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Insulation Testing Concepts
Insulation resistance can be considered by applying Ohm's
Law. The measured resistance is determined from the
applied voltage divided by the resultant
current,
There are two further important factors to be considered.
These are:
(i)
the nature of the current through and/or over
the insulation, and :
(ii)
the length of time for which the test voltage
is applied. These two factors are linked.
The total current that flows is made up of three separate
currents:-
1. Capacitance charging current. This current is initially
high and drops as the insulation becomes charged up
to the applied voltage.
V
R =
I
2. Absorption current. This current is also initially high but
drops at a much slower rate than the charging current.
3. Conduction or Leakage current. This is a small steady
current that can be sub-divided into two:-
(a)
A current flowing along conduction paths
through the insulation material.
(b)
A current flowing along conduction paths
over the surface of the insulation material.
As the total current depends upon the time for which
the voltage is applied, Ohm's Law theoretically applies at
infinite time.
The charging current falls relatively rapidly as the
equipment under test becomes charged up. The actual
length of time depends upon the size and capacitance of
the item under test. Larger items with more capacitance
will take longer e.g. long supply cables. The absorption
current decreases relatively slowly compared with the
charging current. In essence it depends upon the nature of
the insulation material. The conduction or leakage current
builds up quickly to a steady value and then remains
constant for a particular applied voltage under stable
conditions. It is this current that is affected by moisture, dirt
etc. and the degree to which it flows bears a direct relation
to the quality of the insulation, and consequently to the
value of the insulation resistance measured. Increase in the
leakage current is a pointer to possible future problems.
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