Application Note

2 Fluke Corporation Leakage current measurement basics
testing. Avoid twisting the jaws
of the clamp meter as this can
cause erroneous measurements.
The clamp meter detects
the magnetic field surround-
ing conductors such as a single
core cable, a wire armor cable,
a water pipe, etc.; or the paired
phase and neutral conductors
of a single-phase circuit; or all
live conductors (3-wire or 4-
wire) of a three-phase circuit
(like a GFCI or residual current
device).
When testing the grouped
live conductors of a circuit, the
magnetic fields produced by
the load currents cancel each
other out. Any imbalance cur-
rent comes from leakage from
the conductors to ground or
elsewhere. To measure this
current, a leakage clamp meter
should be able to read less
than 0.1 mA.
For example, taking a mea-
surement on a 240 V ac circuit,
with all loads disconnected,
might result in a value of
0.02 mA (20 μA) leakage. This
value represents an insulation
impedance of:
240 V / (20 x 10
-6
) =
12 MΩ. (Ohms Law R=V/I)
If you conducted an insula-
tion test on a circuit that was
powered down, the result
would be in the region of 50
MΩ or more. This is because
the insulation tester uses a dc
voltage for testing, which does
not take the capacitive effect
into consideration. The insula-
mA
A
mA
D-H
Supply Load
mA
A
mA
D-H
Supply Load
mA
A
mA
D-H
Supply Load
mA
A
mA
D-H
Supply Load
tion impedance value is the
actual value that exists under
normal operating conditions.
If you measured the same
circuit loaded with office equip-
ment (PCs, monitors, copiers,
etc.), the result would be sig-
nificantly different due to the
capacitance of the input filters
on these devices. When many
pieces of equipment are oper-
ating on a circuit, the effect
will be cumulative; that is, the
leakage current will be higher
and could well be in the order
of milliamps. Adding new
pieces of equipment to a cir-
cuit protected by a GFCI could
trip the GFCI. And because
the amount of leakage current
varies depending on how the
equipment is operating, the
GFCI may trip randomly. Such
intermittent problems can be
difficult to diagnose.
A clamp meter will detect
and measure a wide range
of alternating or changing
currents passing through a
conductor under test. When
telecommunications equip-
ment is present, the value of
leakage indicated by a clamp
meter may be considerably
more than that resulting from
insulation impedance at 60 Hz.
This is because telecommu-
nications equipment typically
incorporates filters that produce
functional grounding currents
and other equipment that pro-
duces harmonics, etc. You can
only measure the characteristic
leakage at 60 Hz by using a
clamp meter that incorporates a
narrow band-pass filter
for removing currents at
other frequencies.
Measurement of leakage
current to ground
When the load is connected
(switched on), the leakage cur-
rent measured includes leak-
age in load equipment. If the
leakage is acceptably low with
the load connected, then circuit
wiring leakage is even lower.
If circuit wiring leakage alone
is required, disconnect (switch
off) the load.
Test single-phase circuits by
clamping the phase and neutral
conductor. The measured value
will be any current flowing to
ground. (See Figure 1)
Test three-phase circuits by
clamping around all three-
phase conductors. If a neutral
is present, it should be clamped
mA
A
mA
D-H
A
B
C
Load
mA
A
mA
D-H
Phase
Neutral
Load
Figure 1
Figure 2