Guide to Electrical Insulation Testing

Meggar test to insulation test

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. To understand insulation testing you really don’t need to go into the mathematics of electricity, but one simple equation – Ohm’s law – can be very helpful in appreciating many aspects. Even if you’ve been exposed to this law before, it may be a good idea to review it in the light of insulation testing.
The purpose of insulation around a conductor is much like that of a pipe carrying water, and Ohm’s law of electricity can be more easily understood by
a comparison with water flow. In Fig. 1 we show this comparison. Pressure on water from a pump causes flow along the pipe (Fig. 1a). If the pipe were to
spring a leak, you’d waste water and lose some water pressure. With electricity, voltage is like the pump pressure, causing electricity to flow
along the copper wire (Fig. 1b). As in a water pipe, there is some resistance to flow, but it is much less along the wire than it is through the insulation.

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 (more on this later).