Megohmmeter: Function & Operation

The megohmmeter or megger is a portable deflection instrument widely used to check the insulation resistance of electrical cables and equipment. In other words, a megohmmeter can be described as a high resistance measurement instrument. The instrument has a constant high-voltage source, normally produced by a hand-cranked generator. The voltage may range anywhere between 100 V and 5000 V. The diagram below illustrates the circuit diagram of a megger:

Fig: Megohmmeter that uses a hand-cranked generator to produce a high-voltage source. The forces from the control and deflecting coils partially balance each other to give a pointer deflection proportional to the measured resistance.

In reference to the figure above, the pointer on a hand-cranked megger is deflected by a permanent magnet moving coil (PMMC) system with two coils. There is no mechanical controlling force; instead, the coils are connected to oppose each other. One coil is identified as a control coil and the other as a deflection coil.

Voltage is applied to the control coil via a standard resistor R₁, so that the controlling force is proportional to the generator voltage divided by R₁. The deflection coil is supplied via R_x, the resistance to be measured, and R₂, the internal deflection circuit resistance. The deflecting force is proportional to the generator voltage divided by R_x + R₂. Deflection is proportional to the difference between R₁ and R_x + R₂, and the instrument scale can be calibrated to directly indicate R_x.

When the megger is measuring an open circuit, no current flows in the deflecting coil. As a result, the force from the control coil causes the pointer to be deflected to one end of the scale. This end is marked infinity (∞). When measuring a short circuit, the pointer is deflected to the opposite end of the scale from infinity, and this end is marked 0 𝝮. When the pointer is stationary at the center of the scale, the deflecting and control forces are equal and R_x + R₂ = R₁ or R_x = R₁ – R₂. The scale is marked equal to (R₁ – R₂) 𝝮 at this point and is proportionately marked at other points. Range changing can be realized by switching to different values of R₂.

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John Mulindi

John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.