Application Note

Power Quality Troubleshooting Fluke Corporation 31
A Lineup of Power Quality Culprits
From utility source to receptacle
Lightning
Can be extremely destructive if
proper surge protection is not
installed. It also causes sags
and undervoltages on the utility
line if far away. If close by, it
causes swells and overvoltages.
But in the final analysis, light-
ning is an act of nature and not
in the same category as the
damage man does to himself.
Utility automatic
breaker reclosure
Causes short duration sags/
outages, but better than the al-
ternative, a longer-term outage.
Utility capacitor switching
Causes a high-energy voltage
disturbance (looks like an oscil-
lating transient riding on the
wave). If the cap bank is near
the facility, this transient can
propagate all through the
building.
Commercial high rises
without enough distribution
transformers
Trying to cut corners in the
wrong places; running 208V
feeder up twenty stories is
not the road to PQ.
Gen-sets not sized for
harmonic loads
Excessive voltage distortion
affects electronic control
circuits. If SCR converter loads
are present, notching can
affect frequency control circuits.
Applying PF correction
capacitors without consider-
ing the effects of harmonics
Harmonics and caps don’t mix.
Those bulging capacitors are
crying for help.
Inrush currents from high
torque motor loads started
across-the-line
Causes voltage sags if the load
is too large or the source imped-
ance too great. Staggered motor
starts can help.
Undersized neutrals
at panelboard
In the era of the 3rd harmonic,
neutrals can easily carry as
much current or more current
than the phase conductor.
Keeping them undersized leads
to overheated lugs, potential fire
hazards and high N-G voltage.
Running power and signal
cables together
Think of the signal cable as a
single-wire transformer second-
ary and the power cable as the
primary. The opportunities for
coupling are endless.
Loose conduit connections
and lack of green wire
grounding conductor
Causes open or high impedance
ground circuit. Not good for PQ
or safety.
Shared neutrals on
branch circuits
Causes load interaction and
overloaded neutrals.
Laser printers and copiers
sharing branch circuits with
sensitive loads
Guaranteed periodic voltage
sags and switching transients.
Miswired receptacles
(N-G swapped)
Hard to believe, but they are out
there in quantity. Guaranteed to
put return currents on the
ground conductor and create
a noisy ground.
Data cables connected to
different ground references
at each end
Ouch! Shows up as voltage
between equipment case and
the data cable connector.
Hi-frequency noise
The most effective high fre-
quency grounding technique is
the installation of a Signal Ref-
erence Grid (SRG).
And in a class by themselves
Isolated ground rods (below)
They’re a safety hazard because
the earth is a high impedance
path and will prevent enough
current from flowing to trip the
breaker. They also cause ground
loops; after all, every electron
still has to go back where it
came from. One of the great
mysteries of PQ is how some
manufacturers get away with
insisting that their equipment
warranty is void unless an iso-
lated ground rod is installed. My
auto mechanic was so inspired
by this practice that he now
warranties every car he works
on unless it’s driven.
Illegal N-G bonds
(Fig. 2.1, page 8)
Guaranteed to put return cur-
rents on ground. A common
enough problem that the Union
of PQ Consultants wants to
charge piecework rates—say, a
dollar for every N-G bond
found. They’d all get rich. Not
only is it a PQ problem, it’s a
plumbing problem. Circulating
ground currents cause corrosion
of water pipes. That explains
why you can never find the
electrician that put in those N-G
bonds—it’s all being done on
the sly by the plumbers. Guar-
anteed employment, as if they
needed it.
Isolated ground rod can cause ground loops. Common problem with CNC machine tool installations.
Load Current
Neutral Return
Current Error
Panel
Line Line
Neutral Neutral
Ground
Ground
Separately
Derived
System
N - G
Bond
Earth Ground
“Isolated” Ground,
Ground Rod, Cold Water Pipe, Etc.