Application Note

6 Fluke Corporation Power Quality Troubleshooting
N-G voltage exists because
of the IR drop of the current
travelling through the neutral
back to the N-G bond. If the
system is correctly wired, there
should be no N-G bond except
at the source transformer (at
what the NEC calls the source of
the Separately Derived System,
or SDS, which is usually a
transformer). Under this situa-
tion, the ground conductor
should have virtually no current
and therefore no IR drop on it.
In effect, the ground wire is
available as a long test lead
back to the N-G bond.
Figure 1.3. Neutral-to-ground voltage increases with shared neutrals.
Ø1
Ø1
Ø2
Ø2
Ø3
Ø3
To panel
ground buss
N-G voltage
To transformer
X
0
Figure 1.4 Neutral-ground voltage increases as load current goes up.
50 ft
15m
Laser
Printer
Copier
200 ft
60m
1. A rule-of-thumb used by many in the industry
is that N-G voltage of 2V or less at the recep-
tacle is okay, while a few volts or more
indicates overloading; 5V is seen as the upper
limit. There’s obviously some room for judgment
in this measurement.
2.A high reading could indicate a shared branch
neutral, i.e., a neutral shared between more
than one branch circuit. This shared neutral
simply increases the opportunities for overload-
ing as well as for one circuit to affect another.
3.A certain amount of N-G voltage is normal in a
loaded circuit. If the reading is stable at close
to 0V, suspect an illegal N-G bond in the
receptacle (often due to loose strands of the
neutral touching some ground point) or at the
subpanel. Any N-G bonds other than those at
the transformer source (and/or main panel)
should be removed to prevent return currents
flowing through the ground conductors.
4.If N-G voltage is low at the receptacle, you’re in
good shape (see Measurement Note #3 for the
exception to the rule). If it’s high, then you still
have to determine if the problem is mainly at
the branch circuit level, or mainly at the panel
level. Remember, assuming there’s no illegal
N-G bond in intervening panels or receptacles,
your ground “test lead” goes all the way back to
the source, so you’re reading voltage drops all
the way to the source.
Receptacle N-G Voltage
Measurement Notes
Shared neutrals
Some buildings are wired so
that two or three phases share a
single neutral. The original idea
was to duplicate on the branch
circuit level the four wire (three
phases and a neutral) wiring of
panelboards. Theoretically, only
the unbalanced current will re-
turn on the neutral. This allows
one neutral to do the work for
three phases. This wiring short-
cut quickly became a dead-end
with the growth of single-phase
non-linear loads. The problem
is that zero sequence current
from nonlinear loads, primarily
third harmonic, will add up
arithmetically and return on the
neutral. In addition to being
a potential safety problem
because of overheating of an
undersized neutral, the extra
neutral current creates a higher
N-G voltage. Remember that
this N-G voltage subtracts from
the L-N voltage available to the
load. If you’re starting to feel
that shared neutrals are one of
the worst ideas that ever got
translated to copper, you’re not
alone.