Insulation Resistance Test Or Megger Test Procedures With Circuit Diagram

 In this article we will go through Megger test first of all i want to say both insulating resistance tests and megger test are same. which is usually carries out to find the insulation of different machines windings conductor wires generator windings etc.
Megger Testing Instrument

  • Megger testing instrument is a high resistance Ohmmeter with a build in generator

  •  It is equipped with three connections Line Terminal (L), Earth Terminal (E) and Guard Terminal (G)
  • Resistance is measured between the Line and Earth terminals
  • The “Guard” terminal is provided for special testing situations where one resistance must be isolated from another.
  • The generator can be hand-cranked or line-operated to develop a high DC voltage which causes a small current through and over surfaces of the insulation being tested
  • Which is measured by an ohm meter having an indicator scale
Importance Of Insulation Resistance Test Or Megger Test
  • An insulation test is carried out to test the integrity of the insulation between conductors. 
  • Which help to find the short circuit problems in the circuit
  • It also serves as the best guide to indicate the health of the equipment
Procedure To tests the Insulation
  • Test the megger before use, whether it gives INFINITY value when not connected, and ZERO when the two terminals are connected together and the handle is rotated.
  • To do the test you ensure the cable is disconnected from any devices (megger normally work on 500v, 1000v testers for higher voltage testing).
  • Make sure that no eddy current in the device by earthing it.(very important) 
  • Instrument should be in normal working temperature because resistance is related to temperature.
  •  Ensure both ends of the cables are separated from each other (connect one end in terminal strip if necessary). 
  • Now connect the megger terminals to the conductors which has to be meassured
  • Then hand crank the generator and a high DC voltage generated which causes a small current through and over surfaces of the insulation being tested.
  • If the Reading shows Infinity means the conductors having good insulation.
Here it shows a megger circuit diagram



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  1. Very informative and nicely explain...........

  2. How to be sure that by cranking we are developing 500vDC

    1. It is not necessary to produce 500v..
      Deflection of pointer increases with the increases in voltage in the external circuit and also decreases with the increase in current. That is, resultant torque is inversely related to current and directly related to voltage

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