Megger Testing of Transformers: Earth Terminals & Insulation Check

[Ron_04]

hi, in megger test of transformers which terminals we have to Earth and why? How this test is used to check insulation?

 

[vk6kro]

Megger tests on transformers are to test the insulation of the transformer.

So you would measure between windings, and between windings and the metal core of the transformer.

Be aware that on large transformers these areas can have substantial capacitance and they can hold a possibly lethal charge for days after the test. So, it is important to discharge this capacitance by the use of a shorting bar.

 

[Ron_04]

Megger tests on transformers are to test the insulation of the transformer.

So you would measure between windings, and between windings and the metal core of the transformer.

Be aware that on large transformers these areas can have substantial capacitance and they can hold a possibly lethal charge for days after the test. So, it is important to discharge this capacitance by the use of a shorting bar.

thnx for the reply but i want to know the exact process to check the insulation and the theory behind it?...please help

 

[vk6kro]

A transformer should not conduct at all between windings or from any winding to ground.

So, depending on the type on Megger, you attach the Megger to the windings you want to test the insulation between and then push a button to produce a high voltage.

If the insulation is OK then the meter will indicate that there is no conduction or that there is infinite resistance between the windings.

You release the button and wait for the voltage to be dissipated or you may have to short out the leads to make the transformer safe.

Never touch both leads of a Megger at the same time, even if you believe it is safe. They can deliver a vicious shock.

I have used hand-held meggers and they are mostly simple to use. Really old ones used to have a handle which you had to wind to get a high voltage. More recent ones just have a DC-DC converter in them which generates the high voltage.

Mostly, you get a choice of voltage to test with. This voltage might be 500 V, 1000 V or 2000 V.
It is possible to do damage with this feature if you select too high a voltage when testing components that are not rated for such a high voltage, but for testing transformers you could test on 1000 volts safely.

 

[Ron_04]

A transformer should not conduct at all between windings or from any winding to ground.

So, depending on the type on Megger, you attach the Megger to the windings you want to test the insulation between and then push a button to produce a high voltage.

If the insulation is OK then the meter will indicate that there is no conduction or that there is infinite resistance between the windings.

You release the button and wait for the voltage to be dissipated or you may have to short out the leads to make the transformer safe.

Never touch both leads of a Megger at the same time, even if you believe it is safe. They can deliver a vicious shock.

I have used hand-held meggers and they are mostly simple to use. Really old ones used to have a handle which you had to wind to get a high voltage. More recent ones just have a DC-DC converter in them which generates the high voltage.

Mostly, you get a choice of voltage to test with. This voltage might be 500 V, 1000 V or 2000 V.
It is possible to do damage with this feature if you select too high a voltage when testing components that are not rated for such a high voltage, but for testing transformers you could test on 1000 volts safely.

thank u very much for ur xplanation...i have only one doubt tht is which part of the transformer has to be grounded while performin the test and y?

 

[vk6kro]

You may have misunderstood this.

The frame of most transformers is grounded in normal use, so testing the transformer windings for leakage to frame involves grounding one side of the Megger.

 

[Ron_04]

You may have misunderstood this.

The frame of most transformers is grounded in normal use, so testing the transformer windings for leakage to frame involves grounding one side of the Megger.

ok i got it thank u very much i truly appreciate ur help