9
matically subtracts the test lead resistance before displaying the
real continuity resistance value. A full description of how this
feature can be used will be found in paragraph 9.2.
6 The Nature of Insulation Resistance
Live conductors are separated from each other and from earth-
ed metal by insulation, which ensures that the current flowing
between conductors and to earth is kept to an acceptably low
level. In simple theory, insulation is perfect, so that its resistance
is infinite and there is no current flow through it at all. In prac-
tice, there will always be a current between conductors and to
earth. And this is known as leakage current. This current is
made up of three components.
6.1 Capacitive Current
The insulation between conductors at different voltages
behaves as the dielectric of a capacitor, the conductors behav-
ing as the capacitor plates. When a direct voltage is applied to
the conductors, charging current will flow to the system but
will quickly die away (usually in much less than one second) as
the effective capacitor becomes charged. If an alternating volt-
age is applied, there will be alternating charge and discharge
currents so that there will be a continuous flow of alternating
current to the conductors.
Insulation (acting as dielectric)
Conductor
(acting as
capacitor
plates)
Capacitive effect
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