Application Note

Safety checklist
3 Use a meter that meets accepted safety standards
for the environment in which it will be used.
3 Use a meter with fused current inputs and be
sure to check the fuses before making current
measurements.
3 Inspect test leads for physical damage before
making a measurement.
3 Use the meter to check continuity of the test
leads.
3 Use only test leads that have shrouded connec-
tors and finger guards.
3 Use only meters with recessed input jacks.
3 Select the proper function and range for your
measurement.
3 Be certain the meter is in good operating
condition.
3 Follow all equipment safety procedures.
3 Always disconnect the “hot” (red) test lead first.
3 Don’t work alone.
3 Use a meter that has overload protection on the
ohms function.
3 When measuring current without a current
clamp, turn the power off before connecting into
the circuit.
3 Be aware of high-current and high-voltage situ-
ations and use the appropriate equipment, such
as high-voltage probes and high-current clamps.
Meter ratings and capabilities vary by manufacturer.
Before working with a new meter, be sure to familiarize
yourself with all operating and safety procedures for
that meter contained in the users manual.
Independent testing is the key to safety compliance
How can you tell if you’re getting a genuine CAT III or CAT
II meter? It’s not always easy. It is possible for a manufac-
turer to self-certify its meters as CAT II or CAT III without
any independent verification. Beware of wording such as
“Designed to meet specifications...” Designer’s plans are
never a substitute for an actual independent test. The IEC
(International Electrotechnical Commission) develops and
proposes standards, but it is not responsible for enforcing
the standards.
Look for the symbol and listing number of an indepen-
dent testing lab such as UL, CSA, TÜV or other recognized
approval agency. That symbol can only be used if the
product successfully completed testing to the agency’s
standard, which is based on national/international stan-
dards. UL 61010, for example, is based on IEC 61010.
In an imperfect world, that is the closest you can come
to ensuring that the multimeter you choose was actually
tested for safety.
6 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained
Measurement categories
A critically important concept to understand about
electrical safety is the measurement category.
Standards define Categories 0 through IV, often
abbreviated as CAT 0, CAT II, etc.
The division of a power distribution system
into categories is based on the fact that a dan-
gerous high-energy transient such as a lightning
strike will be attenuated or dampened as it travels
through the impedance (ac resistance) of the
system. A higher CAT number refers to an elec-
trical environment with higher power available
and higher energy transients. Thus a multimeter
designed to a CAT III standard is resistant to much
higher energy transients than one designed to
CAT II standards.
Within a category, a higher voltage rating
denotes a higher transient withstand rating, e.g.,
a CAT III 1000 V meter has superior protection
compared to a CAT III 600 V rated meter. The real
misunderstanding occurs if someone selects a
CAT II 1000 V rated meter thinking that it is supe-
rior to a CAT III 600 V meter.
Measurement category In brief Examples
CAT IV Three-phase at
utility connection,
any outdoor mains
conductors
Refers to the “origin of installation,” i.e., where low-voltage connection is made to utility power
Electricity meters, primary overcurrent protection equipment
Outside and service entrance, service drop from pole to building, run between meter and panel
Overhead line to detached building, underground line to well pump
CAT III Three-phase
distribution,
including single-
phase commercial
lighting
Equipment in fixed installations, such as switchgear and poly-phase motors
Bus and feeder in industrial plants
Feeders and short branch circuits, distribution panel devices
Lighting systems in larger buildings
Appliance outlets with short connections to service entrance
CAT II Single-phase
receptacle
connected loads
Appliance, portable tools, and other household and similar loads
Outlet and long branch circuits
– Outlets at more than 10 meters (30 feet) from CAT III source
– Outlets at more than 20 meters (60 feet) from CAT IV source
CAT 0 Electronic Protected electronic equipment
Equipment connected to (source) circuits in which measures are taken to limit transient over-
voltages to an appropriately low level
Any high-voltage, low-energy source derived from a high-winding resistance transformer, such
as the high-voltage