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"
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