electrical question (wood shop)

Finished wiring the wood-shop area. All in conduit, 12 quadplex wall outlets, and four fluorescent lights on three way switches. Also a 220V feed to the air compressor. New sub-panel in wood shop is fed by an other sub-panel 3 feet away on the other side of a wall. Feeder panel has a 50amp breaker feeding the wood shop sub panel. All wiring within conduit is #12 except the 220 which is #10.
Question: As I installed each of the quad (gang of four) outlets, I would go to the breaker, turn it on and check the outlet with a clamp lamp and then turn off the breaker for further work. When I was installing the outlet group # 11, the wires shifted withing the conduit and the hot wire in # 12 touched the metal box. (Yes, I didn't cap the wires while I was working, I simply bent them out of each other's way. Stupid) Anyway, the question is "why did the 50 amp breaker feeding my sup panel break instead of the 20 amp breaker withing the sub panel"?
Later, after wrestling my large air compressor into it's closet the same thing happened. I dislodged the safety shield on the belt, the fan blade could not turn and ergo the motor drew too many amps on start-up. Again, instead of the 30 amp circuit feeding the compressor breaking, it's the 50 amp feeding the panel that broke.
Note: 50 amp breaker is used. 30 amp breaker is used. 20 amp breaker is new.
Is the 50 amp breaker simply too sensitive?
I could live with the above, except when the 50 amp breaker trips I lose everything and am left in the dark. Have to get a flashlight, go outside one building and into another to access the other side of the wall.
All comments and advise appreciated.
Ivan Vegvary
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[...]

[...]
Well, it's *more* sensitive, anyway. That doesn't mean, necessarily, that it's *too* sensitive (i.e. more sensitive than it's supposed to be). It could be that way by design.
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On 8/31/2012 11:21 AM, Doug Miller wrote:

Although I don't have Doug's electrical expertise, I was thinking that the surge was just that, a surge. It's wasn't like the turning the flow from a water faucet from low to high. It was more like turning the flow from low to "Niagara Falls"!
CB'ers supposedly handle motor start-up better than fuses(don't they?), clearly due to a delay. I think it's built into the mechanics of the CB'er. Maybe this delay is a measurable feature of the 2 CB'ers involved?
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On 8/31/2012 12:06 PM, Mike Marlow wrote: ...

Circuit breakers have time response curves, too; there are ones specifically for motor applications as well just as "slo-blo" fuses.
It's possible one of OPs is but it'll be knowable by looking at the actual manufacturer's part no's...
As for tripping a 50A breaker ahead of a 20A when the short is on that 20A circuit--that ain't the way it should operate, no.
I'd suspect the 50A is the culprit here--check that it's not warm to touch when operating normally; they will fail w/ age and that's often a clue.
--
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It's possible that the current drawn would exceed the current ratings of both circuit breakers, and in that case there would be a race condition to see which one trips first. The used 50A breaker might move easier, so it would tend to win that race.
Replacing the 50A breaker might let the 20A breaker "win the race" more often. This doesn't eliminate the race condition, so the main might go first on a dead short every now and again.
Puckdropper
--
Make it to fit, don't make it fit.

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Somebody wrote:

----------------------------------- All bets are off.
A thermal-magnetic, molded case, plug-in cbk'r is a single use device.
They are a single use device that provides inrush (magnetic) or inverse time delay (thermal) protection.
These cbk'rs only have to clear a fault ONCE to meet design spec.
There is no such thing as a USED cbk'r that you can trust.
Lew
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Not necessarily, Mike -- it depends on the speed with which the breaker reacts to the overcurrent. If the overcurrent is, say, 40A, of course the 20A breaker should trip first, because the 50A breaker should not trip at all. But a dead short may result in a brief current of well over 50A, in which case the breaker that trips will be the one that reacts fastest.
In this case, with a used 50A breaker and a new 20A breaker, one would expect the 50A breaker to trip a little faster when subjected to a current of, say, 100A.
Even if they're both brand-new, and even if both have the same *design* trip speed, they still won't respond at *exactly* the same speed, simply due to normal statistical variability in the manufacturing process. Suppose the breakers are designed to trip within 10ms +/- 0.5% when an overcurrent occurs -- if the actual trip delay is 9.95ms for the 50A breaker, and 10.05ms for the 20A, they're both within design specifications, but on a 100A overcurrent the 50A breaker will go first every time.

No, it should not. It should result in the fastest-reacting breaker opening first. The reaction speed of a breaker is *not* dependent on its amperage rating.

I think you owe Puckdropper an apology for that. He's more nearly right than you are.
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On 9/1/2012 7:47 PM, Mike Marlow wrote: ...

