Fluke T+, T+PRO Service Guide

Differences in voltage

testers can be shocking

Walk onto any job site where
electricians are working, and
you’ll probably see some
voltage testers. These handy
devices—which often fit in a
shirt or pants pocket—are
popular because they quickly
give an indication of voltage
presence. That makes them
very handy for general voltage
checks. Thus, voltage testers
are popular with electricians.
Yet, these devices are not all
the same. The differences show
up in safety, reliability, and
convenience.

If you were to look at all the
voltage testers on the market
and note their differences,
you’d quickly see they divide
into two general categories:
solenoid-based testers and
electronic testers. Solenoid­based testers have a long
tradition—they were the first
voltage testers available and
are still widely in use today.

When voltage passes some
threshold, the tester will indi­cate a voltage is present. Below
that threshold, the tester won’t
indicate a voltage at all. The
thresholds are markedly differ­ent between the two catego­ries of testers—and that fact
carries important implications
for safety and convenience.
Let’s compare voltage testers
in these two categories more
closely, so you can draw your
own conclusions about what to
have in your toolbox—or what
to carry in your pocket.

Figure 1: This solenoid tester experienced a cata­strophic failure after receiving an impulse. Even
with an MOV (metal oxide varistor) (see A), the unit
will still self-destruct from overheating (see B).

Solenoid-based
voltage testers

These devices operate, as
their name implies, on solenoid
principles. A solenoid depends
on the movement of a ferrite
core referred to as the slug in
response to the energization
and de-energization of an elec­tromagnetic coil. The indica­tion function of these testers
depends on a spring, which
drives a mechanical pointer.

The spring restrains the slug—
which slides to one end of its
chamber or the other, depend­ing on whether the coil has
enough energy to cause the
slug to overcome the opposing
force of the spring. The amount
of energy required restricts the
sensitivity of solenoid-based
testers. In the US, a useful
solenoid tester will have the
ability to measure voltages up
to 480 V or more. Having the

Figure 2: Current-limiting resistors (see C) protect this elec­tronic tester which results in a predictable failure mechanism
when the tester is exposed to electrical impulses.

Application Note

From the Fluke Digital Library @ www.fluke.com/library

Electronic testers like this Fluke T+ vibrate, beep, and
glow to indicate voltage, giving you the feel of an old­fashioned solenoid in a safer electronic model.

ability to measure higher volt­ages 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 tes­ters means significantly more
heat—enough that the testers
can quickly overheat, even to
the point of damaging the tes­ter if the voltage is measured a
little too long (see Figure 1). In
fact, you must allow for cool­downs (on the order of half a
minute) as you take readings

with solenoid-based testers. If
your programmable logic con­troller (PLC) goes down and
the plant manager is scream­ing 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 volt­age testers.

This high current in sole­noid-based testers has another
downside. Applying Ohm’s Law
to the low impedance solenoid­based 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.

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.

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 tes­ters. Simply apply Ohm’s Law,
and the advantages become
clear. You’re going to be deal­ing with far less input current.
That means more safety. It also
means less time—if any—wait­ing for the instrument to cool
between readings. They work
at lower voltages, and typically
carry an IEC Category rating.
Figure 2 shows the input pro­tection 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 tes­ter 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

2 Fluke Corporation Differences in voltage testers can be shocking