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
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