Application Note

Power Quality Troubleshooting Fluke Corporation 3
Recept.
L.C.
ASD
Recept.
XFMR
480/208Y
Utility
XFMR
MV/480Y
Switch Gear
Lighting Panel
Lighting
Load
Motor
M.C.C.
PF
Correction
Capacitor
Induction
Motor
Receptacle
Load
Start at the scene
of the crime
To troubleshoot PQ problems,
one approach is to start as close
to the “victim load” as possible.
The “victim load” is the sensi-
tive load, typically electronic,
that is somehow malfunction-
ing. Poor PQ is suspected, but
part of your job is to isolate PQ
as a cause from other possible
causes (hardware, software?).
Like any detective, you should
start at the scene of the crime.
This bottom-up approach can
take you a long way. It relies
on making use of a sharp eye
and on taking some basic
measurements.
An alternative is to start at
the service entrance, using a
three-phase monitor, and work
back to the “victim load.” This is
most useful if the problems
originate with the utility. Yet
survey after survey has con-
cluded that the great majority
of PQ problems originate in the
facility. In fact, as a general rule,
PQ is best at the service en-
trance (connection to utility)
and deteriorates as you move
downstream through the distri-
bution system. That’s because
the facility’s own loads are
causing the problems. Another
illuminating fact is that 75% of
PQ problems are related to wir-
ing and grounding problems!
For this reason, many PQ
authorities recommend that a
logical troubleshooting flow is
to first diagnose the electrical
infrastructure of the building,
then monitor if necessary. Our
bottom-up troubleshooting pro-
cedure is designed to help you
do this detective work.
First steps
1. Make a map: Obtain or
create a current one-line
It’s tough to diagnose PQ prob-
lems without having a working
knowledge of the site being
investigated. You can start by
locating or reconstructing a
one-line diagram of the site.
The one-line will identify the
ac power sources and the loads
they serve. The “as built” one-
line, the one with red-lines, is
the one you want.
If you work on-site, the map
might already exist in your
head, but it will be a big help to
everyone, including yourself, if
it’s on paper. If you’re coming to
a work site for the first time,
getting an up-to-date one-line
means identifying new loads or
other recent changes in the sys-
tem. Why go to this effort? Sys-
tems are dynamic; they change
over time, often in unplanned
and haphazard ways. Further-
more, while some problems are
local in origin and effect, there
are many problems that result
from interactions between one
part of the system and another.
Getting Started
Your job is to understand these
system interactions. The more
complete your documentation,
the better off you’ll be.
It’s true, however, that the
sites that need the most help
are the ones least likely to have
a good record of what’s going
on in their system. Many a con-
sultant has earned his fee by
upgrading the documentation
handed him with what actually
exists on-site. So the simple rule
is, at this point in the investiga-
tion, do the best you can to get
good documentation, but don’t
count on it being available.
2. Do a walk around
of the site
Sometimes a visual inspection
will offer immediate clues:
•
A transformer that’s much
too hot
•
Wiring or connections discol-
ored from heat
•
Receptacles with extension
strips daisy-chained to exten-
sion strips
•
Signal wiring running in the
same trays as power cables
•
Extra neutral-ground bonds
in sub-panels.
•
Grounding conductors con-
nected to pipes that end in
mid-air.
At a minimum, you will get a
sense of how the facility is
wired and what the typical
loads are.
3. Interview affected
personnel and keep
an incident log
Interview the people operating
the affected equipment. You will
get a description of the problem
and often turn up unexpected
clues. It’s also good practice to
keep a record of when problems
happen and what the symptoms
are. This is most important for
problems that are intermittent.
The goal is to find some pattern
that helps correlate the occur-
rence of the problem in the
“victim load” to a simultaneous
event elsewhere. Logically, this
trouble-logging is the responsi-
bility of the operator closest to
the affected equipment.
Simplified electrical distribution system typical of commercial and industrial facilities.