Well, only if the two responses are nominally identical...which wouldn't be necessarily so at all.
--
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How much load on the upstream breaker?
Let's say you have a load center with a 50 amp main breaker and four 20 amp circuits.
Now, let's also say that three circuits have 15 amp loads on them already, and you put a 10 amp load on the fourth. What happens? You trip the 50 amp breaker without tripping any of the 20s.
Not saying that's what happened in this case--not enough information was given. But it's something to think about.
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On 8/31/2012 10:21 AM, Doug Miller wrote:

Any consideration given to the 20AMP being defective?
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sensitive (i.e.

While certainly possible, I'd consider it to be pretty unlikely -- the OP stated that the 20A breaker is new, and the 50A is used.
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On 9/2/2012 6:34 PM, Doug Miller wrote:

I agree that it is unlikely, but we BOTH agree that it is possible. Quality Control isn't always controlling<g>
Logically, it's worth looking at and swapping out the breaker to make certain that is NOT the case, especially when a dead short on that circuit won't trip the 20A breaker but will flip the 50A.
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On Thu, 30 Aug 2012 21:04:39 -0700 (PDT), Ivan Vegvary
and four fluorescent lights on three way switches. Also a 220V feed to the air compressor. New sub-panel in wood shop is fed by an other sub-panel 3 feet away on the other side of a wall. Feeder panel has a 50amp breaker feeding the wood shop sub panel. All wiring within conduit is #12 except the 220 which is #10.

to the breaker, turn it on and check the outlet with a clamp lamp and then turn off the breaker for further work. When I was installing the outlet group # 11, the wires shifted withing the conduit and the hot wire in # 12 touched the metal box. (Yes, I didn't cap the wires while I was working, I simply bent them out of each other's way. Stupid) Anyway, the question is "why did the 50 amp breaker feeding my sup panel break instead of the 20 amp breaker withing the sub panel"?

happened. I dislodged the safety shield on the belt, the fan blade could not turn and ergo the motor drew too many amps on start-up. Again, instead of the 30 amp circuit feeding the compressor breaking, it's the 50 amp feeding the panel that broke.

everything and am left in the dark. Have to get a flashlight, go outside one building and into another to access the other side of the wall.

You don't say whether all the breakers are the same manufacturer. Actually had that happen in a computer center where a 125 amp breaker would trip before the 30 amp parking area breaker. Of course it always led to a 2AM call out so we found that problem quickly. Anyway you have two variables which is the short circuit trip time and the overload trip time. Possible this was the result of the dead short so you might not have to do anything. Possible a defective breaker, most likely the one tripping to fast. A possible problem of a poor ground path to the first breaker.
Mike M
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It happens that Mike M formulated :

Can you explain what part a ground plays in a simple 2 terminal breaker?
--
John G



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wrote:

Simple you can have a line to line short, or a short to ground. If it is a short to ground and you have a poor ground path the breaker may sense it more as an overload then a short and not trip or take longer to trip.
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On Mon, 03 Sep 2012 08:49:35 -0700, Mike M

This may be true in a GFI (ground fault interrupter) circuit breaker, but in 120/240v standard breakers, there IS no ground lead.
-- The most powerful factors in the world are clear ideas in the minds of energetic men of good will. -- J. Arthur Thomson
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On Mon, 03 Sep 2012 09:27:50 -0700, Larry Jaques

I don't know what code book your using but you'll never pass a NEC inspection if the entire system isn't grounded. Your thinking of the nuetral or identified conducter.
Mike M
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Problem solved! Thanks for all the help. Scrounged around for another breaker and found a 40 amp double (240 volt). Replaced the suspect 50 with the 40 and all is O.K. If I force a short in the 20 amp circuit the 20 amp now breaks and the upstream 40 amp holds. Obviously the 50 amp breaker was bad. (proved it by putting it back in and forcing a short) BTW, I force a short by plugging in a shorted power cord. (Doesn't everybody in this group cut off the power cord on a tool prior to throwing it away?)
Thanks again, you guys are wonderful!
Ivan Vegvary
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On Fri, 31 Aug 2012 23:21:50 -0700 (PDT), Ivan Vegvary

Replaced the suspect 50 with the 40 and all is O.K. If I force a short in the 20 amp circuit the 20 amp now breaks and the upstream 40 amp holds. Obviously the 50 amp breaker was bad. (proved it by putting it back in and forcing a short)

Jeeze, Ivan. We usually remove the plug from the outlet before cutting the cord. "Forcing a short" indeed! ;)
-- I have the consolation of having added nothing to my private fortune during my public service, and of retiring with hands clean as they are empty. -- Thomas Jefferson, letter to Count Diodati, 1807
Too bad -none- of the current CONgresscritters are willing to do that. -LJ
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