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Old 10-26-2013, 09:54 PM   #1
Banjorear
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Default Breaker didn't trip when wire was cut. Trouble shooting tips...

Replacing the doors in my house. While cutting out the old one, I cut clean through the wire feeding the lights in the room. (Someone routed the feed through the headers on top notching the top corner of the rough opening. Lousy work for sure).

Anyway, curious why the breaker didn't trip as the blade cut through the wire.

Thoughts?
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Old 10-26-2013, 09:58 PM   #2
Charles (in GA)
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

Not by chance a wire that is switched somewhere upstream so it didn't have any power to it?

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Old 10-26-2013, 10:01 PM   #3
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

Quote:
Originally Posted by Charles (in GA) View Post
Not by chance a wire that is switched somewhere upstream so it didn't have any power to it?

Charles
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Old 10-26-2013, 10:05 PM   #4
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

It had power going to it for the lights were on at the time. It sparked, smoked, and arc on the blade of the sawsall.
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Old 10-26-2013, 10:15 PM   #5
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

What you cut through was no doubt just a switch leg. You just turned the light off the hard way! How old was the wiring? Sounds like there was no ground wire.

I am assuming you were using a double insulated saw, one that had only a two prong plug. At the time you cut through the wire the blade and metal casing of that saw were at line potential. If your body was grounded you would have gotten a good jolt, perhaps knocking you off the ladder. If it were a three prong saw, plugged into a grounded outlet, it would have tripped the breaker, and minimized shock danger.

Last edited by justsam; 10-26-2013 at 11:01 PM. Reason: Added shock potential
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Old 10-26-2013, 10:22 PM   #6
Charles (in GA)
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

If its is a switch leg, there is no neutral and thus no way to have a dead short to trip the breaker (esp if no ground wire is present). The saw blade became the switch momentarily as it went thru the wires.

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Old 10-26-2013, 11:17 PM   #7
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

Unless the OP has a Sh*tsco(Zinsco) or an FPE stab-u-in-the-back(stab-lok) panel, then Im also gonna say its a switch leg!!

The arcing and smoke could just be from cutting the wire under load and not a short at all! Have ever disconnected a neutral wire with a load connected through it?

Last edited by wyliesdiesels; 10-28-2013 at 01:36 AM.
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Old 10-27-2013, 03:08 AM   #8
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

Agreed, it could have been a switchleg with no ground wire in the romex. But I have also seen on numerous occasions where a breaker didn't trip after a circuit was shorted and not just the crappy FPE's or Zinsco's. If the short is "short" in duration a breaker will sometimes not trip. I'm betting this is the case, since the OP mentioned it sparked and smoked. The only question in my mind being is this indicitive of a faulty breaker? As cheap as breakers are, it might not be a bad idea to just swap out the one that should have tripped.
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Old 10-27-2013, 01:32 PM   #9
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

I've done this a few times: cut live wires. Sometimes the breaker trips, sometimes not. Couple of factors, but how quickly the blade (circular power saw) moved through the wire and where in the cycle the AC is can affect it.

I suppose if you are worried, you can replace the breaker, but I suspect it's OK.
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Old 10-27-2013, 01:47 PM   #10
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

I think you are correct in wondering if you seen smoke and sparks. There may be an issue, with an arc something should have happened?
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Old 10-27-2013, 01:53 PM   #11
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

My Experience is similar to nehog's, you can monetarily short a wire and not trip a breaker. But not for long... A moving saw blade may make and break the connection enough that the breaker never trips. More so if the blade is coated.

When I was epoxying my garage floor I stepped on the lead to the vacuum cleaner with my spiked shoes. Nice noise, a quick flash and that was it. The wire melted back from the arc before the breaker tripped. Seems likes it takes a bit to make a good connection when jabbing live wires.

I think the wires arced back faster than the spike could penetrate. The saw blade may have done the same thing.
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Old 10-27-2013, 01:56 PM   #12
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

A concern here is the possibility of a missing bond to a 3 wire fed panel. I have had them flash when making an arc and so small you could barely see a spark and knock off a 30A breaker This may be worth a little investigation.
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Old 10-27-2013, 10:49 PM   #13
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

No breaker can trip in under around 1.5 cycles. That's about 1/40th is a second. If the short is broken in less time than that the breaker will. It trip.
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Old 10-27-2013, 11:23 PM   #14
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

Time current curve

All circuit protection devises have a time/current curve (as linked above). Like rlitman said you probably did not create a short of enough time and magnitude to trigger the breaker. You may not have created a dead short at all and may have only broke the current carrying line to the load and only the current of the load created the spark, smoke, and burn on the blade.

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Old 10-28-2013, 01:02 AM   #15
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

The OP has never confirmed if there was a ground or neutral conductor in the Romex. There may have been no "short" at all.
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Old 10-29-2013, 06:50 AM   #16
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

Quote:
Originally Posted by justsam View Post
The OP has never confirmed if there was a ground or neutral conductor in the Romex. There may have been no "short" at all.
Sorry guys. I was away for a few days. It was the older Romex that has what appears to be that coated cloth-type covering. It did have a ground wire which was very thin compared to new Romex.

Thanks for all the replies.
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Old 10-29-2013, 12:27 PM   #17
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

I have encountered, in the past, situations that especially with only a brief arc, the resistance of the air gap and tool grounding is enough to limit current and keep from tripping a breaker or fuse.

I've seen many times in autos a break in the wire that's shorting through corrosion or otherwise doesn't make a good connection. Sure clean wire to a bare ground would blow the fuse but corroded wire to painted body makes it just enough current to be a pain in the butt. Cars don't run so well when the 5v reference voltage drops to 4v.
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Old 10-29-2013, 01:15 PM   #18
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Default Re: Breaker didn't trip when wire was cut. Trouble shooting tips...

Still not enough information to really know, but I have what sounds like the same wire in my house (cloth-covered romex), and the "with ground" is used randomly in the house along with "without ground".

I'd suspect a switch leg as others have said and you didn't create a dead-short to trip the breaker (load was still limited by the light fixture). I know all the bedrooms in my house are wired with the feed from the panel coming into the celing box and then branching from there for the light switch and all outlets in the room (think spider web).
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