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Contact resistance test producing CT secondary current

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    Contact resistance test producing CT secondary current

    I have a utility customer that experienced a differential trip on an extremely sensitive scheme from a contractor performing a Contact Resistance test on an out of service, but un-isolated breaker.

    The theory is pretty simple. The test set is plugged into 120vac, uses either a half or full wave rectifier to convert to DC. That rectification isn't perfect, especially the half wave. It allows enough AC ripple, or trash, that a small current is induced in the CT.

    My problem is that he needs a peer reviewed article to submit to management. Is anyone aware of a white paper or technical manual that states this? My next step is to hook a spare CT up to a relay for a waveform capture while running Ductor leads through it.
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    How did they test the breaker? I am assuming it was connected in normal position with line bus de-energized and contact-resistance was measured from line bus to load bus.

    As far as finding a peer reviewed article is concerned, I don't know of any but I can ask some relay guys if they have ever seen anything like what your describing. You might be able to contact the manufacturer of the test set for an answer. Your experiment sounds interesting and I would love to see the result.
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    So after talking with some of the guys at my shop, everyone seems to agree that your theory is plausible but more information is needed about the scheme to know what really happened. It's also possible that there is a wiring problem somewhere which caused the breaker to trip.

    Can you provide a sketch of the differential protection scheme along with relay settings and the location of the DLRO test leads? This will help to better understand the various zones of protection.
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    Differential Trip

    Quote Originally Posted by alexlounsbury View Post
    I have a utility customer that experienced a differential trip on an extremely sensitive scheme from a contractor performing a Contact Resistance test on an out of service, but un-isolated breaker.

    The theory is pretty simple. The test set is plugged into 120vac, uses either a half or full wave rectifier to convert to DC. That rectification isn't perfect, especially the half wave. It allows enough AC ripple, or trash, that a small current is induced in the CT.

    My problem is that he needs a peer reviewed article to submit to management. Is anyone aware of a white paper or technical manual that states this? My next step is to hook a spare CT up to a relay for a waveform capture while running Ductor leads through it.
    Review your SLD, check the Zone of protection what equipment are associated with the differential scheme.
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