Insulation Testing Concepts - Megger BM25 Guia Del Usuario

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,
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.
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 that can
be sub-divided into two:-
18
R = V
I
(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.