5 Insulation tests
11
Warning: Ensure that circuits to be tested are not live.
Disconnect the instrument from the circuit under test before
operating the function switch.
To select the insulation resistance range select ‘INSULATION’.
5.1 The nature of insulation resistance
Live conductors are separated from each other and from earth metal by
insulation, which has a resistance which is high enough to ensure that the
current between conductors and to earth is kept at an acceptably low
level. Ideally insulation resistance is infinite and no current should be able
to flow through it. In practice, there will normally be a current between
live conductors and to earth, and this is known as leakage current. This
current is made up of three components, which are:-
1.
capacitive current
2.
conduction current, and
3.
surface leakage current.
5.1.2 Capacitive Current
The insulation between conductors which have a potential difference
between them behaves as the dielectric of a capacitor, the conductors
acting as the capacitor plates. When a direct voltage is applied to the
conductors, a charging current will flow to the system which will die away
to zero (usually in less than a second) when the effective capacitor
becomes charged. This charge must be removed from the system at the
end of the test, a function which is automatically performed by the KT62.
If an alternating voltage is applied between the conductors, the system
continuously charges and discharges as the applied voltage alternates, so
that there is a continuous alternating leakage current flowing to the
system.