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Figure 4.1 – Two most common contributing factors in
insulation failure

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Excessive Moisture
On air-cooled electrical machinery, the moisture content of the
air is very important. With aging of the insulation, small hairline
cracks will appear in the insulation. Moisture will seep into these
cracks and allow an electrical path to short-circuit between
adjacent turns of wire.
Although the voltage between the turns is quite small, when
they short together, a closed loop to the magnetic flux is provided,
and this causes tremendous currents to flow in the shorted loop.
This usually destroys the electrical machine, and it has to be
removed and re-wound/replaced.

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