Open Circuited CT Failure Mode 3

[MxGxK]

I need some help. I work for a large industrial plant and have seen a large motor trip that is being attributed to an Open Circuited CT. I would post pics but the company prohibits it.

The CT was located in the connection box and is not used for protection. It was connected to a current transducer and then to a DCS system.

The relay indicated a ground fault using a 3phase core balance CT. This, I do not question.

When the connection box was opened, we found a CT (purpose described above) laying in the bottom of the connection box. The phase cables were hanging and half of the lugs were burnt off. Copper droplets were found on the arrester below. The lugs were connected to the neutral bus bar (ungrounded) and located ~6" from the CT. The micarta (sp?) board around the CT was burnt very bad, more damage on the lug side. The side of the connection box was burnt between the lug and CT, not on the opposite side of the CT. Six inches of the insulation was burnt back from the lugs. The DCS loss of current coincided with the relay trip time.

Question: Did the lug get hot or did we have an open circuited CT?

There was no evidence of arcing or burns on a Weidmuller block were a loose wire (CT Sec) was said to have been found.

Question: How would the CT fail? Could it cause fire at open circuit or does core get hot and melt?

Any thought would be appreaciated.

 

[scottf]

Oh, I didn't know it was an ABB CT. I change my mind...it probably was the CT!!