Re: Ground potential differences
The worst case of ground potential difference that I have heard of is a case where guy got a shock from the grounded metal reflector of his trouble light while working outside on his car. Turns out the pump in his neighbor's well (fed from the same transformer) had a short from from line to ground. The potential difference between the ground tied to line at the neighbor's and the ground tied to neutral at this guy's house caused a voltage gradient along ground between the properties high enough to cause a shock when enough distance was covered (by the cord of the trouble light). Don Borowski Schweitzer Engineering Labs Pullman, WA This e-mail may contain SEL confidential information. The opinions expressed are not necessarily those of SEL. Any unauthorized disclosure, distribution or other use is prohibited. If you received this e-mail in error, please notify the sender, permanently delete it, and destroy any printout. Thank you. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on browse and then emc-pstc mailing list
Re: Ground potential differences
I've no publications, but I did have personal experience with the more destructive results with ground potential differences. At another company, I witnessed the insulation of a coax cable melting after being connected between equipment in a lab where I worked. The problem being a difference in ground potential of 15vac. That in and of itself wasn't the problem. The amperage capacity through ground of the mains system was the culprit. Circa 1983 or so. Turned out that the building was supplied by two electrical substations at opposite ends of the building. And for some strange reason, the substations each shared about half the outlets in the lab. And the potential difference was caused by the electricians connecting to slightly different points on the transformers for ground. The electricians were told and were able to fix the problem. And as far as anything in the field showing a similar problem, I have only dealt with one such instance. And relating the substation transformer ground tap story fixed it for them. Regards, Doug McKean --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on browse and then emc-pstc mailing list
RE: Ground potential differences
Most earth potential differences come from load currents. Leakage currents don't usually add up to very much. Electrical installations should have one neutral to earth connection at the source of the system (at the transformer or service entry). However many electrical installations have branch panels with earth and neutral incorrectly connected together at the panel. This means the neutral and earth paths are parallel returns to the source. The IR drop generates potential differences between different building areas. These stray currents can amount to tens or hundreds of amps in big installations. Occasionally these same currents come from miswired plugs or receptacles with neutral and earth switched. To look for stray currents, put a clamp on ammeter around all mains conductors together (phases and neutral). Often they will show up even with the meter around the conduit. The resulting voltage differences in grounds typically amount to 2 to 5 volts. It depends on load currents, neutral resistance, ground resistance, current paths, etc. Transients and noise of course include all those present on the load currents. In residential applications where several services, each with their own ground, are fed from a single transformer (also with its neutral grounded). Several amps typically flow in the parallel earth connections between houses, but this is irrelevant. The important thing to achieve is an equipotential earth environment around the particular house involved. Bob Johnson ITE Safety -Original Message- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of John Woodgate Sent: Saturday, October 12, 2002 12:53 PM To: emc-p...@majordomo.ieee.org Subject: Re: Ground potential differences I read in !emc-pstc that Bailey, Jeff jbai...@mysst.com wrote (in B115DFA26896D511BAB600105AA3493275EA3F@SSTMAIL) about 'Ground potential differences' on Fri, 11 Oct 2002: I am interested to know what the actual magnitude of ground differences may be from one end of a plant to another as well as where the numbers come from. It depends on what sort of equipment is present. Some things have very high leakage current, putting a lot of amps in total into ground conductors. Then, in old plants, there may be bad ground wiring that doesn't show up as a fault. Have they been calculated or actually measured? Both. Usually after the problem has been discovered by chance. I've measured 9 V over a distance of 20 m, but there are reports of much higher voltages. If shields are connected directly to chassis at each node of a network will there be an effect of equalizing the ground levels through the network or will enough current flow to melt the shield of the cable? Both are possible. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on browse and then emc-pstc mailing list attachment: Robert Johnson.vcf
