OCPD Size for Harmonic Filter

[padawan learner]

We have a 1400kVAr switched passive harmonic filter fed from a 2500A circuit breaker recently installed. The rated capacitor current for the unit is 1680A = (1000 X 1400kVAr/(1.73 X 480V)). According to the NEC Article 460 and the mfg. installation manual, conductor size to feed the unit should be a minimum of 135% X 1680A. The installation guide says a fused disconnect should be sized at least 165% of the rated capacitor current of 1680A, but says nothing about sizing a circuit breaker and the NEC says the OCPD should be as low as practicable. The wire was sized correctly for a 2500A breaker. The breaker keeps tripping after about 2 hours on what appears to be an overload. The highest current measured on the load side of the breaker was 1920A (notably about 15% higher than nameplate FLA) which is still less than 80% of the breaker rating. The electrician tried disabling 200kVAr (about 240A), but still the breaker tripped.

Question: Is a 2500A molded case, 80% rated CB with digital trip unit too small for 1400kVAr? Since the breaker also tripped with only 1200kVAr engaged, could the breaker have an internal problem??

Thanks for your help!!

 

[T.M.Haja Sahib]

Yes. The breaker may have an internal problem. Replacing the trip unit of the breaker by another one may solve the problem.

 

[steve66]

Are there settings on the breaker? Have you checked them? Does the breaker tell you what function caused the trip?