Kewtech KT64, Instruction Manual, Page: 21 / 44

18
8 . 1 . 4 Surface Leakage Current
Where insulation is removed, for the connection of conductors and so on,
current will flow across the surfaces of the insulation between the bare
conductors. The amount of leakage current depends on the condition of
the surfaces of the insulation between the conductors. If the surfaces are
clean and dry, the value of the leakage current will be very small. Where the
surfaces are wet and/or dirty, the surface leakage current may be significant.
If it becomes large enough, it may constitute a flashover between the
conductors.
Whether this happens depends on the condition of the insulation surfaces and
on the applied voltage; this is why insulation tests are carried out at higher
voltages than those normally applying to the circuit concerned.
8 . 1 . 5 Total Leakage Current
The total leakage current is the sum of the capacitive, conduction and surface
leakage current described above. Each of the currents, and hence the
total leakage current, is affected by factors such as ambient temperature,
conductor temperature, humidity and the applied voltage.
If the circuit has alternating voltage applied, the capacitive current (
8 . 12 ) will
always be present and can never be eliminated. This is why a direct voltage is
used for insulation resistance measurement, the leakage current in this case
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 insulation and cause
flashover due to surface leakage (see
8 . 1 . 4 ), thus showing up potential faults
which would not be present at lower levels.
The insulation tester measures the applied voltage level and the leakage
current through the insulation. These values are internally calculated to give
the insulation resistance using the expression:-
Fig. 14
Test voltage (V)
Insulation resistance (M
Ω
)
＝
Leakage current (
μ
A)
Insulation