Re: Ground potential differences
Jeff, I used to develop network interfaces for printers at IBM and Lexmark. As I recall, the Ethernet 10BASE2, 10BASE-T, and 100BASE-Tx standards required 1500V isolation between the network cabling and the product. This was no problem for 10BASE-T and 100BASE-Tx, because the transformers easily gave us this much isolation. But we had to put TransGuards on several 10BASE2 products because the circuitry required both a capacitive and a weak resistive connection between shield ground and chassis ground to work. As part of our product qualification tests, we would hit 10BASE-T and 100BASE-Tx ports with +/-400V surges, and 10BASE2 ports with +/-800V surges. These could be rather noisy when the 40 joules of available energy blasted weak components right off a prototype card... (This was not a test to run if you were tired or careless-- it could kill you with one zap.) I've never seen a document that mentioned/specified the maximum AC or DC voltage that you could expect between the network cabling and the product under normal conditions. But I don't think that it is very high, because the BNC connectors and BNC T's used for 10BASE2 have exposed bare metal connected to the shield of the cable. So when you are connecting/disconnecting a 10BASE2 cable from a product, you can have one hand on network voltage and the other hand on chassis ground, with any leakage current going through your heart. IEC 950 specifies a maximum of 30 Vrms for alternating current, or 42.4 Vdc (60 Vdc for a telecom port), for exposed metal in Safety Extra-Low Voltage (SELV) circuits. So I expect that normal network-to-product voltage are well under these limits. John Barnes KS4GL SM dBi Corporation http://www.dbicorporation.com/ --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on browse and then emc-pstc mailing list
Re: Ground potential differences
I read in !emc-pstc that Bailey, Jeff jbai...@mysst.com wrote (in B115DFA26896D511BAB600105AA3493275EA3F@SSTMAIL) about 'Ground potential differences' on Fri, 11 Oct 2002: I am interested to know what the actual magnitude of ground differences may be from one end of a plant to another as well as where the numbers come from. It depends on what sort of equipment is present. Some things have very high leakage current, putting a lot of amps in total into ground conductors. Then, in old plants, there may be bad ground wiring that doesn't show up as a fault. Have they been calculated or actually measured? Both. Usually after the problem has been discovered by chance. I've measured 9 V over a distance of 20 m, but there are reports of much higher voltages. If shields are connected directly to chassis at each node of a network will there be an effect of equalizing the ground levels through the network or will enough current flow to melt the shield of the cable? Both are possible. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on browse and then emc-pstc mailing list
Ground potential differences
Greetings all, I have become involved in a discussion regarding potential ground voltage differences between opposite ends of long network lines in industrial locations. As I understand it these differences are the reason for some network types floating cable shields or connecting them through snubbing networks to chassis instead of connecting directly. In the past I have simply accepted this explanation however I am becoming less comfortable with just accepting it. Can someone provide me with or point me to where I can obtain information on this subject? I am interested to know what the actual magnitude of ground differences may be from one end of a plant to another as well as where the numbers come from. Have they been calculated or actually measured? If shields are connected directly to chassis at each node of a network will there be an effect of equalizing the ground levels through the network or will enough current flow to melt the shield of the cable? Thanks in advance for any replies. Jeff Bailey Compliance Engineering SST - A Division of Woodhead Canada Phone: (519) 725 5136 ext. 363 Fax: (519) 725 1515 email: jbai...@mysst.com Web: www.mysst.com --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on browse and then emc-pstc mailing list
Re: Ground potential differences....
Hi Doug: Without knowing the issue, we connected two different machines with a coax. Each machine eventually connected to different substations by way of differently sourced outlets. And we watched with amazement as the rubber jacket of the coax melted. All with a 15V difference. The electricians were notified I'll bet it wasn't the voltage, but the current through the resistance of the coax that caused the voltage difference. I'll bet the coax was carrying part of the neutral current. If the neutral is grounded at more than one point, then the protective ground conductors are in parallel with the neutral and will carry some of the neutral current. Best regards, Rich --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re: Ground potential differences....
