What is Megger : Construction and Its Working Principle

Devices which directly utilize electrical energy to provide desired or expected output or a result is known as Electrical devices. During the process of utilization of electrical energy, i,e, the negatively charged particles which are electrons not only flow from one end to another end in a current-carrying conductor but also changes its state from one form to another like heat to gain expected results. There are many electrical components and devices like a transformer, circuit breaker, transistors, resistors, electric motor, and refrigerators, gas fireplace, electric water heater tank, etc. In any electrical system, there may be losses based on the material of metal used (Losses α Degraded Output). Therefore losses should be maintained less. In order to protect these electrical systems from losses, there are certain parameters that are to be maintained and also certain instruments are used to keep track of the electrical systems to safeguard them. This article discusses what is a megger and its working.

What is Megger?

An instrument that is used to measure insulation resistance is a Megger. It is also known as meg-ohm-meter. It is used in several areas like multi-meters, transformers, electrical wiring, Etc. Megger device is used since the 1920s for testing various electrical devices which can measure greater than 1000meg-ohms.

Insulation Resistance

Insulation resistance is resistance in ohms of wires, cables, and electrical equipment, which is used to safeguard the electrical systems like electrical motors from any accidental damages like electrical shocks or sudden discharges of current leakages in wires.

Principle of Megger

The principle of Megger is based on moving coil in the instrument. When current is flowing in a conductor, which is placed in a magnetic field, it experiences a torque.

Where vectored Force = strength and direction of the current and magnetic field.

Case (i) Resistance of insulation = High;   pointer of moving coil = infinity,

Case (ii) Resistance of insulation = Low;   pointer of moving coil = zero.

It is the comparison between Insulation resistance and the known value of resistance. It provides the highest accuracy in measurement than other electrical measuring instruments.

Construction of Megger

Megger is used to measure a high value of resistance. Megger consists of the following parts.

  • DC generator
  • 2 Coils (Coil A, Coil B)
  • Clutch
  • Crank handle
  • terminal X & Y

Block Diagram of Megger

  • Crank handle present here is rotated manually, and the clutch is used to vary the speed. This arrangement placed between magnets, where the entire set-up is called a DC generator.
  • A Resistance scale is present towards the left of the DC generator, which provides the value of resistance ranging from 0 to infinity.
  • There are two coils in the circuit Coil-A and Coil-B, which are connected to the DC generator.

The two testing terminals X and Y which can be connected in the following manner

  • To calculate the resistance of the winding of the transformer, then the transformer is connected between the two testing terminals X and Y.
  • If we want to measure the insulation of the cable, then the cable is connected between the two testing terminals A and B.

Working of Megger

Megger here is used to measure

  • Insulation resistance
  • Machine windings

According to the principle of DC generator, whenever a current-carrying conductor is placed between the magnet fields, it induces a certain amount of voltage. The magnetic field generated between the two poles of the permanent magnet is used to rotate the rotor of the DC generator using the crank handle.

Whenever we rotate this DC rotor, some voltage and current are generated. This current flows through the Coil A and Coil B in an anti-clockwise direction.

Where coil A carries current = IA and

Coil B carries current = IB.

These two current produces fluxes ϕA and ϕB in two coils A and B.

  • On one side motor requires two fluxes to interact and produce reflecting torque, then the only motor runs.
  • Whereas on the other side the two flux’s ϕA and ϕB which are interacted with each other and then the pointer which is presented will experience some force by the production of deflecting torque “Td”, where the pointer shows the resistance value on the scale.


  • The pointer on the scale initially indicates infinity value,
  • Where ever it experiences a torque, the pointer moves from infinity position to zero position on the resistance scale.

  Why the Instrument Initially shows infinity and Finally moves towards zero?

According to Ohm’s law

R = V / I ;          ——– (2)

If the current is maximum in the instrument, resistance is zero,

R α 1/I;            ——– (3)

If the current in minimum in the instrument, resistance is maximum.

