Case Studies

Application Note
Problem description
This case history came from
an electric utility engineer
assigned to maintain the power
systems for several small towns
in the western U.S. The engi-
neer carries a Fluke 43B on
all trouble calls, because it
provides a graphical display
that allows customers to see
a “picture” of the problem.
The case history began
with a call from a local police
department. The officer
explained that the traffic light
at the main intersection of town
was randomly malfunctioning.
Sometimes it worked fine,
while other times it generated
unacceptably long delays in
one direction or the other.
Normally when a traffic light
fails, it will not work at all. In
this case, however, the problem
was intermittent. Therefore, the
engineer suspected the problem
was due to an interaction with
the power system, rather than
a failure within the traffic light
Malfunctioning
traffic light
From the Fluke Digital Library @ www.fluke.com/library
itself. The engineer’s investiga-
tion showed that three facilities
(a barbershop, a small café and
an automotive machine shop)
shared the transformer second-
ary that supplied power to the
traffic light. See Fig. 1.
One of the keys to trouble-
shooting a problem is deter-
mining what changed just prior
to the appearance of the prob-
lem. Visits to the barbershop
and café didn’t reveal anything
meaningful. The machine shop,
however, had a different story.
The shop had just installed a
new brake lathe they were
using to resurface brake drums
and rotors for cars and trucks.
The lathe was equipped with a
dc motor powered by a dc
adjustable speed drive.
Machine
Shop
Traffic
Light
Utility
Barber
Shop
Café
Fig. 1 One-line diagram showing power supplied to the traffic light
Operator: Electric utility engineer
Measuring tools: Fluke 43B Power
Quality Analyzer
Features used: Voltage, transient
capture
Power
Quality
Case
Study