Fusing/ Circuit Breaker Sizing

[crthompson]

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!!

 

[VEBill]

You should be able to assume that the 120VAC 60Hz outlet has a suitable breaker or fuse (typically 15A or 20A) installed upstream. You shouldn't have to worry about the fault current from a normal outlet exceeding 10kA.

Stand by for others' input.

 

[MiketheEngineer]

Have you considered GFI's??

 

[crthompson]

@VE1BLL I was worried about a fault of between 5ka to 10ka in which my breaker would try to clear before the other breaker cleared. I am afraid that if that were to happen, it could destroy my breaker.

@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!

 

[VEBill]

I think that if you calculate/estimate/measure the milliohms here and there from the source to your circuit breaker, you'll soon find that 5kA is physically impossible due to Ohms law (24 milli-ohms).

(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.)

 

[waross]

Determine the impedance of your connection cord or cable. That will probably put you below 5 kA.
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

 

[DRWeig]

I'm going to go along with Bill. A 10kA breaker is easily found.

Best to you,

Goober Dave

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[crthompson]

Thanks gentlemen! The cord I am using has a DC resistance of .5 Ohms. That is way more than I needed to limit the fault current. I figured everything would be ok but I wanted to be certain. Thanks again!

 

[djs]

Be aware that most GFCI recptacles are only rated for 2KA interrupting capacity. Hubble is not producing one that is rated 10KA.

 

[crthompson]

Thanks guys! I'm going to go with the 5Ka interrupting breaker.

I am an engineer, so the glass is twice as big as it needs to be.