Megger WM6, User Manual

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, 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.

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.

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