High resistance measurement using Megger

What is meggar instrument?

Megger is used to measure very high resistances, such as those found in cable insulation, between motor windings, in transformer windings.

The Megger is essentially a portable ohmmeter with a built-in high voltage source. Meggers may read resistances of several hundred or even thousands of megaohms. They have the advantage, as compared to an ordinary ohmmeter, of applying a high voltage to the circuit under test, and this voltage causes a current if any electrical leakage exists.

Resistance measurement using meggar:

The Megger, has two main elements, a magnet-type de generator to supply current for making measurements, and an ohmmeter which measures the resistance value. As shown:

The generator armature is turned by a hand crank usually through step-up gears, to produce an output voltage of 500 V. When the crank is turned, the gears turn the generator at high speed to generate a high output voltage.

Coil A is the CUITentcoil with one terminal connected to the negative output and the other connected in series with R, to the test lead P2

Coil B is the voltage coil and is connected to the generator output through the resistance R 2.

When an extremely high resistance appears across the terminals, such as in an open circuit, the pointer reads infinity. On the other hand, when a resistance of relatively low value appears across the test points, such as when the cable
insulation is wet, the current through the series winding causes the pointer to move towards zero (resistance short-circuited).

However, the pointer stops at a point on the scale determined by the current through the series resistor, which in turn is governed by the value of the resistance being measured.

When an unknown resistance R; is connected across the test leads, the current flow in coil A. The corresponding torque developed moves the pointer away from the infinity position, into a field of gradually increasing strength, until the torque fields between coils A and B are equal.

Variations in the speed of the hand-cranked generator do not affect the Megaohmmeter readings since charges in generator voltage affect both coils in the same manner.

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