26 Aug
2015
26 Aug
'15
2:01 p.m.
On Aug 26, 2015, at 4:07 PM, Noel Chiappa <jnc at
mercury.lcs.mit.edu> wrote:
I can't vouch for the truth of the story; I heard it a long time ago from a fairly
reliable source.
But consider this theory. Suppose you have two service drops, fed from transformers off
the utility high voltage line. The neutral is simply defined by the ground rod at the
transformer and at the service entry to the building. If the building is a steel frame
and all service entries are bonded to the steel, and the steel is generally conductive,
you have a single neutral. But if some aren't bonded, or the building isn't
conductive, then you have two separate ground references. Also, the green wire
(protective ground) is connected to the neutral at the service entry.
Now suppose that you have unbalanced phases, which will generally be the case. That
produces a neutral current, which dissipates through the ground rods. If you have two
services, the resulting neutral voltages will not be in phase.
If you now tie these two neutrals (grounds) through an unplanned wire, as in this story,
the phase unbalance voltage (the difference between the two neutrals) will produce a
current that's split between that wire, and the ground around the building, in
proportion to the impedance of the two paths. The wire impedance is likely to be far
lower than that of the ground (especially in New England), and the phase unbalance current
in a large building might well amount to a lot of amps.
paul
From: Paul Koning
What happened is that the "grounds"
were offset enough, and with enough
of a current supply, that the ground strap that's supposed to connect
the row of RP06 drives melted.
This sort of thing is a major electric code violation: you can certainly have
multiple services, but all the grounds are required to be connected by
substantial wire; you're not allowed to stick ground rods in at
multiple places and leave it at that.
I'm pretty blown away that the various grounds could be offset by that much,
to produce that kind of current when they were tied together. Wow.