What is circuit faults? how it is tested and effects

  • An Open circuit fault will not allow the motor to start. If the fault occurs in 1 phase when the motor is running, the current in other two phases will increase and burn out the motor.
  • A Short circuit fault will cause high current to flow in the winding's which may burn out.
  • An earth fault provides an easy path for current to flow through the wires and earth.

How to check these faults for a removed motor:

OPEN Circuit: Earth one end of the wire and using multimeter or megger switched to res. Value, check between wire at different points and earth. Resistance should show minimum value.

EARTH fault: Open circuit where possible and check the res. Between wire near “open” ends and earth. Resistance should show maximum value.

SHORT circuit fault: Remove the terminals at the panel. Turn all switches for that circuit to ON position. Check for resistance between positive and negative sides of the conductor. A reading of less than infinite indicates the fault.

Types of circuit Faults:
  • Three basic circuit faults can occur:
  • An open-circuit faults is due to a break in the conductor, as at A, so that current cannot flow.
  • An Earth Fault is due to a break in the insulation, as at B allowing the conductor to touch the hull or an earthed metal enclosure.
  • A Short-Circuit fault is due to a double break in the insulation, as a C, allowing both conductors o be connected so that a very large current by-passes or short circuits the load.


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