Application Notes
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,
V
R = I
There are two further important factors to be considered.
These are:
(i) the nature of the current through and/or over the
insulation
(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.
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. An increase in the leakage current is a pointer to
possible future problems.
1 0 0
1 0
1
0 . 1
1
1 0
C O R R E N T E
Corrente de
Absorcão
Corrente Total
Corrente de
Carga de Capacitãncia
Corrente de
Conduçao
ou Dispersão
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