11
6.4 Total Leakage Current
The total leakage current is effectively the sum of the capaci-
tive, conduction and surface leakage currents described above.
Each current, and hence the total leakage current is affected by
factors such as ambient temperature, conductor temperature,
humidity and the applied level of voltage. If a circuit is fed with
an alternating voltage, the capacitive current (6.1) will always be
present, and can never be eliminated. This is why direct voltage
is applied to test insulation, the capacitive current quickly falling
to zero so that it has no effect on the measurement. A high
voltage is used because this will often break down poor insula-
tion or surface leakage paths and thus show up insulation faults
which would not be present at lower voltage levels. The insula-
tion tester measures the applied voltage and the resulting leak-
age current flow, displaying the resistance which is obtained by
an internal calculation based on Ohm's Law:
Test Voltage (V)
Insulation Resistance (M
Ω
) =
Leakage Current (µA)
As the effective capacitance of the system charges up, so the
leakage current reduces. A steady insulation resistance reading
indicates that the system capacitance is fully charged and that
the capacitive component of current has fallen to zero. It
should be noted that the system charges up to the test voltage
used (250V, 500V, or 1000V). Thus, it can be dangerous for
people or animals to make contact with an electrical installation
which is under test. Even when the test voltage is removed, the
wiring system may remain charged for a significant time unless
steps are taken to provide a path for discharge current. The
KT35 automatically connects a discharge resistor across the cir-
cuit when the test button is released to provide a path for dis-
charge current.
If a wiring system is wet and/or dirty, the surface leakage com-
ponent of test current will be high, giving a low insulation resis-
