Application Note

4 Fluke Corporation Power Quality Troubleshooting
Part I: Facility Distribution System
Many PQ problems show up at
the branch circuit level. There’s
a simple reason for this: that’s
where most of the sensitive
loads (and sensitive employees)
are located. It’s also the “end of
the line” of the electrical sys-
tem, and the place where short-
comings can’t be hidden. Let’s
assume you’ve been called in
to solve the problem. You’ve
already talked to the people in-
volved, have a rough idea of the
symptoms (equipment lock-ups,
intermittent resets or crashes,
etc.) and as much sense of the
timing and history of the prob-
lems as you can get. So it’s time
to gather hard evidence: it’s
time to take measurements.
Our primary focus with
troubleshooting at the recep-
tacle level is to determine if the
Line-Neutral (L-N) voltage avail-
able is of sufficient stability and
amplitude to supply the needs
of the load(s).
Measurement
1. Waveform
The waveform gives us quick
snapshot information. An ideal
waveform would be a sine
wave. In this case, (see Fig 1.1)
the voltage waveform is flat-
topped, which is typical of a
building with many non-linear
loads such as computers and
other office equipment (see
“Flat-topped voltage,” page 5).
Our other measurements will
tell us whether this flat-topping
is excessive.
2. Peak voltage
The peak value is critical to
electronic loads because the
electronic power supply charges
its internal capacitors to the
peak value of the line voltage.
If the peak is too low, it affects
the ability of the caps to charge
fully and the ability of the
power supply to ride through
momentary dips in the line volt-
age. For an RMS voltage of
115V, the peak value would be
1.414 x 115V = 162.6V, if the
waveform were a sine wave.
However, as we just saw from
the flat-topped waveform, what
we have is far from a sine wave
and will have a lower peak
value.
3. RMS voltage
Nominal line voltage is measured
in RMS (root-mean-square)
which corresponds to the effec-
tive heating value. Equipment is
rated in RMS, not peak, because
their main limitation has to do
with heat dissipation.
RMS voltage can be too high
or too low, but it is usually the
low voltage that causes prob-
lems. Low RMS voltage com-
bined with flat-topping (low
peak) is a deadly combination
for sensitive loads.
Voltage drop is a function
of both the loading of the circuit
and the source impedance,
which in effect means the
length and diameter (gauge)
of the wire run. The NEC (210-
19.a, FPN No. 4) recommends a
limit of a 3% voltage drop from
the branch circuit breaker to
the farthest outlet, and a total
voltage drop of less than 5% in-
cluding the feeder and branch
circuit.
4. Recording (short-term)
The limitation of the above
measurement is that it is static.
Many loads require more cur-
rent, usually referred to as in-
rush current, when they are first
turned on. This momentary high
current may cause a momentary
low voltage (sag) because of the
additional IR drop through the
conductors. Such sags are often
caused by loads drawing inrush
currents on the same branch
circuit, or on the same
panelboard.
You can measure a worst-
case sag of 100 ms or more
(about 6 cycles at 60 Hz) by us-
ing the MIN MAX function of the
Fluke 87 while energizing the
load. What if you want to know
if there are recurring sags? The
Sags & Swells trending feature
of the Fluke 43 Power Quality
Analyzer will continuously cap-
ture sags of as little as single
cycle duration (17 ms). A four-
minute to a one-hour recording
time (i.e., anywhere from a
single cup of coffee to a lunch
break) may be enough to tell
you if there are recurring sags
and swells.
Table 1.1Measurements on receptacle branch circuits.
Voltage Measurements Look for Instrument
1. Waveform Snapshot of severity of 43 PQ Analyzer
voltage distortion 41B Harmonics Analyzer
2. Peak voltage Excessive flat-topping 43 PQA, 41B
87 DMM (Peak MIN MAX)
3. RMS voltage Low rms (steady-state low 43 PQA (Sags/Swells)
rms or intermittent/cyclical 41B (MIN MAX)
sags) 87 DMM (MIN MAX)
4. Recording (short-term) Sags, swells, interruptions 43 PQA
while troubleshooter remains (Sags/Swells or Transients)
on-site (4 minutes to 1 hour
typical recording time)
5. Recording (long-term) Up to 4,000 sags, swells, VR101S
outages, transients
6. Neutral-ground N-G voltage too high 43 PQA, 87 DMM
(or close to zero)
Figure 1.1 Flat-topped voltage at receptacle.
Section 1
Receptacle Branch Circuit