Circuit Breakers in Parallel

[jimmy2times]

we have shore supply panel on a ship that has three 400A MCCBs in parallel, to effectively make 1200A capability.

For marine code there is no definitive statement to prohibit this. I've also looked local code (Australia) wiring rules and they are silent on the matter also. It just doesn't look right to me.

What are the issues with this (if any)

 

[itsmoked]

During a fault, the breakers will trip sequentially leaving a single one to trip out on the entire fault. I could see where this might leave the last one trying to interrupt fault larger than it's rated for, depending on the network's short circuit capability. But wait and see what others say as this is only a guess.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Sharpie8]

Have another look at AS3000. Having parallel conductors protected by different protection devices that do not operate simultaneously (e.g. separate fuses or in your case parallel MCCBs) is not allowed.

 

[jimmy2times]

Sharpie8, could you send me the reference paragraph, I looked again searching "parallel", "simultaneous" and "separate fuse" and cannot find such reference. I am looking at AS/NZS 3000: 2007

 

[Zogzog]

Pretty common setup for shore power. Funny story, I was on a sub and we were on shore power in the middle of a Rx shutdown, some cub scout troop was on the pier for a tour or something and one kid pushed the trip button on one of the breakers, which overloaded the other 3 so they tripped shortly after, this greatly increased the time of our shutdown (Rx trip). Our engineer (Who hated kids) went out there and saw them looking guilty went on a screaming rampage, those poor kids.....

 

[jimmy2times]

Yeah same happen one breaker found tripped. Hot cable on other remaining two. Only found as a result of ships rounds.

My thoughts were if at high load there is a motor start there could be a case where the inrush or other transient takes out the most sensitive breaker of three leaving two in circuit. Then you are in overload, granted after time it should trip other two.

Was thinking there should at least be indication one has tripped.

With reference to marine class rules the best, clutching at straws, reference i can find is protection shall be complete and coordinated to effectively eliminate fault

Reading another thread online i saw US code states ""240.8 Fuses or Circuit Breakers in Parallel. Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel."

Any other thoughts much appreciated