Doug: Just to enhance your observation, at RF ( i.e. broadcast frequencies 520-1800 Khz) there is a pseudo resonant effect that places RF ground at different places in a building , home etc. The RF ground is dependant on the intereception of direct radiation and is related to how much wiring is exposed and the angle of exposure. It would make an interesting problem to model. In one case, a CD system on a second floor was completely disabled by RF common mode) and the only way to re establish a ground reference was tying the ground lead from the CD player to the bedprings. It may sound far fetched, but it worked. Sometimes plugging into a different AC outlet in the same room , cleared the problem. Ground potential difference can also occur when transformer insulation becomes leaky , mainly due to age. Cheers. Ralph Cameron EMC Consultant for Suppression of Consuemr Electronic Equipment (After Sale) - Original Message - From: Doug dmck...@gte.net To: emc-p...@majordomo.ieee.org Sent: Saturday, May 06, 2000 1:18 PM Subject: Re: Ground potential differences I'm going to stick my neck out here and say from two experiences with this topic, there's a couple of things to consider ... First, ideally, any potential difference between GROUNDS should be zero. If you had asked about NEUTRALS, I'd have to say - unknown. The physical connection is a function of the connection to the transformer by the electricians and imperfections in the transformer. Second, it's a function of what's sourcing the difference. I worked at a place where the outlets in one half of the lab were supplied by a different substation than the outlets at the other end of the lab. Without knowing the issue, we connected two different machines with a coax. Each machine eventually connected to different substations by way of differently sourced outlets. And we watched with amazement as the rubber jacket of the coax melted. All with a 15V difference. The electricians were notified and the problem was solved but a potential difference of some sort was still there. I don't think you'll ever get away from it. So, I guess what I'm saying is that you should not only consider the voltage difference, but the power involved. And that would be have to tested some other way. - Doug McKean Kelly Tsudama wrote: Hi gang! I have been asked to look into ground potential differences by one of the teams that I support. Can any of you provide any insight on how I can determine the maximum potential difference between different ground circuits within a building??? I've heard numbers ranging from 2V to 50V!!! Even with all the bonding requirements in the NEC, there must be some voltage differential between grounding points??? Thanks for any help you can provide. Kelly --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re: Ground potential differences....
I'm going to stick my neck out here and say from two experiences with this topic, there's a couple of things to consider ... First, ideally, any potential difference between GROUNDS should be zero. If you had asked about NEUTRALS, I'd have to say - unknown. The physical connection is a function of the connection to the transformer by the electricians and imperfections in the transformer. Second, it's a function of what's sourcing the difference. I worked at a place where the outlets in one half of the lab were supplied by a different substation than the outlets at the other end of the lab. Without knowing the issue, we connected two different machines with a coax. Each machine eventually connected to different substations by way of differently sourced outlets. And we watched with amazement as the rubber jacket of the coax melted. All with a 15V difference. The electricians were notified and the problem was solved but a potential difference of some sort was still there. I don't think you'll ever get away from it. So, I guess what I'm saying is that you should not only consider the voltage difference, but the power involved. And that would be have to tested some other way. - Doug McKean Kelly Tsudama wrote: Hi gang! I have been asked to look into ground potential differences by one of the teams that I support. Can any of you provide any insight on how I can determine the maximum potential difference between different ground circuits within a building??? I've heard numbers ranging from 2V to 50V!!! Even with all the bonding requirements in the NEC, there must be some voltage differential between grounding points??? Thanks for any help you can provide. Kelly --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
Re: Ground potential differences....
Hi Kelly: First, I presume you are considering the TN power distribution system. The answers to your question are dependent upon the type of power distribution system, whether TN, TT, or IT. My response does not apply to the TT and IT systems. The first question that must be asked in dealing with ground potential differences is: What is the reference? I will assume it is the body of the earth, a non-current-carrying earth rod. The second question that must be asked is: What is the source of current that causes the voltage drop across the resistance (due to the resistance of the wire in question)? The potential difference between different PE conductors arises when the neutral has two or more earth connections. This occurs when the same service has two or more service entrances, where each service entrance is required by the NEC to have the neutral connected to earth. These multiple earth connections allow the earth to conduct part of the neutral current (because the two or more connections make the earth a parallel path to the neutral). Kirchoff's Laws. (I'm too lazy to draw an ASCII schematic diagram to show the circuit.) The potential difference between two PE conductors cannot be pre-determined because it is a function of the neutral current, the earth resistance between the two or more ground rods, and the number of ground rods. The problem of connecting equipment signal grounds of two different service entrances means that some of the neutral current will flow in the ground of the signal leads. Often, this current will corrupt the signal or, worse, cause an open in the ground side of the signal lead. When the neutral has two or more grounding points, then there will ALWAYS be a potential difference between PE wires (grounds) of the two service entrances. The PE wire (ground) is at earth potential only when the neutral is grounded at a single point. Good luck, and best regards, Rich --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson:pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
