Faults with CTs

[simisteven]

Hi Guys here is an unusual one I had never experienced before. We shut down a plant for 2 weeks and upon start up one 350 kW 6600V pump motor tripped on start up, showing short circuit on Micom P225 relay. We did all the usual checks and inspections, mechanical and electrical. Then the motor was replaced (an unnecessary step because motor was not at fault). The SC setting was increased but to no avail. Called out cable specialists who test cable but gave it clean bill of health. At that stage I was called in from holiday and found that fault appeared to be confined to phase C. On inquiry it appeared that ammeter had been removed from one set of 40/5 A metering Cts and the CTs had been been shorted.
The CT in question is a composite 200/5 protection CT 10 VA and a 40/5 A metering CT 10 VA.
Question: is it possible that leaving the 40/5 A CT leads open circuit could have resulted to damage to the 200/5 A protection CT ? I would imagine the two CTs are electrical separate but share a common core.

 

[scottf]

So were the 40:5A windings shorted or left open?

The 40:5A metering winding and the 200:5A protection winding would not be on a common core. However, a CT left open-circuit can generate a lot of heat and could damage other core/coil assemblies in the same unit.

 

[simisteven]

That sounds likely scenario, ,many thanks Scott