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 transform-
ers, which are used to measure
ac current only, and Hall-Effect
probes, which are used to mea-
sure 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 con-
verted to 100 mV ac. The probe
leads are connected to the “V” and
“COM” jacks. Set the meter func-
tion switch to the “V” or “mV”
scale, selecting V~ for ac current
or V for dc current measure-
ments. The meter displays 1 milli-
volt 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 par-
ticular attention to the WARNING
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 Cat-
egory 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 harming 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 standards. (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 condition
no longer exists. After the
overload is removed, the DMM
automatically returns to nor-
mal 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).
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 mea-
sured and converting this reduced current to a volt-
age. The DMM displays 1 mV for every amp.
5 Fluke Education Partnership Program
ABCs of DMMs: Multimeter features and functions explained