Application Note

2 Fluke Corporation Understanding capacitive voltage sensors
This test gives us a clue about
how the sensor can detect an
open neutral in a branch circuit.
Let’s assume the circuit you are
testing is a 120 V wall outlet.
When you plug in a load, noth-
ing happens. A quick check of
the panel show
s that the correct
circuit breaker is on and your
multimeter measures 120 V
between the hot and ground at
the outlet. Next, you take out
your sensor and insert the tip
into the hot side of the outlet - it
indicates live voltage. Then, you
insert the tip into the neutral
side of the outlet with the same
results - a live voltage indica-
tion. How can this be? If the
neutral were in contact with the
hot conductor, wouldn’t we have
a short circuit? Wouldn’t the
breaker be tripped? If we think
carefully about capacitive cou-
pling, the answer will be obvi-
ous. The hot and neutral
conductors are lying side by side
for the complete distance from
the outlet back to the panel. In
other words, they are capaci-
tively coupled together: each
wire is one “plate” of the capaci-
tor and the conductor insulation
is the dielectric. If the neutral is
open at the panel, and therefore
not grounded, the neutral con-
ductor will float up to nearly the
same voltage as the hot. That’s
why the voltage sensor indicates
live voltage on the neutral.
electrodes as earth grounds).
The dielectric for the second
capacitor is the carpet plus your
shoes and socks. This second
capacitor is much larger than the
first. A very small ac current will
flow because there is 120 V
across the series c
ombination.
(As an aside, this current must
be way below the shock thresh-
old or we wouldn’t be living in a
world of ac power-we definitely
would not be turning on lights
in the bathroom.)
Capacitive voltage
sensors
But how does the voltage divide
between the two caps in series?
This answer is critical to under-
standing how the capacitive
voltage sensor works. Let’s
briefly return to our electrical
circuit theory again. In a series
circuit, the largest voltage will
develop across the largest
impedance (Ohm’s Law). With
capacitors, the smaller the
capacitor, the larger the imped-
ance (known as capacitive reac-
tance). It’s a little tricky, because
it’s the opposite of how resistors
behave, but keeping this twist in
mind, the rest is straightforward.
When two capacitors are in
series, the largest voltage will
develop across the smallest
capac
itor. In the above example,
only a few volts will develop
between your feet and the floor
(the large capacitor) while the
remainder of the 120 V will be
between your head and the light
bulb (the small capacitor). This
may sound bizarre because we
normally don’t think of the car-
pet and floor as parts of an elec-
tric circuit but in fact they are
and they will obey Ohm’s Law
and Kirchoff’s rules if we apply
them correctly.
The capacitive voltage sensor
works because when you hold
the barrel in your hand and
place the tip near a live conduc-
tor, you are inserting the high
impedance sensing element into
a capacitively coupled series cir-
cuit. As in the previous example,
your hand and body form a rela-
tively large capacitor coupled to
the floor. The sensor tip is a
small capacitor coupled to the
live voltage. The sensing circuit
detects the voltage and turns on
a light or sounds the buzzer.
Try it and see
To prove the theory for yourself,
try this simple test: Find a metal
desk lamp that has a two-prong
power cord, i.e., a lamp that is
not grounded. Plug the cord into
a live outlet and with the sensor
in your hand, touch the tip to
the metal frame of the lamp. The
sensor should indicate live volt-
age because the metal frame of
the lamp is near (capacitively
coupled to) the hot side of the
line cord and there is no
grounding conductor to “draw
down” the voltage. In other
words, the sensor detects the
“stray” voltage coupled to the
light frame by the “stray” capac-
itance between the frame and
the hot side of the line. Now,
rest the sensor on a stack of
books or other non-conductive
object so that the tip remains in
contact with the lamp frame
while you take your hand away.
The sensor will no longer indi-
cate live voltage because its
capacitively coupled circuit was
broken when you took your
hand away!