Page 133 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

For insulation resistance test

Between windings: As shown in Fig 4, one end of the Megger lead is connected to any one terminal of the RYB
and the other is connected to F1 or F2 of the field winding. If the Megger reads one megohm or more, then the
insulation resistance is accepted as okay.

If there is short, between the armature and field windings, the Megger reads zero ohms. If it is weak, it shows
less than one megohm.
Testing insulation resistance between body and windings: As shown in Fig 5, one lead of the Megger is
connected to one of the leads of the RYB, and the other lead of the Megger is connected to the body. If the
insulation between the windings and the frame is all right, the Megger reads more than one megohm.

The field is tested by connecting one terminal of the Megger to F1 or F2 of the field and the other terminal to the
body as shown in Fig 6. If the insulation between the field and the frame is all right, the Megger reads more than
one megohm. A lower reading than one megohm shows weak insulation and leakage to the ground.

Caution
While conducting the insulation resistance test, if the Megger reads zero, then it should be concluded that the
insulation of the winding has failed completely and needs thorough checking.
The permissible insulation resistance should not be less than 1 megohm.

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CITS : Power - Electrician & Wireman - Lesson 76-85 CITS : Power - Electrician & Wireman - Lesson 76-85