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Author Topic: How a circuit breaker works  (Read 1082 times)

Jonathan Johnson

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How a circuit breaker works
« on: February 06, 2019, 04:40:37 pm »

How a circuit breaker works, in slo-mo:

https://www.youtube.com/watch?v=wGFnooeA6Iw

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Corey Scogin

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Re: How a circuit breaker works
« Reply #1 on: February 06, 2019, 05:56:08 pm »

I didn't realize that common breakers combined both thermal and magnetic functions. I knew that magnetic-only breakers existed for specific uses but I thought all common residential/commercial ones were thermal-only.
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Keith Broughton

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Re: How a circuit breaker works
« Reply #2 on: February 07, 2019, 11:49:04 am »

Great vid!
Thanks for posting.
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Jonathan Johnson

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Re: How a circuit breaker works
« Reply #3 on: February 07, 2019, 12:59:43 pm »

I didn't realize that common breakers combined both thermal and magnetic functions. I knew that magnetic-only breakers existed for specific uses but I thought all common residential/commercial ones were thermal-only.

What it shows on the video is a Square-D type QO plug-on breaker. CORRECTION: The panels shown are Square-D type QO, but the breaker demonstrated is an Eaton/Cutler Hammer type BR. Square-D QO is commonly found in commercial installations, and only somewhat common in residential in my area (Vancouver, Washington). The largest electrician shop around here uses Square D QO; the smaller shops seem to use Eaton (Cutler Hammer type BR), Siemens, or GE. A few of the smaller shops might use Square D.

Some other residential breakers might not have magnetic trip -- but I haven't researched that. I know the old Zinsco Fire Starters had magnetic trip.
« Last Edit: February 11, 2019, 01:05:27 am by Jonathan Johnson »
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John Roberts {JR}

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Re: How a circuit breaker works
« Reply #4 on: February 09, 2019, 01:41:18 pm »

I didn't realize that common breakers combined both thermal and magnetic functions. I knew that magnetic-only breakers existed for specific uses but I thought all common residential/commercial ones were thermal-only.
IIRC magnetic breakers were more common in EU... This was an issue with big SR audio amps at peavey last century where amp inrush current could trip magnetic breakers (in europe) that (US) thermal breakers would tolerate.

====

OK I have a new question for our experienced breaker users (my house still has fuses). I have been enlisted to help an old neighbor's widow troubleshoot an electrical problem... 3 outlets on the same branch in her house are dead as a doornail, no response at all from NCVT. The fuse box has resettable breakers.

They are all set on and appear to have power per the NCVT but I suspect even a bad breaker might register energized that way, and/or suffer from a loose wire despite being energized.

I did not pull the breaker box cover to inspect wiring and advised her to get a real electrician to check for a bad breaker/loose wire.

The other alternative is squirrel activity in the attic, but no recent rodent activity reported.

My question, how common is it for old breakers to just fail? Do they have common physical symptoms when bad, besides no power coming out?

If more serious I should have opened up the box, but I already do them too many favors (I am already waiting on parts to repair their dining room ceiling lamp with broken socket/switch), and I have no personal experience with replacing circuit breakers. 

JR
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Dave Garoutte

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Re: How a circuit breaker works
« Reply #5 on: February 09, 2019, 06:07:01 pm »

You could just pop the cover plate on the breaker panel and check the switched side of the breaker with a volt meter with the breaker on.
If there is full voltage, the breaker is passing power, and the problem is elsewhere.
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John Roberts {JR}

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Re: How a circuit breaker works
« Reply #6 on: February 09, 2019, 07:16:14 pm »

You could just pop the cover plate on the breaker panel and check the switched side of the breaker with a volt meter with the breaker on.
If there is full voltage, the breaker is passing power, and the problem is elsewhere.
Thanks, that agrees exactly with what I suggested they have an actual electrician do...

not my job mon.

JR
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Stephen Swaffer

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Re: How a circuit breaker works
« Reply #7 on: February 09, 2019, 07:56:08 pm »

IIRC magnetic breakers were more common in EU... This was an issue with big SR audio amps at peavey last century where amp inrush current could trip magnetic breakers (in europe) that (US) thermal breakers would tolerate.

====


My question, how common is it for old breakers to just fail? Do they have common physical symptoms when bad, besides no power coming out?


JR

Common?  Not very often, but a few weeks ago I replaced a bad Square D QO breaker on a neighbors stove-after they replaced their "bad" stove.   :)  Absolutely no visible indication-just no power passed through.  Most often it happens on relatively heavily loaded circuits-as you would expect.
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Adam Kane

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Re: How a circuit breaker works
« Reply #8 on: February 10, 2019, 10:11:34 pm »

The video showed a bunch of shots of QO panels, but the breaker that he opened and filmed was a Cutler Hammer BR breaker... Not a QO.

I've seen the guts of most of the popular breakers on the market (this side of the pond anyway) and they all employ some variation of what was shown inside the BR.
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John Roberts {JR}

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Re: How a circuit breaker works
« Reply #9 on: February 23, 2019, 03:18:03 pm »

For the rest of the story about my neighbor's possible bad breaker, a real electrician showed up and determined it had tripped at some point and was not reset back on hard enough.

 it looked like it was on but wasn't. I didn't try exercising them all... Glad it was relatively painless for her.  Sad that I didn't catch it, but I have old school fuses in my panel.

JR 
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