Application Note

Power Quality Troubleshooting Fluke Corporation 9
Solutions
Table 2.2 Service panel recommendations.
Recommendation Reason
Limit length of 208V feeder runs to 120V Minimize source impedance and chance
subpanels to 200 ft. (65m). of voltage sags.
Don’t cascade (daisy chain) subpanels off Upstream loads can cause voltage sags that
of other subpanels if possible, and will affect all downstream loads.
especially if the upstream panel is heavily
loaded or has loads with high inrush
currents.
Install a green wire ground conductor Maintain a continuous, low impedance
(don’t rely on conduit connections). ground.
Reduce the load on the panel if necessary. Minimize heat, voltage sags.
Redistribute branch circuit loads to improve Reduce neutral return current (of the
balance of the three phases. fundamental current).
Upsize the feeder neutral if necessary, to Prevent overloading and heating of feeder
accommodate the third harmonic. This can neutral. Will reduce N-G voltage.
be done by running another neutral
in parallel.
Install 3rd harmonic filter. Reduce neutral current.
Nonlinear load panel. Manufacturer-designed for nonlinear loads.
Measurements
1. Feeder phase current
Check each phase to make sure
it is not overloaded. Also check
for excessive unbalance.
2. Feeder neutral current
Measure the feeder neutral
conductor for cumulative neu-
tral current. Third harmonic
currents from all three phases
will add arithmetically in the
neutral.
3. Feeder neutral-to-ground
voltage test
As at the receptacle, excessive
N-G voltage indicates overload-
ing. A N-G voltage at or very
near zero indicates the exist-
ence of an illegal N-G bond in a
subpanel.
4. Phase-to-neutral
voltage test
Phase-to-neutral voltages are
measured and recorded. They
can be compared with recep-
tacle L-N voltages to measure
voltage drop.
5. Branch neutral current
Measure each branch neutral
for overloading. The neutrals
are measured instead of the
phase conductors because they
might share the return current
of several phase conductors,
yet they are not protected
by breakers.
6. Circuit breaker
voltage drop
The voltage drop across a set
of breaker contacts will give
you a quick measure of the
wear of those contacts. Ideally,
the voltage drop should be
zero. In practice, there will be
some voltage drop in the mV
range, with the exact value be-
ing dependent on the load cur-
rent. As a general rule, the
voltage drop should not exceed
20-100 mV, depending on load.
Replace worn breakers.
Circuit breaker
voltage drop
Branch
L-N
voltage
N-G voltage
Branch neutral current
Feeder
neutral
current
Feeder
phase
current(s)
Figure 2.2 Panel with clamps, probes to show measurements.