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Good Insulation
Every generator winding, motor and extension cord have electric wire that is covered with some form
of insulation. Electrical wire is normally copper and it is a good conductor of the electric current that
powers motors. The wires insulation must be the opposite of a conductor; it should resist the current
and keep it in its path along the conductor.
The purpose of the insulation around a conductor is similar to that of pipe carrying water. Pressure on
water from a pump causes flow along the pipe. If the pipe was to spring a leak you'd waste water and
lose some water pressure. With electricity, voltage is like the pump pressure causing current to move
along the copper wire. As with the flow of water in a pipe, there is resistance to flow of current but it is
much less along the conductor than through its insulation.
It should be noted that no insulation is perfect; meaning has infinite resistance, so some electricity
does flow along the insulation or even through it to ground. The current passing through the insulation
may only be a millionth of an ampere (one microampere) but it is the basis of insulation testing. A
higher voltage tends to cause more current leakage through the insulation. This current leakage
would become a problem if the insulation has deteriorated.
All this leads us to determine "what is good insulation". Under normal conditions "good" means a
relatively high resistance to current. Or it can also be stated that a good insulation has the ability to
keep a high resistance. A suitable way of measuring resistance can tell us how good the insulation is.
Also if regular insulation measurements are made you can track trends towards its deterioration.
Causes of Bad Insulation
When generators, welders and electric motors are new the insulation should be at its highest level of
resistance. During equipment use, insulation is subject to many effects which can cause it to fail.
These causes can be mechanical damage, vibrations, excessive heat or cold, dirt, oil, corrosive
vapors and even moisture from humidity. During the life of a conductor's insulation all of these causes
are at work in combination with electrical stresses. If a pin hole or even a crack in the insulation
develops, moisture and foreign matter can penetrate the surfaces of the insulation. This provides for a
low resistance path for leakage current. Once the insulation has begun to deteriorate all elements of
causes tend to combine until excessive current leakage is allowed through the insulation.
At times the drop in insulation resistance can be sudden, such as occurs if the equipment is flooded.
Normally insulation resistance drops gradually and gives plenty of warning if checked as a
preventative maintenance. Periodic checks would allow planned reconditioning prior to operation
failure and the ability to remove from service if the insulation resistance became dangerously low.
Equipment with no checks may not only be dangerous to touch with voltage applied but also be
subject to total burn out. A failed insulator becomes a partial conductor.
For more information on Meg-Ohm-Meters visit our website at www.multiquip.com look at Technical
Information bulletins under Meg-Ohm-Meters.
Multiquip Inc. ◦ GA Series Generators ◦ Manual No. GAGENSM
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