Insulation Tests; The Nature Of Insulation Resistance; Capacitive Current - KYORITSU KEW 6016 Instruction Manual

8. INSULATION TESTS

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'.

8.1.1 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.
8.1.2 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 KEW6016. 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.
Fig.15
20