Testing earth fault resistance

A three-phase induction motor has conductors which are insulated to withstand the full rated supply voltage.

This insulation may break down so that the insulation resistance falls below certain values.

The conductors must have an insulation resistance of greater than one hundred kilohms to function at all, and greater than one megohm ideally.

In a typical three phase induction motor circuit, an appropriate place to test is at the load side of the direct-on-line motor starter.

Figure 1

      O  O  O
       /  /  /
      /  /  /
      O  O  O
      A  B  C         Earth

If the motor system has satisfactory motor coil resistance, the test should be made between A, B, or C and Earth.