Megger BM11D Guide De L'utilisateur page 15

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.
1 0 0
1 0
Conduction
or Leakage
Current
1
0 . 1
Components of insulation test current
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
R =
Capacitance
Charging Current
Total Current
Absorption
Current
1 0
1
T I M E
that can be sub-divided into two:-
(a) A current flowing along conduction paths through
the insulation material.
V
I
(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. An
increase in the leakage current is a pointer to possible future
problems.
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