Application Note

Application Note
Your partner just bought one of those pen-shaped ac
voltage detectors. He calls it a “tick-tracer” or a
“glow-tip”. You’ve seen him carry it in his shirt pocket
wherever he goes. He must like it because he won’t
loan it out. When you asked him about it, he claimed
that it can detect live ac voltage inside an insulated
wire. He also says he has used it to quickly detect an
open neutral in a branch circuit or in some cases spot-
ted a bad ground connection for a metal enclosure.
How does this thing work anyway? How can it
detect voltage without making a metallic contact?
Will it detect live conductors inside a grounded metal
conduit?
Understanding capacitive
voltage sensors
How voltage detectors use your body’s conductivity
From the Fluke Digital Library @ www.fluke.com/library
Capacitive coupling
AC voltage detectors work on
the principle of capacitive cou-
pling. To understand this, let’s
return momentarily to electrical
circuit theory and recall how a
capacitor works. A capacitor has
two conductors or “plates” that
are separated by a non-conduc-
tor called a dielectric. If we con-
nect an ac voltage across the
two conductors, an ac current
will flow as the electrons are
alternately attracted or repelled
by the voltage on the opposite
plate. There’s a complete ac cir-
cuit even though there’s no
“hard-wired” circuit connection.
The electrical “field” inside the
capacitor, between the two
plates, is what completes the ac
circuit.
We often think of capacitors
as individual circuit components
such as motor starting caps, but
in reality, the world is full of
small “stray” capacitors that we
don’t normally realize are pres-
ent. Here’s an example. Suppose
you are standing on a carpeted
concrete floor directly under a
120 V light fixture and the light
is on. Your body is conducting a
very small ac current because it
is part of a circuit consisting of
two capacitors in series. The
two conductors or plates for the
first capacitor are the live ele-
ment in the light bulb and your
body. The dielectric is the air
(and maybe your hat) between
them. The two conductors for
the second capacitor are your
body and the concrete floor
(remember that concrete is a
good conductor, as is shown by
the use of concrete encased