relationship between peak surge voltage and probability of occurrence strongly favors larger spacings. Doubling the spacings may reduce the probability of breakdown by a factor of ten. Regarding spark gaps, there are at least two known limitations for spark gaps that are simply added in the copper layout: *The actual firing voltage is highly variable * Copper traces are poor electrodes for high current surges. If the gap fires for a lightning surge, you will likely see considerable erosion of the copper electrodes and some carbon tracking across the gap. The first limitation may not be a major issue for you in this application. The firing voltage can be made less variable using pointed electrodes to produce an inhomogeneous field in the gap. The second limitation should be considered very carefully with respect to your goals. If your view is that you know that breakdown will occur in rare circumstances and you simply want to control where it occurs, adjusting the layout accordingly might be okay. However, any spark gap that you create in the copper layout will be damaged by the very first surge that breaks down over the spark gap, and the board may need to be replaced anyway. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com Our engineers are working on an AC Mains Distribution PCB. Like most electronic devices, we have seen the damage caused by lightning strikes. So we are increasing our creepage and clearance distances as wide as we can and still meet other requirements. But no matter what spacing you design to, there is a lightning bolt out there that will exceed the design and it will arc somewhere. So the question came up to whether it makes sense to deliberately make a weak spot, or an area where the clearance is slightly smaller to control where a lightning/surge pulse will arc and/or discharge, like a Spark-Gap. I have seen spark-gap lay outs on PC boards on I/O connectors; usually for ESD protection, but not on AC Mains. Is this a bad bad idea or something worth doing? Pros and Cons? Other suggestions?? Thanks to all for your help. The Other Brian LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher j.bac...@ieee.org David Heald dhe...@gmail.com - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to emc-p...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@radiusnorth.net Mike Cantwell mcantw...@ieee.org For policy questions, send mail to: Jim Bacher j.bac...@ieee.org David Heald dhe...@gmail.com
Re: [PSES] Spark Gap PCB Layout on AC Mains
Hi Gert: Do you know of a source for a small, inexpensive gas tube with a rated breakdown voltage in the range of 6 KV to 10 KV? I don't, but I'd like to know of a suitable source. As I noted, conventional gas tubes are typically rated at a few hundred volts. Putting a 400 volt gas tube across a barrier that breaks down at 6 KV to 10 KV invites surge currents that otherwise would not occur. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com An air gap is a gas tube without the tube And the air properties vary, and so the performance with altitude , humidity and pollutions. So just use a commercial available gas discharge tube .! I have seen and recommended ( in that order ;) mains wire coiled into a 10 cm coil before being connected to a spark device., thus adding a bit of inductance to reduce the current. That fits nice with your observations on high breakdown voltages. Gert Gremmen Van: Joe Randolph [mailto:j...@randolph-telecom.com] Verzonden: vrijdag 6 september 2013 21:06 Aan: EMC-PSTC@LISTSERV.IEEE.ORG Onderwerp: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: You mention that there are companies that make air gaps. Are these devices anything like what I described? What I have in mind is an inexpensive component about the same physical size and cost as a gas tube, with two tungsten electrodes separated by an air gap of 5 to 10 mm to achieve a nominal air breakdown levels in the range of 6 KV to 10 KV. I don't think corona would be an issue in an application where the normal working voltage is just 120/240 VRMS AC main voltages. The intended use would be what I think Brian had in mind, which is to provide a known path for surges that exceed the rating of the insulation. If the size of the air gap is coordinated properly with a good insulation barrier, the gap could be expected to trigger very rarely (if ever) in the product's lifetime. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) j...@randolph-telecom.com http://www.randolph-telecom.com Joe, Yes we are in agreement, and you make several valid points regarding gas discharge tubes. As is always the case in any design, there are trade-offs. I have found it beneficial to use a combination of the very good ideas being discussed on this thread. For example, solid insulation barriers in combination with EMI filtering and surge suppression. The solid insulation an be judicially placed with thickness sufficient to prevent punch-thru and sufficient creepage distance to prevent flashover. An EMI filter adds come level of series impedance to high frequencies and surge suppression devices behind this are less likely to fail. Gas discharge tubes have another problem in that a poorly selected voltage breakdown may cause the gas to glow under normal operating conditions and these devices are nothing like the old neon lamps, they will fail as a result. In addition, when they do fire, they are unlike MOVs in that they clamp to near zero volts and the only way to extinguish the plasma within the tube is a zero crossing of the line voltage. MOVs are always suspect because of the leakage current heating problem and catastrophic failure mode where they sputter metal on nearby surfaces. There are companies who make air gaps and these are viable so long as they do not have a problem with corona when the electrodes erode (due to arcing) into ragged edges. Sharp points can lower the breakdown voltage (e-fields) just as the ice pick experiment did back in high school. You would also need a way to replenish the air supply within the gaps since corona can build up and eventually arc over without requiring a surge event. In any design it is useful to test your mitigation attempts in the actual product design. thanks, doug Douglas E Powell doug...@gmail.com http://www.linkedin.com/in/dougp01 From: Joe Randolph Sent: Friday, September 06, 2013 11:34 AM To: Doug Powell ; EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Spark Gap PCB Layout on AC Mains Hi Doug: I think we are in agreement about some of the pitfalls of spark gaps that are simply added in the copper layout. Regarding spark gap components, the devices you cite (also known as gas tubes) typically fire at just a few hundred volts. Adding these to enhance the surge tolerance of an air gap can have unintended consequences. My philosophy for lightning protection is that the designer has only two options: 1) Block the surge current using adequate insulation 2) Direct the surge current to a known, safe path My preference is to use Option 1 whenever possible. As I noted in my earlier posting, the probability of experiencing a given surge drops off dramatically with increasing voltage. So, if a product is designed with spacings that break down at 10 KV, the probability of a surge exceeding that threshold is fairly low. If you then add a 400 volt gas tube across the barrier
