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I was installing a light and tripped the breaker (in retrospect, should have turned the breaker off before starting). When I finished installing it I reset the breaker and tried the light switch. The light still had a short, however, instead of the same breaker tripping it ended up tripping the main breaker.

I disconnected the light, turned off all breakers, reset the main breaker, and turned the local breakers back on. Everything is now humming along perfectly. My current belief is that the local breaker must have been busted by the first short and failed to catch the second short.

Questions:

  • Does this diagnosis seem correct (broken local breaker)?
  • Does this mean I should have the main breaker replaced, could it be worn out now?
  • Is there any way to test breakers other than making a short?
  • Is it a good idea to replace breakers when buying a house (not a new one of course)?
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1  
as a side note, most code says one wire per breaker screw if you decide to replace - don't double tap with another circut. –  Mark Schultheiss Jun 1 '11 at 14:37
    
One thing to keep in mind is that if two breakers protect the same circuit, either could trip in case of a short, regardless of amp rating. When the current load heads towards infinity, it's overcurrent for any breaker. –  Jay Bazuzi Jan 16 '12 at 18:24
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4 Answers

up vote 11 down vote accepted

Breakers are supposed to be good for a fairly large number of triggers, and they are supposed to fail open when they fail. Is there a chance that you were running at a heavier overall load when the second incident occurred? I'm thinking that if you were say within 10 amps of max on the main breaker just due to normal load (air conditioners, perhaps?), and caused a short on the secondary (a 15 amp circuit, perhaps), you might have triggered the main to trip first by simply overloading it, before the secondary could draw enough to trip.

If you really think the secondary might be bad, then by all means replace it, but no, you don't have to replace the main just because it tripped. As long as it feels normal when you move it back into position (flip some other breakers to see how they feel - there shouldn't be any slop in the movement), you'll be fine.

The last several times I've seen breakers fail they've all failed open.

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The only ones I've heard of that have failed closed, is FPE Stab Lok –  Brad Gilbert Jun 25 '11 at 5:16
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@Brad Gilbert - ouch, talk about massive design failure! –  Michael Kohne Jun 25 '11 at 21:58
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That's 4 questions. You should split it up.

1,2) Michael Kohne has answered these.

3) You could plug in a kettle and hairdryer into the same circuit, but it seems dangerous -- particularly if the breaker is iffy.

4) It's not a bad idea, but it's not necessary. A breaker costs about 7-8 bucks, so if you're doubtful, go ahead, it won't hurt. You can do it yourself, just turn off the main breaker first. It is however, an excellent idea to have your electrical system inspected by a qualified, licensed, and insured electrician when buying a house. It won't cost that much. A good electrician will also tell you what you can do yourself (receptacles etc.) and what you should have done professionally.

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1. Not necessarily.

2. No

3. Breakers have two different trip modes. One is the "thermal trip" where it accumulates heat due to being in an overload state (somewhat higher than the rating) and will eventually trip. The time depends on the load. A 30 amp load on a 15 amp breaker should be within a minute. A 20 amp load might give you 10 minutes. A 16 amp load may well be within the range of not ever tripping. The other mode is "magnetic trip". This is designed to be an instant trip if the current exceeds some high amount. This should be high enough that a motor start should not trip it and that can be 6 times the current or more. I'd expect a 15 amp breaker to have a 90 to 150 amp magnetic trip point.

An intentional overload would only trip the thermal half. A short trips the magnetic half. You should not intentionally do the latter due to the arc hazard.

4. That would be dictated by the age of the breakers. You should hire an electrician to inspect the breaker panel. The breakers can be removed and their attachment contacts inspected to look for arc trails and heat damage due to bad contacts.

Also:

The springs for breakers should be checked. An experienced electrician can tell if the springs are weak when she switches them on as they won't feel as stiff as they should. I recommend all breakers, including the main breaker, be manually switched off and back on every 2 to 3 years. Do this often enough and you could likely tell if a breaker feels wrong. I do this myself about once a year, usually when the power goes out so I'm not powering off stuff more often than needed.

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Don't even guess if it's worn out. Just replace it. They are so cheap. Don't take a chance with your safety. Have the circuit tested by a professional.

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