R α 1/ I↓            ——— (4)

Which means, resistance and current are inversely proportional

R α 1/I;             ———-5

If we rotate the crank handle at a particular speed. This, in turn, leads to the production of voltage in this rotor, and the high value of current also flows in anti-clockwise, through the two coils A and B.

Where this flow of current leads to the generation of deflecting torque like Td in the circuit. Hence the pointer varies the resistance ranges from infinity to zero.

  Why Pointer is Initially at Infinity?

Due to the non-rotation of the crank handle, hence there I no rotation in the DC motor.

(E) Emf of rotor = 0,                              ——– ( 6)

Current I = 0                                             ——–  (7)

The two flux’s ϕA and ϕB = 0.              ——–  (8)

Deflecting torque Td = 0.                     ——–  (9)

Therefore the pointer is at rest (infinity).

We know that

            R α 1/ I ;                                          ——– (10)

Since I = 0, it means we get a high value of resistance which is infinity.

Practical Application Condition of AC and DC Motor

  • A DC motor consists of 4 terminals out of which 2 are rotor winding and the remaining 2 are stator winding. Out of which 2 rotor windings are connected to X terminal (+ve) and the remaining two are connected to Y terminal (-ve).If we move the crank handle, deflecting torque is produced which indicates a resistance value.
  • An AC motor consists of 6 terminals out of which 3 are rotor winding and the remaining 3 for stator winding. Out of which 3 rotor windings are connected to X terminal (+ve ) and the remaining two are connected to Y terminal (-ve). If we move the Crank handle, deflecting torque is produced which indicates a resistance value.

In both AC and DC motor

Case (i): If R = infinity, there is no interconnection between the winding, which is known as an open circuit.

Case (ii): If R = infinity, there is an interconnection between the winding, which is known as a short circuit. It is the most dangerous condition; hence we have to disconnect the supply.

Types of Meggers








  • Analog Display,
  • Hand Crank,
  •  Wire Terminals.

  • Digital Display,
  • Wire Leads,
  • Selection Switches,
  • Indicators.





  • No, External Power source is required to operate,
  • Low cost



  • Easy to handle,
  • Safe
  • Less time consumption.








  • Time consumption is high
  • Accuracy is not high
  • compared to Electronic type


  • The external Power source is required to operate,
  •  The initial cost is high.



Megger for Insulation Resistance Test / IR Test

Let us consider a wire, which contains conducting material at the center and insulating material surrounding it. Using this wire we test the insulation- resistance test with the help of megger.

Why Insulation Resistance Test to be Performed?

A wire contains conducting material at the center & insulating material at the surrounding of it. For instance, if the wire has the capacity of 6 Amps, there will be no damage if we provide 6 Amps of input current. In case if we provide input greater than 6 Amps then the wire will get damaged, and cannot be used further.


Units of Insulation = Mega Ohm’s

Measurement of the High Resistance Value

The device which is used for measuring is Megger. To measure the insulation of the wire, one end of the wire terminal is connected to a positive terminal and the end is connected to the ground terminal or megger. When the crank handle is rotated manually, which induces emf in the instrument where the pointer deflects indicating the resistance value.


Applications of Megger

  • The electrical resistance of insulator can also be measured
  • Electrical systems and components can be tested
  • Winding installation.
  • Testing of battery, relay, ground connection…etc


  • Permanent magnet DC generator
  • The resistance between the ranges zero to infinity can be measured.


  • There will be an error in reading value when the external resource has low battery,
  • Error due to sensitivity
  • Error due to a change in temperature.

Megger is an electrical instrument used to determine the range of resistances between zero to infinity. Initially, the pointer is at the infinite position, it gets deflected when an emf is generated from infinity to zero, which depends on Ohm’s law. There are two types of meggers, manual and electrical megger. The main concept of megger is to measure insulation resistance and machine windings. Here is a question, which condition leads to a dangerous situation in megger operation, and what is done to overcome, state it with an example?

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