Know the working of a megger

  • Megger is used for the insulation resistance.
  • The parts of megger are as shown in figure below.
  • The armature of the generator is reotated by the hand driven crank lever.
  • The clutch mechanism is designed to slip at a predetermined speed.
  • This facilitates the generator to maintaine a constant speed and hence the constant voltage while testing.
  • The two coils A and B constitute a moving coil voltameter and an ammeter.
  • Both are combined to form one instrument.
  • The "HOT" terminal of the equipment whose inulation has to be measured is connect to the testing terminal X.
  • The terminal Y is connected to the body of the equipment, which is generally grounded.
  • When the crank handle is rotated, a voltage is generated in the generator.
  • The generated voltage is applied across the voltage coil A through a resistance R1.
  • When the terminals X and Y are free initially, no current flows through coil B.
  • The torque produced by the coil A rotates the moving element to show infinity.
  • While testing, the terminals X and Y are connected across the terminal and body of the machine for measurement.
  • Now the current passes through the deflecting coil B.
  • The deflecting torque produced by coil B intracts with the torque of coil A and rotates the moving element to indicate the resistance valve.
  • Voltage generated by this instrument is around 500 volts.
  • Meggers are available to generate 1000 volts, 2500 volts and 5000 volts also.
  • High voltage meggers are either motor operated or power operated.

What is Insulation resistance? Why it's need to be checked?
As we relate safety of crew on board, safety of machinery and power efficiency the insulation resistance come ls under critical parameters.
Due to aging, mechanical and electrical stress, contamination, temperature, atmosphere, humidity etc,. The IR value of the electrical devices changes.
So, to avoid the fatal accident due to electrical shocks, the seafarer should check the parameters regularly.

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