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Difference between AC and DC megger

Difference between AC and DC megger


We need to test a new medium voltage vacuum breaker.
Manual says to us an AC megger.  We have a DC meggar.

Anyone know the reason for the difference of
AC vs DC meggering?


RE: Difference between AC and DC megger


All meggers output only DC.

What the manual means that it is a ac power operated megger (could be ac motor operated or eletronic) as against the hand-cranked one.

* I would go green if only I were not yellow *

RE: Difference between AC and DC megger

I think they are refering to an AC hipot, vs DC Hipot.  Generally to test the breaker you megger it, then to see if the vacuum is in the bottles the bottles are hipoted.  They recommend AC though DC can be used.  See the manual for voltage levels.  The vacuum bottles I have found bad were found with a 5kV megger, it looked ok at 2.5kV, but showed bad at 5kV.  Generally the levels they recommend are about 40kVDC across the bottles.  

RE: Difference between AC and DC megger

pwrengrds is right but I need to add the caution of X-rays being emmited during this test, the area should be roped pff and signs posted during the test, check the inst book for the safe distance but it is usually 4 ft.

I would say if you dont own a Hipot you have not done this type of testing before and are not qualified to do this test, you may want to consider a qualified testing agency, this test will cost you less than the hipot rental. You also need to do contact resistance (Do you have a DLRO?) and contact erosion inspection. A power factor test should also be done if it is new to establish baseline data.

RE: Difference between AC and DC megger

My experience of vacuum bottle testing in accordance with manufacturers recomendations indicates the following test voltages:-
11kv bottles tested at 20kvrms
33kv bottles tested at 58kv rms
both for 5-seconds
dc testing is no longer recomended but these values were used before the change:-
11kv bottles tested at 30kv
33kv bottles tested at 70kv

RE: Difference between AC and DC megger

The test voltages vary from manufacturer and the bottle design, not all 11kV bottles are the same.

C-H VCP-W is 27/40kV for up to 17.5 kV for example

RE: Difference between AC and DC megger

I have never worked with vacuum breakers, so please forgive a basic question.

Where do the X-ray's come from?

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RE: Difference between AC and DC megger

High DC voltages (over about 30kV) between 2 electrodes a very short distance apart produce x-rays, unsure exactly why.  I noticed in the dental office they use 60kVDC 7mAmps yesterday.  GE recommends 10 feet from the bottles.  DC can be used to verify that the bottles are good by withstanding the test voltage, but the manuals I have read do not allow DC to fail the bottles, only an AC test can fail the bottle.

RE: Difference between AC and DC megger

If DC will generate X rays when the electrons impinge on one electrode (contact) then AC will probably generate X rays at both contacts.
The X ray tube in early commercial X ray machines was basically a vacuum tube diode with the plate anode mounted at a 45 degree angle to direct the X rays. It was fed with about 100kV AC and acted as a rectifier as well as an X ray generator.

"Why not the best?"
Jimmy Carter

RE: Difference between AC and DC megger

Hi All,

Thanks for your responses.

Yes manual does shed some light as well, say to use an AC highpot.
Also warns about Xrays - so the last few comments cleared up my next question.

Last question - "A power factor test should also be done if it is new to establish baseline data. " from Zogzog.

What do you mean by this?  I was reviewing the NETA testing standard and there is no mention of this?  Please provide me some enlightenment.

Thanks again,

RE: Difference between AC and DC megger

What standard are you looking at?

2007 ATS and Pf are both Pf tests.

RE: Difference between AC and DC megger

Hi ZogZog,

Yes I missed it - here is what I found.

*9.    Perform power-factor or dissipation-factor tests on each pole with the breaker open and each phase with the breaker closed.
*10.    Perform power-factor or dissipation-factor tests on each bushing. Use hot collar procedures if bushings are not equipped with a power factor tap.

I looked up what Power-factor testing was.

(learned something new last 2 days)

Thanks lots.

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