Megger test / Sheath resistance test procedure for cables

instrumentation

#1

A Megger test or sheath resistance test is used to diagnose cable faults, either short circuit problem, water penetration, moisture or current leakage. A Megohmmeter is used to measure the resistance of the crossed-link to an applied DC voltage.

A Megohmmeter applies current for 1 minute through the cable before being measured and recorded. The voltage applied varies depending on cables.

Megger tester

  • Megger tester consists of an inbuilt generator. Rotating the pedal in the generator produce current in the circuit.

  • Megger normally work on 500v, 1000v tester for higher voltage testing.

  • There are three connection line terminals in the tester, Gaurd terminal, Line terminal and Earth terminal.

  • Resistance is measured between Line and Earth terminals. Gaurd terminal is used for special situations.

  • The high DC voltage passes a small current through and over the surface of the insulation being tested. Which is indicated in the ohm meter’s indicator scale.

Procedure:

  • Before going to test the cable disconnect the cable with all the devices.Ensure both ends of the cable are separated from each other.

  • Make sure that there is no Eddy current the device by earthing it.

  • Now connect the Megger terminals to the cable which has to be measured, as shown in the diagram

  • Hand crank the Megger to generate a high voltage, which causes a small current through and over the surface of the cable.

  • If the current reading shows infinity in the ohm meter then the cable as good insulation.