Application Note

6 Fluke Corporation ABCs of multimeter safety
Multiple categories
There’s one scenario that some-
times confuses people trying to
apply categories to real world
applications. In a single piece of
equipment, there is often more
than one category. For example,
in office equipment, from the
120 V/240 V side of the power
supply back to the receptacle is
CAT II. The electronic circuitry,
on the other hand, is CAT 0. In
building control systems, such
as lighting control panels, or
industrial control equipment
such as programmable con-
trollers, it is common to find
electronic circuits (CAT 0) and
power circuits (CAT III) existing
in close proximity.
What do you do in these
situations? As in all real-world
situations, use common sense.
In this case, that means using
the meter with the higher
category rating. In fact, it’s not
realistic to expect people to be
going through the category-
defining process all the time.
What is realistic, and highly
recommended, is to select a
multi meter rated to the highest
category in which it could pos-
sibly be used. In other words,
err on the side of safety.
Shortcuts to
understanding categories
Here are some quick ways to
apply the concept of categories
to your every day work:
• The general rule-of-thumb
is that the closer you are to
the power source, the higher
the category number, and the
greater the potential danger
from transients.
• It also follows that the greater
the short-circuit current avail-
able at a particular point, the
higher the CAT number.
• Another way of saying the
same thing is the greater
the source impedance, the
lower the CAT number. Source
impedance is simply the total
impedance, including the
impedance of the wiring,
between the point where you
are measuring and the power
source. This impedance is
what dampens transients.
• Finally, if you have any
experience with the applica-
tion of transient voltage surge
suppression (TVSS) devices,
you understand that a TVSS
device installed at a panel
must have higher energy-
handling capacity than one
installed right at the com-
puter. In CAT terminology, the
panelboard TVSS is a CAT III
application, and the computer
is a receptacle-connected
load and therefore, a CAT II
installation.
As you can see, the concept
of categories is not new and
exotic. It is simply an exten-
sion of the same common-sense
concepts that people who work
with electricity professionally
apply every day.
Work safely
Safety is everyone’s responsibility but ulti-
mately it’s in y our hands.
No tool by itself can guarantee your safety.
It’s the combination of the right tools and
safe work practices that gives you maximum
protection. Here are a few tips to help you in
your work.
• Work on de-energized circuits whenever
possible. Use proper lock-out/tag-out
procedures. If these procedures are not
in place or not enforced, assume that the
circuit is live.
• On live circuits, use protective gear:
– Use insulated tools.
– Wear safety glasses and arc rated face
shield if required
– Wear insulated gloves; remove watches
or other jewelry.
– Stand on an insulated mat.
– Wear approved clothing, not ordinary
work clothes.
• When making measurements on live
circuits:
– Hook on the ground clip first, then make
contact with the hot lead. Remove the
hot lead first, the ground lead last.
– Hang or rest the meter if possible. Try to
avoid holding it in your hands, to mini-
mize personal exposure to the effects of
transients.
– Use the three-point test method, espe-
cially when checking to see if a circuit
is dead. First, test a known live circuit.
Second, test the target circuit. Third,
test the live circuit again. This verifies
that your meter worked properly before
and after the measurement.
– Use the old electricians’ trick of keeping
one hand in your pocket. This lessens
the chance of a closed circuit across
your chest and through your heart.
Applying categories to your work
Always wear approved personal protective
equipment (PPE), including arc rated clothing,
leather over rubber gloves, safety glasses, and
an arc-rated face shield or hood, both with hard
hat and hearing protection.