Insulation tests
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5.1.5 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 (5.1.2)
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 5.1.4), 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:-
Insulation resistance (MΩ) =
Test voltage (V)
Leakage current (µA
As the capacitance of the system charges up, so the charging current falls
to zero and a steady insulation resistance reading indicates that the
capacitance of the system is fully charged. The system is charged to the full
test voltage, and will be dangerous if left with this charge. The KT62
provides an automatic path for discharging current as soon as the test
button is released to ensure that the circuit under test is safely discharged.
If the wiring system is wet and/or dirty, the surface leakage component of
the leakage current will be high, resulting in low insulation resistance
reading. In the case of a very large electrical installation, all the individual
circuit insulation resistances are effectively in parallel and the overall
resistance reading will be low. The greater the number of circuits
connected in parallel the lower will be the overall insulation resistance.
5.2 Damage to Voltage-Sensitive Equipment
An increasing number of electronic-based items of equipment are being
connected to electrical installations. The solid state circuits in such
equipment are likely to be damaged by the application of the levels of
voltage used to test insulation resistance. To prevent such damage, it is
important that voltage-sensitive equipment is disconnected from the
installation before the test is carried out and reconnected again
immediately afterwards. The devices which may need to be disconnected
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