I have a ground fault test that keeps tripping at half of the setting value. The system has one main circuit breaker, with one CT around the neutral. No trip test comes out fine (current in line side of CB, out of load side, into load side of neutral, through CT, and out of line side of CT). Trip test reversing though CT yields have of the setting value (about 217A vice 400A).
NETA Level III
Posted October 31, 2017
Pro Subscriber
What is the CT ratio and what is the relay set to? My first thought is polarity but this cant be the case if no trip came out fine. What was neutral to ground megger readings?
It would be easy to analyse if you could provide a simple sketch of what exactly the connection is..? and are you performing a test using secondary injection kit.?Originally Posted by joenavymctest
NETA Level III
Posted November 7, 2017
Pro Subscriber
Any verdict on this one? It's been about a week since OP. I agree a sketch would make it easier to analyze the situation.
You stated this is a ground fault test and it sounds like it is connected as a neutral test. Are these settings the same?
Junior Member
Posted March 4, 2020
Do you have multiple grounds? (1 in CT ckt, 1 in test set?)
Experienced Member
Posted March 6, 2020
Pro Subscriber
Sounds like the test went according to expected. With the test set set up on the line side of the breaker and Neutral CT and jumper on Load side of both the breaker and Neutral CT he experienced a No Trip on Ground Fault as expected.
Putting the test set on the line side of the breaker and load side of the Neutral CT, with a jumper from the load side of the breaker to the line side of the breaker will cause a trip of the breaker at half the value of the ground fault setting.
This is because you have now placed 2 CT's in series, thus doubling the measured current making the breaker think it sees double the amount of current. Where the No Trip current caused the CT's to cancel the current seen by the breaker making it think no ground current is present.
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