Application Note

Always make sure the
power is off before cutting
or unsoldering the circuit
and inserting the DMM for
current measurements.
Even small amounts of
current can be dangerous.
Never attempt a voltage
measurement with the test
probes in the current jack.
Meter damage or personal
injury may result.
There are two basic types of
current probes: current trans-
formers, which are used to
measure ac current only, and
Hall-Effect probes, which are
used to measure ac or dc current.
The output of a current trans-
former is typically 1 milliamp per
amp. A 100 amp value is reduced
to 100 milliamps, which can be
safely measured by most DMMs.
The probe leads are connected to
the “mA” and “COM” input jacks,
and the meter function switch is
set to mA ac.
The output of a Hall-Effect
probe is 1 millivolt per amp, ac
or dc. For example, 100 A ac is
converted to 100 mV ac. The
probe leads are connected to the
“V” and “COM” jacks. Set the
meter function switch to the “V”
or “mV” scale, selecting V~ for
ac current or V
for dc current
measurements. The meter
displays 1 millivolt for every amp
measured.
Safety
Multimeter safety
Making measurements safely
starts with choosing the proper
meter for the application as well
as the environment in which
the meter will be used. Once the
proper meter has been chosen,
you should use it by following
good measurement procedures.
Carefully read the instrument
user manual before use, paying
particular attention to the WARN-
ING and CAUTION sections.
The International Electrotechni-
cal Commission (IEC) established
safety standards for working on
electrical systems. Make sure you
are using a meter that meets the
IEC category and voltage rating
approved for the environment
where the measurement is to be
made. For instance, if a voltage
measurement needs to be made
in an electrical panel with
480 V, then a meter rated Cate-
gory III 600 V or 1000 V should
be used. This means the input
circuitry of the meter has been
designed to withstand voltage
transients commonly found in
this environment without harm-
ing the user. Choosing a meter
with this rating which also has a
UL, CSA, VDE or TÜV certification
means the meter not only has
been designed to IEC standards,
but has been independently
tested and meets those stan-
dards. (See Independent Testing
sidebar on page 6.)
Common situations that lead
to DMM failure:
1. Contact with ac power source
while test leads are plugged
into current jacks
2. Contact with ac power source
while in resistance mode
3. Exposure to high voltage
transients
4. Exceeding maximum input
limitations (voltage and
current)
Types of DMM protection
circuits:
1. Protection with automatic
recovery. Some meters have
circuitry that detects an over-
load condition and protects
the meter until the condi-
tion no longer exists. After
the overload is removed, the
DMM automatically returns to
normal operation. Usually used
to protect the ohms function
from voltage overloads.
2. Protection without
automatic recovery. Some
meters will detect an over-
load condition and protect the
meter, but will not recover
until the operator performs an
operation on the meter, such
as replacing a fuse.
Look for these safety
features in a DMM:
1. Fused current inputs.
2. Use of high-energy fuses
(600 V or more).
3. High-voltage protection in
resistance mode (500 V or
more).
4. Protection against voltage
transients (6 kV or more).
5. Safety-designed test leads
with finger guards and
shrouded terminals.
6. Independent safety organiza-
tion approval/listing (e.g., UL
or CSA).
Keep yourself away from
dangerous panels
Your DMM can also protect you
be keeping you away from
hazardous situations. DMMs that
communicate wirelessly with
personal computers, smartphones
and other wireless test tools can
be placed safely inside electrical
panels with the power shut off.
When the panel is closed and
reenergized, measurements can
be made remotely, saved and
shared, all without putting your-
self in front of a live electrical
panel. Diagnosing and solving
problems has never been easier.
Figure 5.
A transformer-type current probe, such as the one
depicted above, scales down the current being
measured. The DMM displays 1 mA for every amp
being measured.
A Hall-Effect probe safely measures high-current
ac or dc values by scaling down the current being
measured and converting this reduced current to a
voltage. The DMM displays 1 mV for every amp.
5 Fluke Education Partnership Program ABCs of DMMs: Multimeter features and functions explained