Application Note

4 Fluke Corporation ABCs of multimeter safety
Protection against two major electrical hazards
Transients–the hidden
danger
Let’s take a look at a worst-case
scenario in which a technician
is performing measurements
on a live three-phase motor
control circuit, using a meter
without the necessary safety
precautions.
Here’s what could happen:
1. A lightning strike causes a
transient on the power line,
which in turn strikes an arc
between the input terminals
inside the meter. The circuits
and components to prevent
this event have just failed or
were missing. Perhaps it was
not a CAT III rated meter.
The result is a direct short
between the two measure-
ment terminals through the
meter and the test leads.
2. A high-fault current–possibly
several thousands of amps–
flows in the short circuit
just created. This happens
in thousandths of a second.
When the arc forms inside
the meter, a very high-pres-
sure shock wave can cause a
loud bang—very much like a
gunshot or the backfire from
a car. At the same instant,
the tech sees bright blue arc
flashes at the test lead tips–
the fault currents superheat
the probe tips, which start to
burn away, drawing an arc
from the contact point to the
probe.
3. The natural reaction is to
pull back, in order to break
contact with the hot circuit.
But as the tech’s hands are
pulled back, an arc is drawn
from the motor terminal to
each probe. If these two
arcs join to form a single arc,
there is now another direct
phase-to-phase short, this
time directly between the
motor terminals.
4. This arc can have a tempera-
ture approaching 6000 °C
(10000 °F), which is higher
than the temperature of
an oxyacetylene cutting
torch! As the arc grows, fed
by available short circuit
current, it superheats the
surrounding air. Both a
shock blast and a plasma
fireball are created. If the
technician is lucky, the
shock blast pushes him away
and removes him from the
proximity of the arc; though
injured, his life is saved. In
the worst case, the victim
is subjected to fatal burn
injuries from the fierce heat
of the arc or plasma blast.
In addition to using a multi-
meter rated for the appropriate
measurement category, anyone
working on live power circuits
should be protected with flame
resistant clothing, should wear
safety glasses or, better yet, a
safety face shield, and should
use insulated gloves and shoes.
A lightning strike causes a transient
on the power line, creating an arc
between the meter’s input terminal
and resulting in loud noises.
Then, a high current flows in the
closed circuit which is formed.
An arc starts at the probe tips.
If those arcs join, the
resulting high-energy
arc can create a life-
threatening situation
for the user.
When you pull the probes away
as a reaction to the loud noise,
arcs are drawn to the motor
terminals you’re probing.
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Figure 2. A worst-case scenario—potential arc blast sequence.