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Suppose I want to reduce risk of a circuit breaker malfunctioning and failing to disconnect a shorted circuit.

What if I install two identical breakers sequentially (one after another) so that if one of them fails the other one steps in and disconnects the circuit? Is there any problem with such installation?

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As I recall, circuit breakers aren't there to disconnect shorts. They're to disconnect based on excessive load. As it happens, when there is a short, it will frequently cause excessive load, but that's incidental. –  The Evil Greebo Oct 7 '13 at 12:37
    
@TheEvilGreebo Yes, but isn't a short almost always the source of the excessive load? An appliance rarely decides to reduce its internal resistance unless circuits cross. –  bib Oct 7 '13 at 13:20
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Often? Yes. Almost always? Hardly. Circuit overloading is all too common. –  The Evil Greebo Oct 7 '13 at 13:27
    
@TheEvilGreebo Most modern circuit breakers offer both overcurrent and short-circuit protection (as well as ground- and arc-fault protection). You'll see two ratings on the breaker, the normal current rating on the handle that everybody pays attention to. This is the overcurrent protection rating, and it is usually a time-delayed mechanism. The other rating is the short-circuit rating, and is typically 10kA or so. During a short-circuit, current in the circuit will raise very fast. A time delay could be dangerous, so the short-circuit protection is usually "instantaneous". –  Tester101 Oct 7 '13 at 13:56
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One problem I see is that if the circuit ever loses power, you'll have to check in two places to figure out which breaker tripped. –  Tester101 Oct 7 '13 at 13:59

3 Answers 3

There is no problem here.

In fact, this is often how subpanels are configured; the main panel has a breaker that feeds the subpanel, and the subpanel has individual circuits protected with their own breakers. The subpanel will have an amp rating. The minimum of the subpanel rating and the ampacity rating of the wire feeding the subpanel will dictate the appropriate breaker for the main panel.

Appliances that require their own local disconnects (such as air conditioners, water heaters, etc.) frequently use a small enclosure with space for just one circuit breaker. This could be a good option for whatever application you are thinking of.

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As another example: Many workshop power strips have their own local circuit breaker, which is in series between the house's breaker (of course) and the load. These are often "fast-blow" breakers, designed to fire before the main breaker does and thus save you a trip to the basement. –  keshlam Aug 21 at 4:04

Two breakers in series would reduce the risk of one failing in a way that stays in a closed (conducting) position. But modern breakers today have an extreme low risk of this happening. If I were that paranoid, I'd put a breaker and a fuse in series. But I'm not.

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Logically, this makes some sense.

Practically, there is no breaker panel configured to let you do this. You cannot modify the panel, physically, without invalidating the U/L certification.

Actually the system availability goes down for mechanisms in series, because you multiply the individual available availabilities.

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Isn't that what he wants? A higher chance of shutdown (either or both would disable), just in case one breaker doesn't trip as fast as he would like. –  bib Oct 7 '13 at 13:18
    
What he wants is merely a single-circuit subpanel -- and there's nothing in the NEC or any UL listing that forbids such; in fact, single circuit breakers in dedicated enclosures are standard catalog items. –  ThreePhaseEel Mar 11 at 2:22

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