KYORITSU 6010A - Manual (Page 15)

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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. Model 6010A 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 before the test include:-