Power practice problem - camara

[ros]

4. A diffrential relay, designed to protect a zone of distribution system, uses current transformers connected to auxiliary CTs. the auxiliary ct must match both the phase shift and the turns ratio of the differential relay cts. if the values do not match, which of the followin is the least likely outcome?

options. trip could occur at a lower level than expected.

trip could occur at a higher level than expected

trip could occur under non-fault conditions.

the trip will occur only when the CTs are wired in parallel...

Ans given is : the trip will occur only when the CTs are wired in parallel.

Can any one provide help with this problem. thank you

 

[cableguy]

I think the key words are "least likely to occur". Those are the words you'd need to underline on the exam and think about.

Keep in mind that some of the questions may be worded strangely, and you need to be able to pick out exactly what they want. They'll give you a bunch of detailed information about "something" and then ask something basic.

If you mismatch the CT's, then, well, yes... a trip at a higher than expected current could occur. A trip at a lower than expected current could occur. Yes, a trip under non-fault conditions could occur. So to me, the "LEAST LIKEY" answer would be the trip occurring when they're wired in parallel (wiring CTs in parallel is like wiring 2 current sources in parallel - you just end up summing the currents, assuming they're properly phased).