30-11-2012, 06:31 PM
ABCs of multimeter safety
ABCsofmultimeter safety.pdf (Size: 493.63 KB / Downloads: 20)
Voltage spikes–an unavoidable hazard
As distribution systems and
loads become more complex,
the possibilities of transient
overvoltages increase. Motors,
capacitors and power conversion
equipment such as variable
speed drives can be prime
generators of spikes. Lightning
strikes on outdoor transmission
lines also cause extremely hazardous
high-energy transients.
If you’re taking measurements
on electrical systems, these
transients are “invisible” and
largely unavoidable hazards.
They occur regularly on lowvoltage
power circuits, and can
reach peak values in the many
thousands of volts. In these
cases, you’re dependent for protection
on the safety margin
already built into your meter.
The voltage rating alone will
not tell you how well that meter
was designed to survive high
transient impulses.
Transient protection
The real issue for multimeter
circuit protection is not just the
maximum steady state voltage
range, but a combination of
both steady state and transient
overvoltage withstand capability.
Transient protection is
vital. When transients ride on
high-energy circuits, they tend
to be more dangerous because
these circuits can deliver large
currents. If a transient causes
an arc-over, the high current
can sustain the arc, producing
a plasma breakdown or explosion,
which occurs when the
surrounding air becomes ionized
and conductive. The result is
an arc blast, a disastrous event
which causes more electrical injuries
every year than the better
known hazard of electric shock.
(See “Transients–the hidden
danger” on page 4.)
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 measurement
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-pressure
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.