What is “Good” Insulation?

Every electric wire in your plant – whether it’s in a motor, generator, cable,
switch, transformer, etc. – is carefully covered with some form of electrical
insulation. The wire itself is usually copper or aluminum, which is known to
be a good conductor of the electric current that powers your equipment. The
insulation must be just the opposite from a conductor: it should resist current
and keep the current in its path along the conductor.

Common sense tells us that the more voltage we have, the more current
there’ll be. Also, the lower the resistance of the wire, the more current for
the same voltage.

Actually, this is ohm’s law, which is expressed this way in equation form:

E = I x R

where,

E = voltage in volts
I = current in amperes
R = resistance in ohms

note, however, that no insulation is perfect (that is, has infinite resistance) so
some electricity does flow along the insulation or through it to ground. Such
a current may only be a millionth of an ampere (one microampere) but it is
the basis of insulation testing equipment. note also that a higher voltage
tends to cause more current through the insulation. This small amount of
current would not, of course, harm good insulation but would be a problem
if the insulation has deteriorated.
now, to sum up our answer to the question “what is ‘good’ insulation?”
We have seen that, essentially, “good” means a relatively high resistance
to current. used to describe an insulation material, “good” would also
mean “the ability to keep a high resistance.” So, a suitable way of
measuring resistance can tell you how “good” the insulation is. Also, if you
take measurements at regular periods, you can check trends toward its
deterioration.

Factors Affecting Insulation Resistance Reading

Remember that the measured resistance (of the insulation) will be
determined by the voltage applied and the resultant current (R = e/I).
There are a number of things that affect current, including temperature
of the insulation and humidity, as mentioned in the previous section.
Right now, let’s just consider the nature of current through insulation
and the effect of how long voltage is applied.
Current through and along insulation is made up partly of a relatively
steady current in leakage paths over the insulation surface. electricity
also flows through the volume of the insulation. Actually, as shown in
the figure above, our total current comprises three components:

1. Capacitance Charging Current
Current that starts out high and drops after the insulation has been
charged to full voltage (much like water flow in a garden hose when
you first turn on the spigot).

2. Absorption Current
Also an initially high current which then drops (for reasons discussed
under the section Time-Resistance Method).

3. Conduction or leakage Current
A small essentially steady current both through and over the
insulation.