Why would anyone prohibit meggering of CT's
Why would anyone prohibit meggering of CT's
(OP)
We have CT's feeding electromechanical relay circuits.
Our ancient CT test procedure says in capital letters 'DO NOT PERFORM DIELECTRIC TESTS ON CURRENT TRANSFORMERS'
I'm sure that procedure is wrong. We have done 250vdc test on CT and associated circuits before (with neutral ground lifted).
I can't phathom where the requirement came from. I can think there are 2 lines of thought, but neither gets very far:
1 - maybe someone was thinking about the fact that certain types of higher current dc testing could cause residual magnetism, but megger is a very low current test (just insulation capacitance charging current). I can't imagine it would cause any problems in that respect (agreed?).
2 - some devices fed from ct's may be susceptible to damage from dc, but we have only electromechanical relays and in any case we should at least be able to isolate the ct and test it separately.
Can anyone think of other reasons why anyone would object to dc testing of CTs?
Our ancient CT test procedure says in capital letters 'DO NOT PERFORM DIELECTRIC TESTS ON CURRENT TRANSFORMERS'
I'm sure that procedure is wrong. We have done 250vdc test on CT and associated circuits before (with neutral ground lifted).
I can't phathom where the requirement came from. I can think there are 2 lines of thought, but neither gets very far:
1 - maybe someone was thinking about the fact that certain types of higher current dc testing could cause residual magnetism, but megger is a very low current test (just insulation capacitance charging current). I can't imagine it would cause any problems in that respect (agreed?).
2 - some devices fed from ct's may be susceptible to damage from dc, but we have only electromechanical relays and in any case we should at least be able to isolate the ct and test it separately.
Can anyone think of other reasons why anyone would object to dc testing of CTs?
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(2B)+(2B)' ?






RE: Why would anyone prohibit meggering of CT's
I guess there is not much question except I am curious if anyone has heard this type of warning before and what is the reason given. Before deleting a big warning in capital letters I'd like to be able to say I understood where it came from.
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(2B)+(2B)' ?
RE: Why would anyone prohibit meggering of CT's
RE: Why would anyone prohibit meggering of CT's
I did have one incident many years ago. This CT was a frame CT used to detect ground faults in a cable and the secondary was connected to a rectifier with a DC coil (needed for sensitivity) which activated a relay when leakage current was more than a set (milliamps) limit. A 500 V Megger had been used and that rectifier (1N914 type bridge, yes before 1N4148) had blown.
We tried to reconstruct what had happened and found that the insulation between card connector (a DIN 41612 64 pole connector) and the frame it was mounted in actually wasn't specified for more than 60 V. There was a long cable going from CT to card rack and we think that the flash over discharged the cable via the rectifier bridge, which killed the diodes.
But that is the only case I know about where meggering a CT circuit has caused any damage.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Why would anyone prohibit meggering of CT's
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(2B)+(2B)' ?
RE: Why would anyone prohibit meggering of CT's
We have talked about the hazards for meggering the main insulation of the transformer under vacuum. I guess it makes sense the same consideration should apply for ct secondary insulation which might be inside the transformer. That would perhaps also include hot spot CT's.
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(2B)+(2B)' ?
RE: Why would anyone prohibit meggering of CT's
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(2B)+(2B)' ?