Application Note

ABCs of multimeter safety Fluke Corporation 5
Transients aren’t the only source of
possible short circuits and arc blast
hazard. One of the most common
misuses of handheld multimeters
can cause a similar chain of events.
Let’s say a user is making
current measurements on signal
circuits. The procedure is to select
the amps function, insert the leads
in the mA or amps input termi-
nals, open the circuit and take a
series measurement. In a series
circuit, current is always the same.
The input impedance of the amps
circuit must be low enough so that
it doesn’t affect the series circuit’s
current. For instance, the input
impedance on the 10 A terminal
of a Fluke meter is .01 W. Compare
this with the input impedance on
the voltage terminals of 10 MW
(10,000,000 W).
If the test leads are left in the
amps terminals and then acciden-
tally connected across a voltage
source, the low input imped-
ance becomes a short circuit! It
doesn’t matter if the selector dial is
turned to volts; the leads are still
physically connected to a low-
impedance circuit.* That’s why the
amps terminals must be protected
by fuses. Those fuses are the only
thing standing between an incon-
venience–blown fuses–and a
potential disaster.
Use only a multimeter with amps
inputs protected by high-energy
fuses. Never replace a blown fuse
with the wrong fuse. Use only
the high-energy fuses specified
by the manufacturer. These fuses
are rated at a voltage and with a
short circuit interrupting capacity
designed for your safety.
Overload protection
Fuses protect against overcur-
rent. The high input impedance of
the volts/ohms terminals ensures
that an overcurrent condition is
unlikely, so fuses aren’t necessary.
Overvoltage protection, on the other
hand, is required. It is provided
by a protection circuit that clamps
high voltages to an acceptable
level. In addition, a thermal protec-
tion circuit detects an overvoltage
condition, protects the meter until
the condition is removed, and then
automatically returns to normal
operation. The most common ben-
efit is to protect the multi meter from
overloads when it is in ohms mode.
In this way, overload protection
with automatic recovery is provided
for all measurement functions as
long as the leads are in the voltage
input terminals.
A
mA
COM
V
TEMPERATURE
A
V
A
mA
COM
V
TEMPERATURE
A
V
TRUE RMS MULTIMETER
189
400mA
FUSED
10A MAX
FUSED
CAT
1000V
A
COM
Figure 3. Misuse of DMM in Ammeter Mode.
While most people are aware of the
danger from electric shock, few real-
ize how little current and how low a
voltage are required for a fatal shock.
Current flows as low as 30 mA can
be fatal (1 mA=1/1000 A). Let’s look
at the effects of current flow through
a “typical” 68 kilogram (150 pound)
male:
• At about 10 mA, muscular paraly-
sis of the arms occurs, so that he
cannot release his grip.
• At about 30 mA, respiratory paraly-
sis occurs. His breathing stops and
the results are often fatal.
• At about 75 to 250 mA, for exposure
exceeding five seconds, ven-
tricular fibrillation occurs, causing
incoordina tion of the heart muscles;
the heart can no longer function.
Higher currents cause fibrillation at
less than five seconds. The results
are often fatal.
Now let’s calculate the thresh hold for
a “hazardous” voltage. The approxi-
mate body resistance under the skin
from hand to hand across the body
is 1000 W. A voltage of only 30 V
across 1000 W will cause a current
flow of 30 mA. Fortunately, the skin’s
resistance is much higher. It is the
resistance of the skin, especially the
outer layer of dead cells, that protects
the body. Under wet conditions, or if
there is a cut, skin resistance drops
radically. At about 600 V, the resis-
tance of the skin ceases to exist. It is
punctured by the high voltage.
For multimeter manufacturers and
users, the objective is to prevent
accidental contact with live circuits at
all costs.
Look for:
• Meters and test leads with double
insulation.
• Meters with recessed input jacks
and test leads with shrouded input
connectors.
• Test leads with finger guards and a
non-slip surface.
• Meter and test leads made of high-
quality, durable, non-conductive
materials.
Use the right hi gh-energy fuses
Electric shock
* Some multimeters, such as the Fluke 80 Series,
have an Input Alert which gives a warning beep
if the meter is in this configuration.
Arc blast and electric shock