Application Note

2 Fluke Corporation Differences in voltage testers can be shocking
ability to measure higher volt-
ages limits the capability to
detect voltages below about
100 V due to the poor dynamic
range of the magnetics, an
unfortunate weakness of
solenoid testers. Try using
one on 24 V or 48 V control
circuits, and you may as well
be using a stick of wood.
An important concern with
solenoid-based testers is their
relatively low input impedance
—10 kilohms at the upper end,
but often as low as 1 kilohm.
Applying Ohm’s Law, you can
see solenoid-based testers can
easily make their presence
felt in a circuit as loads – and
subsequently interfere with
the operation of that circuit.
The relatively high current
draw of solenoid-based tes-
ters means significantly more
heat—enough that the testers
can quickly overheat, even to
the point of damaging the tes-
ter if the voltage is measured a
little too long (see Figure 1). In
fact, you must allow for cool-
downs (on the order of half a
minute) as you take readings
with solenoid-based testers. If
your programmable logic con-
troller (PLC) goes down and
the plant manager is scream-
ing about production being lost
forever, you’re at the mercy of
this limitation. Even receptacle
testing can become dicey. Of
course, you could carry around
half a dozen types of testers
and rotate them in and out of
service—but that defeats one of
the reasons for using a small
tester to begin with.
Solenoid-based testers are
generally unable to comply to
IEC 61010 due to excessive
current draw, poor dielectric
withstand performance and
impulse destruction due to
transients originating from
the mains. This is one reason
many companies forbid the use
of voltage testers in general
on anything but 24 V control
circuits and some forbid them
altogether. In a moment, we’ll
look at reasons to reconsider
those restrictions, at least for
category rated electronic volt-
age testers.
This high current in sole-
noid-based testers has another
downside. Applying Ohm’s Law
to the low impedance solenoid-
based tester shows that you
can easily carry a lethal current
through the tester. Wearing
insulated gloves can reduce the
shock hazard, but, you’ll also
be risking an arc hazard each
and every time. Yes, there are
riskier things you can do than
use a solenoid-based tester.
But, there are also safer things
you can do—such as using
an electronic voltage tester,
instead.
Electronic voltage testers
The first noticeable advantage
of electronic voltage testers is
their rugged, compact design,
relative to their old-technology
counterparts. Thus, they are
easier to carry around and
less likely to break. But these
advantages pale beside the
significant safety advantages
that come from the far higher
input impedance of electronic
voltage testers. Some of these
have an input impedance of
one megohm—about 100 times
that of the best solenoid-based
testers. Even at the low end,
you’re looking at 20 kilohms
for an electronic voltage
tester-still twice as good as
the best solenoid-based tes-
ters. Simply apply Ohm’s Law,
and the advantages become
clear. You’re going to be deal-
ing with far less input current.
That means more safety. It also
means less time—if any—wait-
ing for the instrument to cool
between readings. They work
at lower voltages, and typically
carry an IEC Category rating.
Figure 2 shows the input pro-
tection part of the circuit that
makes the IEC Category rating
possible. They allow you to
troubleshoot a wider range of
problems—safer and faster.
This higher impedance has
a downside: an electronic tes-
ter might indicate voltage on
a non-energized conductor
(e.g., ghost voltages). This can
happen when one conductor
induces a voltage in another
conductor parallel to it. This
voltage indication can be a
disadvantage by showing a
Electronic testers like this Fluke T
+
vibrate, beep, and
glow to indicate voltage, giving you the feel of an old-
fashioned solenoid in a safer electronic model.
NFPA 70E as the deal-breaker
The 2009 edition of the NFPA 70E Standard for
Electrical Safety in the Workplace now mandates
the use of only IEC-rated tools. That means
if you’re still using an unrated solenoid tester,
you’re out of compliance with the NFPA.