Fusing/ Circuit Breaker Sizing
Fusing/ Circuit Breaker Sizing
(OP)
Hello gentlemen, I have what "should" be a relatively simple question that I need answered. I have been tasked with designing five test boxes that will be used in the field to verify the proper operation of a component we produce. I already have the circuit designed but I ran into a problem when deciding on how to protect the box and its output. This box will be fed from several different power sources(120VAC 60Hz), in various states, in various facilities, and with varying short-circuit currents. How can I be sure my breaker will clear a short-circuit if I don't know what the available fault current is? How do appliances and such get around this with their fusing? The breaker I originally sized was more than capable of protecting the internal wiring from an overload but it can only interrupt 5ka of fault current... If I recall, most of the single phase CBs I have seen have an interrupting rating of 10ka. My concern is that if my test box has an internal short circuit, that the breaker I had in mind will try to clear the fault and won't be able to. I don't want anyone to get hurt. Is there an industry standard(US) somewhere that deals with this topic? Any help will be greatly appreciated!!





RE: Fusing/ Circuit Breaker Sizing
Stand by for others' input.
RE: Fusing/ Circuit Breaker Sizing
RE: Fusing/ Circuit Breaker Sizing
@MikeTheEngineer I hadn't considered GFIs... That is a good idea. The only concern I would have about GFIs, is that I've heard you can't feed a GFI with a GFI(I don't know how true it is)...
I had one engineer I work with recommend using a 1:1 isolation XFMR. That would work but I would really like to stay away from adding more components if at all possible.
Thanks for the responses guys!
RE: Fusing/ Circuit Breaker Sizing
(Assuming the voltage is remaining at 120 VAC. If the lines get crossed and the applied voltage suddenly rises to 2500 volts then all bets are off.)
RE: Fusing/ Circuit Breaker Sizing
Use a 10 kA rated breaker. There are lots of them available.
GFIs are always good (Well almost always) but they only work if the fault is to ground. They will do nothing for you if a component fails in such a way as to produce a line to neutral fault. And a GFI should be rated to interrupt the the available fault current.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Fusing/ Circuit Breaker Sizing
Best to you,
Goober Dave
Haven't see the forum policies? Do so now: Forum Policies
RE: Fusing/ Circuit Breaker Sizing
RE: Fusing/ Circuit Breaker Sizing
RE: Fusing/ Circuit Breaker Sizing
I am an engineer, so the glass is twice as big as it needs to be.