Page 143 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

The torque exerted on the meter movement is proportional to the value of current flowing through the current coil.
The current through the current coil, which is under the influence of the permanent magnet, develops a clockwise
torque. The flux produced by the voltage coils reacts with the main field flux, and the voltage coils develop a
counter-clockwise torque.
For a given armature speed, the current through the voltage coils is constant, and the strength of the current coil
varies inversely with the value of resistance being measured. As the voltage coils rotate counter-clockwise, they
move away from the iron core and produce less torque.
A point is reached for each value of resistance at which the torques of the current and voltage coils balance,
providing an accurate measurement of the resistance. Since the instrument does not have a controlling torque to
bring the pointer to zero, when the meter is not in use, the position of the pointer may be anywhere on the scale.
The speed at which the armature rotates does not affect the accuracy of the meter, because the current through
both the circuits changes to the same extent for a given change in voltage. However, it is recommended to rotate
the handle at the slip speed to obtain steady voltage.

Because megohmmeters are designed to measure very high values of resistance, they are frequently used for
insulation tests.
Connection for measurement
When conducting insulation resistance test between line and earth, the terminal ‘E’ of the insulation tester should
be connected to the earth conductor.
Precautions
• A megohmmeter should not be used on a live system.

• The handle of the megohmmeter should be rotated only in a clockwise direction or as specified..
• Rotate the handle at slip speed.
Uses of a megohmmeter
• Checking the insulation resistance

• Checking the continuity.
Specification of Megger :
Nowadays electronically operated, Meggers are available, called as push button type for general application and
for industrial application motorised megger are also available. Hence a megger is basically specified based on
the voltage generated by it .
Example: 250 V, 500V, 1KV, 2.5KV, 5KV.

Earth resistance tester

Objectives: At the end of this lesson you shall be able to:
• state the precautions to be followed while selecting a site for the earth electrode
• define earth resistance tester
• explain the principle constructon and working of an earth resistance tester
• explain the method of measuring the earth resistance
• state the IE rules pertaining to earthing.

Precautions to be followed while selecting the site for earth electrode: However, even the earth electrode,
either rod or plate type, implanted properly in the earth according to the specified recommendations is found to
have high resistance resulting in failure of safety. The earth electrode resistance could be kept at a reasonable
level.

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CITS : Power - Electrician & Wireman - Lesson 20-25