What is Megger & Its Operation?

In this tutorial we are going to learn about what is Megger & Its Operation?

Megger : –

Resistance of the order of 0.1 Mῼ and upwards are classified as high resistances these high resistances are measured by portable , instrument known as Megger it is also used for testing the insulation resistance of cables .

Principle of Operation

It is based on the principle of electromagnetic which  shows in the image ,  the consumption of Megger. When a current carrying conductor is placed in a uniform magnetic field it experiences n mechanical force whose magnitude depends upon the strength of current and magnetic field while its direction depends on the direction of current and magnetic field.

Construction

Megger construction , It is a permanent magnet that give  the field for both the generator  and ohmmeter the moving element of the ohmmeter consist of three coil pressure or control coil and compensating coil these coil are mounted on central shaft which are free to  rotate over a stationary c-shaped iron core the coils are connected to the circuit through flexible leads called ligaments which do not produce a restoring torque on the moving element consequently the moving element takes up any position over the scale when the generator handle is stationary. The current coil is connected in series with resistance in series with resistance R1 between one generator terminal and the test terminal T2 The series resistance RA± protects the current coil in the event of the terminals getting short circuited and also controls the range of the instrument the range of the instrument the pressure coil in series with a compensating coil and protection resistance R2 Is connected across the generator terminals the compensating coil is included in the circuit to ensure better scale proportions the scale is calibrated reversely means the normal position of pointer indicates infinity when full scale deflection indicates zero resistance

Working

When the current flows from the generator through the pressure coil the coil tends to sell at the field of the permanent magnet when the test terminals are open corresponding to infinite resistance no current flows through deflection coil thus the pressures coil governs the motion of the moving element making it move to its extreme anticlockwise position the pointer comes to rest at the infinity end of the scale when the test terminals are short circuited i.e. corresponding to zero resistance the current from the generator flowing through the current coil A s large  enough to produce overcome the pressure coil due to this pointer moves over a scale showing zero resistance to be tested is connected between terminals T1 and T2 the opposing torques of the coils balance each other so that pointer attains a stationary position at some o=intermediate point at some intermediate point on scale the scale is calibrated in mega ohms so that the resistance is directly indicated y pointer .

The guard ring is provided to eliminate the error due to leakage current the supply to the meter is usually given by hand driven permanent magnet D.C.

Applications

It is used to determine whether there is sufficiently high resistance between the conducting part of a circuit and the ground this resistance is called insulation resistance.  The Megger can also be used to test continuity between any two points when connected to the two points when connected to the two points if pointer shows full deflection then there is an electrical continuity between them.

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