Application Note

Power Quality Troubleshooting Fluke Corporation 5
5. Recording (long-term)
For longer term recording, the
VR101S Voltage Event Record-
ers will record sags, swells, out-
ages, transients and frequency
deviations while plugged into
the outlet (see “Recording at the
Receptacle Outlet,” page 7). The
device can be left on-site, unat-
tended, for days and weeks, all
the time catching intermittent
events (4000 event buffer). Now
you can see why it’s so impor-
tant to ask the user to keep a
troubleshooting log: correlation
of equipment malfunction with
voltage events is hard evidence
of a PQ problem.
6. Neutral-to-ground voltage
Let’s say that you make a
simple L-N measurement at the
outlet and get a low reading.
You can’t tell if the reading is
low because the feeder voltage
is low (at the subpanel), or if the
branch circuit is overloaded. You
could try to measure the voltage
at the panel, but it’s not always
easy to tell which panel feeds
the outlet you’re measuring and
it’s also sometimes inconvenient
to access a panel.
N-G voltage is often an easier
way of measuring the loading
on a circuit. As the current trav-
els through the circuit, there is a
certain amount of voltage drop
in the hot conductor and in the
neutral conductor. The drop on
the hot and neutral conductors
will be the same if they are the
same gauge and length. The to-
tal voltage drop on both con-
ductors is subtracted from the
source voltage and is that much
less voltage available to the
load. The greater the load, the
greater the current, the greater
the N-G voltage.
Think of N-G voltage as the
mirror of L-N voltage: if L-N
voltage is low, that will show
up as a higher N-G voltage
(see Fig. 1.4).
The flat-topped waveform is
typical of the voltage in a com-
mercial building with computer
loads. What causes flat-topping?
The utility supplies ac power,
but electronic equipment runs
on dc power. The conversion
of ac into dc is done by a power
supply. The PS has a diode
bridge which turns ac into pul-
sating dc, which then charges a
capacitor. As the load draws the
cap down, the cap recharges.
However, the cap only takes
power from the peak of the
wave to replenish itself, since
that’s the only time the supplied
voltage is higher than its own
voltage. The cap ends up draw-
ing current in pulses at each
half-cycle peak of the supplied
voltage. This is happening with
virtually all the electronic loads
on the circuit. Now that we see
what the loads are demanding
from the source, let’s take a look
at what the source can supply.
If the source were perfectly
“stiff,” meaning that it had an
infinite capacity to supply all
the current that was required,
then there would be no such
thing as flat-topping (or sags
or any voltage distortion). Think
of it this way: if you had all the
money in the world, you
wouldn’t get distorted either
when the bills came in. But in
the real world there are practi-
cal limits to what a source can
supply. This limit is usually
described by a concept called
source impedance, which is the
total impedance from the point
you’re measuring (or the point
where the load is located) back
to the source. There are two
major contributors to this source
impedance. One is the wiring;
the longer the conductor and
the smaller the diameter (higher
gauge), the higher the imped-
ance. The other factor is the
internal impedance of the trans-
former (or other source equip-
ment). This internal impedance
is simply a way of saying that a
transformer of a given size/rat-
ing can only supply so much
current.
The source impedance is
naturally greatest at the end
of a branch circuit, the farthest
point from the source. That’s the
same place where all those
electronic loads are demanding
current at the peak of the wave.
The result is that the voltage
peak tends to get dragged
down—in other words, flat-
topped. Maybe you’ve felt the
same way when all the bills
come in at the same time of the
month. The more loads there
are (the more the bills), the
greater the flat-topping. Also,
the greater the source imped-
ance (the less the cash), the
greater the flat-topping.
Flat-topped voltage
Figure 1.2 Flat-topped voltage.
Diode-Capacitor
Input Circuit
Switching
Power Supply