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, 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.
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Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com
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