4 A STITCH IN TIME
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).
WHAT MAKES INSULATION GO BAD?
When your plant electrical system and equipment are new, the electrical
insulation should be in top notch shape. Furthermore, manufacturers of wire,
cable, motors, and so on have continually improved their insulations for
services in industry. Nevertheless, even today, insulation is subject to many
effects which can cause it to fail – mechanical damage, vibration, excessive
heat or cold, dirt, oil, corrosive vapors, moisture from processes, or just the
humidity on a muggy day.
In various degrees, these enemies of insulation are at work as time goes
on – combined with the electrical stresses that exist. As pin holes or cracks
develop, moisture and foreign matter penetrate the surfaces of the
insulation, providing a low resistance path for leakage current.