Application Note

8 Fluke Corporation Power Quality Troubleshooting
Section 2
Service Panels
Check-out the service panel
as follows:
•
Visual inspection
•
Feeder conductor current test
•
Neutral conductor current test
(feeder and branch)
•
Phase-to-neutral voltage test
(feeder and branch)
•
Neutral-to-ground voltage
test (feeder)
•
Circuit breaker voltage drop
and current on branch phase
conductors
The service panel is where the
effects of single-phase harmonic
loads are easy to measure. A
true-rms meter ensures accurate
readings of non-linear voltages
and currents (see “Why True-
rms, page 27).
Visual inspection
•
Look for an illegal Neutral-
Ground bond in subpanels.
This is a violation of the NEC
as well as of PQ wiring. It is
also extremely common. If an
illegal N-G bond is found in
one panel at a site, it is likely
to be in any number of them.
Who knows why they’re
there: perhaps the installer
was thinking that all panels
are wired like residential ser-
vice panels; or that the
quickest way to reduce N-G
voltage was to install a
jumper, or that the more
grounds the better. In any
case, remove all illegal
N-G bonds—no exceptions.
•
Look for signs of overheat-
ing, such as discolored con-
necting lugs. Loose con-
nections and excessive load-
ing show up as heat. High
levels of harmonic current
that were not accounted for
in the original wire sizing
can also cause overheating.
Infrared sensors are the
preferred method for
non- contact temperature
measurement.
•
Of particular concern is the
size of the feeder neutral
conductor. It has long been
understood that any funda-
mental current resulting from
the unbalance of single phase
loads among the three phases
will return on the neutral, but
a relatively recent phenom-
enon is the third harmonic
(triplen) currents generated by
nonlinear single-phase loads
that all return on the neutral.
The 1996 NEC for the first
time stated that “On a 4-wire,
3-phase wye circuit where the
major portion of the load con-
sists of nonlinear loads, there
are harmonic currents present
in the neutral conductor, and
the neutral shall be considered
to be a current-carrying con-
ductor.” (Article 310, “Notes to
Ampacity Tables of 0 to 2000
Volts,” Note 10.c). In effect, this
requires that the neutral con-
ductor at least equal the size of
the phase conductor. This re-
quirement is based on solid re-
search: a 1990 survey of 146
sites nation-wide found that
22.6% of them had neutral
current in excess of 100% of
phase current!
Many experts would recom-
mend that the neutral be
double the size of the phase
conductor.
•
Check for shared branch
neutrals. Count neutral con-
ductors for branch circuits: if
there are fewer than the phase
conductors, there are shared
neutrals.
•
Check tightness of conduit
connections, especially if the
conduit is being used exclu-
sively as the grounding con-
ductor (not recommended).
Figure 2.1 Sub-panel N-G bonds cause load return currents to flow on ground conductors.
This causes corrosion of pipes in grounding system as well as noisy grounds.
Panel Sub-Panel
Line
Neutral
Ground
N-G
Bond
Table 2.1 Service panel measurements.
Measurement Look for Instrument
1. Feeder phase current Overloading and balance. 43, 41B, 87 w/80i-400,
True-rms ClampMeter
2. Feeder neutral current High currents from unbal- 43, 41B for spectrum.
anced fundamental and 87 to find dominant
3rd harmonics. frequency.
3. Feeder N-G voltage High voltage indicates Same
excessive current, near-zero
indicates possible subpanel
N-G bond.
4. Branch L-N voltage Low voltage. Same
5. Branch neutral current Shared neutrals. Same
6. Voltage drop across Worn contacts. Breakers in 43, 87
breaker contacts. need of replacement.
Hot breakers.