Back when I used to install PBX's, the manufacturer REQUIRED a #6 (min.)
ground wire be run from the PBX service panel in the PBX room to, with no
taps, the xformer that provided the power. If we simply wired the ground to
the panel that provided power to the PBX room or building ground then we
would void the warranty. Also, nothing that was fed from the PBX panel could
be in contact with building ground. With some of the installs being in the
range of millions of dollars, we were told that we could loose our job if we
didn't get the electrician to wire the ground "correctly."
There were many times that this would lead to a heated argument between us
and the contracted electrician. We did mostly new construction.
To put things into perspective, some of the larger installs would require as
much as up to 6-30 amp 220 volt feeds (if I remember correctly) and would
power, truck sized banks of batteries and refrigerator sized inverters and
rectifier units. The PBX's ran on 48 to 54 volts DC and each of the up to 6
rectifier units could pump out up to 50 amps DC.
Was it simply a matter of the manufacturer not wanting to put trust in the
Was it a matter of trying to eliminate any chances of ground loops?
Was it a matter of the manufacturer trying to get the cleanest power
Was it just a Nortel nuance?
Yes... <g> Probably the best place to ask would be the maker of the
equipment. Most will be glad to explain exactly what their reasoning
is, but isolation of that ground would be an important issue to them,
For a while back in the 70s, the state of the art in electronics
was very susceptible to noise, especially ground mode noise. I can
remember having an electrician completely rewire half a panel's
worth of circuits because he tied the isolated ground recepticles
to the flex-duct, even though that totally destroyed the concept of
the isolated ground. Even today, one is more likely to find
'mission-critical' equipment being feed with isolated ground
circuits, especially in things like medical equipment were they are
dealing with extremely lov voltage inputs.
An example: In my younger days I used to be part of the IT staff
at a major automotive engine plant. We had a T1 rack in the
computer room feed off of the dedicated service panel for the room.
(This was in the days before T1 = SDSL) We had been having problems
for about six months with one or more of the cards locking up
and/or apparently failing in the rack. I was in the process of
pulling together the documentation to demand that the phone company
replace the whole rack when one of my associates found an old Sola
Power Conditioner in a corner of an old store room. For lack of
anything better to try, we plugged it in and used it to feed the T1
rack. Lo and Behold, not another problem.
Question - Did you really have to run that line all the way back to
the transformer itself, or were you allowed to tie to the neutral
at the main service entrance/disconnect?
I called one of my old co-workers to ask him about this today and he pointed
me to a URL that provided some of this information.
It looks as though I got part of it wrong, the voltages are 208 or 240 not
The grounding section starts on page 16 and indicates that the green #6
ground can be connected to a dedicated service panel
In part it says (from page 23) . . .
"The isolated ground bus within the dedicated AC service panel serves as the
"system" SPG [single point ground]. The dedicated AC service panel should be
supplied from the buildings principle ground source, usually the transformer
which is located within the building."
So it looks like the ground didn't have to be run all the way to the
transformer if the panel was a dedicated panel and the ground for the panel
is a dedicated run to the transformer.
I now wonder if the senior install technicians didn't make the electricians
do this simply to make sure that the ground was actually a dedicated one and
not simply tied to the building frame once the wires disappeared behind the
Sorry for the confusion.
Just an observation based on my own experiences and electronics education.
Sensitive electronic equipment and its power supply should not use the same
grounding line, if feasible, as basic electrical equiipment. But, you
probably know that and why...
I'm starting to feel like I am being slowly honed in the direction of
answering my own question, not a bad thing, simply an observation.
If I remember correctly, and this is based on almost no electronics or
building construction education, a building ground may not be at zero volts.
If it is connected to other devices which bleed some voltage to it then the
"ground reference" my actually be at a level above earth ground potential.
Also, the ground could be used by other devices as a dumping ground (no pun
intended) for static signals or RFI and may be contaminated.
This all makes sense to me.
Dam it, the more I think about how to reply to your question "... why...",
the more I answer my own questions.
First, electricity is never same at both ends of a wire. Wire is
an electronic device. Therefore where grounds meet is important.
For numerous reasons, all incoming utilities must share the same
single point earth ground. If the PBX uses the same earth ground as
AC electric, then all phones inside the plant see the same ground
potential. Then both AC electric and telephone lines share the same
single point ground.
Well, this assumes AC electric and incoming phone lines are properly
installed - both connect short to the same earth ground as required by
Second principle - eliminate ground loops. Another reason why
everything inside the building should share that single point earth
ground. Even the floor, air ducts, or pipes can be a conductor that
must connect to that single point ground to eliminate ground loops.
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