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
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