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  Low Insulation Resistance of Electrical Equipment

The causes of Low Insulation Resistance of Electrical Equipment and ways to rectify them

Low insulation is approximately always the result of absorption of moisture through the insulated windings, because most of the insulating materials used are hygroscopic (moisture absorbent).

To keep high insulation values, the following care should be taken.

a) Don't permit dust to collect on the motor windings. Dust and dirt absorb and keep moisture, leading to leakage of electricity that may at last result in a breakdown. Clean up the motor windings at regular intervals via blowing compressed air, and wipe the outer surface of windings clean and bright through a dry cloth.

b) Oil and grease are uniformly bad; because they are much harder to remove once they reach the windings and soak them, the result of worn-out bearings, over-oiling, leaky gaskets of oil level indicators, etc. Oil and grease not only create the equipment messy but are good places for dust to settle in. This must be prevented.

c) Guard large motors, rotary convertors, etc. against inclement weather while they are idle, through covering them over with a large tarpaulin and keeping the windings warm through connecting up some electric radiators or infra red lamps all round. Alteration over the working and stand-by sets regularly to preserve both in good condition.

d) Despite all precautions, occasionally motors do get submerged within water. Retrieve them as soon as probable and blow the wet surfaces with compressed air. Dry out through putting them in a hot chamber.

Dampness in windings can be separated by drying out the equipment carefully in a hot chamber or in an impregnating plant, the inside of that is kept at a temperature of 800C to 1000C. The heating should be performed for various hours and in the case of large equipment for one or two days, if needed until all the moisture has been driven out. This can be determined through recording the insulation resistance readings at regular intervals of one or two hours.

Table: Winding insulation resistance at different temperatures

 

Winding voltage

Megaohms resistance at

200C

300C

400

500C

600C

66 KV and above

1200

600

300

150

75

22 KV to 44 KV

1000

500

250

125

65

6.6 KV to 19KV

800

400

200

100

50

Below 6.6 KV

400

200

100

50

25

The insulation resistance should be calculated while motor is hot, when possible. The reason is that though a damp winding will provide a relatively higher reading if taken while cold and so such a reading is not reliable. The reading will be lower while taken 'hot'. Like already stated, the resistance of any insulation falls while the temperature increases that are directly reverse to that of any metallic conductor the resistance of that increases as the temperature increases. While recording insulation resistance of a winding, it is so very significant to note down the temperature of the winding through a thermometer. It is wanted to have a standard technique of recording the readings, say at 750C. All megger readings might after that convert to this common base, through multiplying or dividing them by the suitable conversion factor.

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