An Insulation Resistance Test measures the total resistance of a product’s insulation (in this case, a pump). The test involves applying a pre-determined level of voltage to the pump and there will be a minimum acceptable value of resistance for a pump to pass the test.
There are two types of insulation in a pump; insulation of the motor and insulation of the cable which is providing power to it.
The Insulation Resistance Test measures the health of the insulation between the copper wiring and the earth. Motor coils are generally coated in shellac which insulates the copper wire, protecting it from itself and from the motor casing. This insulation prevents the voltage/power supply from being lost to earth i.e. keeps it contained. A Residual Current Device (RCD) is used to measure the earth leakage in a circuit.
Low insulation resistance will cause the motor to draw higher current (it thinks it needs more because its losing electricity) and will eventually result in motor fail due to an open circuit on the motor stator (static bit) coils.
Any electrical appliance/machine will always degenerate over time due to heat.
There are two key reasons why insulation resistance reduces;
Age and heat (heat generated by the motor)
Moisture ingress into the motor – this degrades it faster and is why you should always have a cover for your pump – especially if it is outside.