Should i use megger or not ?

[panthripu]

One more silly question but i really need some one to make it clear.
I always use a megger to check the IR value of a insulation/insulator. and i am sure most of people are using megger to check the Insulation Resistance. Correct ?
If i check an insulator used in a AC system with megger and get very high value , then i should not be worried about the condition of the insulator.
Another view :-
As the megger uses DC voltage to check the insulation resistance , it does not give right condition of insulator.As we use this insulator on AC system ,and if the insulator is old enough , there could be leakage current due to capacitive reactance with respect to ground and can cause flash over.
Is it correct or not?

 

[SG-1]

A BIL test is also basically a DC test.

The critical issue would be to use the correct level of DC voltage & watch for trends over time. With a one time test for a single insulator it would be essential to know the minimum value that is acceptable.

You could purchase an AC unit. The main draw back is price & weight. Oil filled transformers are heavy. AC tests are distructive, where as DC is not - unless the insulator fails.

 

[zog]

One more silly question but i really need some one to make it clear.
I always use a megger to check the IR value of a insulation/insulator. and i am sure most of people are using megger to check the Insulation Resistance. Correct ?
If i check an insulator used in a AC system with megger and get very high value , then i should not be worried about the condition of the insulator.
Another view :-
As the megger uses DC voltage to check the insulation resistance , it does not give right condition of insulator.As we use this insulator on AC system ,and if the insulator is old enough , there could be leakage current due to capacitive reactance with respect to ground and can cause flash over.
Is it correct or not?

It depends on what rating insulation you are asking about. For LV a megger test will suffice, but for MV systems you should do an dielectric strength test. There are many types and the best choice again depends on what you are refering to. Transfomers, insulators, bushings, breakers, bus, motors, cables will all have different dielectric strength tests recommended. Please be more specific. Insulation/insulator leaves a lot of possibel things you could be asking about.

As far as the megger tests, there are 3 different currents you are reading with a megger, leakage, absorbtion, and capacitive charging. They decay at different rates and time intervals and ratios (PI and DAR) can be used to determine differnt properties of the insulation. This thread should help explain http://forums.mikeholt.com/showthread.php?t=122150&highlight=polarization

 

[panthripu]

capacitance

capacitance

What are the factor on which the ground capacitance of insulators ( for example mica insulators)depends.
If i say the ground capacitance has become more resulting decrease in capacitive reactance .Over a time , this phenomenon may occur and our regular meggering of the system will not give us the clear picture....and finally we find one day the insulator got flashed.

 

[zog]

What are the factor on which the ground capacitance of insulators ( for example mica insulators)depends.
If i say the ground capacitance has become more resulting decrease in capacitive reactance .Over a time , this phenomenon may occur and our regular meggering of the system will not give us the clear picture....and finally we find one day the insulator got flashed.

I don't understand this post, perhaps you can rephrase the question, if there is one.

It helps if you give us some information on the system you have, at this point I don't know if these are 600V or 600,000V insulators.

 

[76nemo]

A BIL test is also basically a DC test.

The critical issue would be to use the correct level of DC voltage & watch for trends over time. With a one time test for a single insulator it would be essential to know the minimum value that is acceptable.

You could purchase an AC unit. The main draw back is price & weight. Oil filled transformers are heavy. AC tests are distructive, where as DC is not - unless the insulator fails.

Not necessarily size and weight any longer. Price,...sure.

This is one of the testers we use at work. Not much bigger than a bench type DMM and weighs less than a twelve pack

http://www.hipot.com/products/4320.shtml

 

[zog]

Not necessarily size and weight any longer. Price,...sure.

This is one of the testers we use at work. Not much bigger than a bench type DMM and weighs less than a twelve pack

http://www.hipot.com/products/4320.shtml

That is cute.

What would you ever use a 6kV hipot for?

 

[76nemo]

That is cute.

What would you ever use a 6kV hipot for?

Were you expecting something the size of a thumper?:grin:


I work at a manufacturing plant with a wide variety of electronic assemblies, just about all of it gets hipotted per customer request before it leaves the door.


Zog, it's not the size that matters:grin: Hehehe