high resistance short?

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wirenut2

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Columbus, OH
Was on a service call this week for nuisance tripping of a 15A breaker. I narrowed the down the problem to somewhere between a single switch and lighting outlet in the ceiling. When the switch was thrown there was a gradual climb in amperage up to 70A before the breaker tripped - all in all in about two to three seconds. The wiring method is AC cable - looking a little worse for the wear and the bldg is about 60 to 70 years old.

I was wondering about it taking that long for the breaker to trip and the gradual climb to 70A before the breaker tripped. The amperage on the return was around 25A, but it would be difficult to determine if there are shared neutrals, etc. I'm thinking a high resistance short to ground along the somewhat corroded cable? Has anyone else seen something similar?

Thanks,

Wirenut2
 
If the AC does not include a shorting wire a high resistance path to ground would not be a surprise.And again it could have been bad splices

where there wire nuts or tape and solder??

Was the armor getting hot??


Charlie
 
The trip curve thqat Im looking at shows that the breaker should trip instantaneously (magnetically) is less than 2 sec at 6-8X the breaker rating.
It's almost a toss up though whether the breaker is tripping on thermal of magnetic at that point. Again, if I'm to understand you correctly that the amperes increases to 70a within 2 seconds and 15a x 6=90a which falls in line with the TC of the breaker.
Regarding the length of time it takes the breaker to trip you had advise that the current increase steadly to 70a over a 2 sec time which indicates to me that the breaker didn't trip until the current reached the trip point. If the current was instantaneous to 70a then, yes, the breaker would have tripped instantaneously.
And "high resistance short to ground." 70a does not represent a high resistance short to me.
Dave
 
Most overcurrent devices (including fuses) will easily carry about 7 times their rating for at least 20 secs. This is just higher than the time vs. current requirement for a normal motor overload element.

Dave, which breaker curve are you looking at?
 
Cpal...There was some splices with solder/tape and some with wirenuts. As far as the cable that was accessible, there was no heat to speak of. Of course, it could be finding another path to ground other than the AC sheath.

Templdl...Yes, the increase to 70A is gradual over the 2 to 3 second time period, followed by the tripping.
 
I agree with Minute man, just replace the section of cable and be done with it. Either your customer is paying you for waisted time, or you are loosing money waisting time trying to figure it out. I would bet you would get your answer once you remove the bad section and replace it.
 
I agree with minuteman. In addition, to be thorough, make sure the problem is not the light fixture (load) itself, if you haven't already.
 
Are you using a digital meter, or an analog meter. If its a digital, there is a good chance its not really taking 2 sec. to get up to 70 A. It probably just takes the meter 2 sec to catch up with the sudden change in current.

Try an ohmmeter reading. If you are lucky, the short will show up on that. (You may have to turn off switches, or unscrew light bulbs.) But that will probably only work if the short is really constant, and doesn't really take 2 seconds to draw 70A.

I would also check the load before changeing the cable. Nothing worse than going to all the trouble to replace something, and then still have the same original problem.
 
My apologies for supplying mis-leading information about tripping times.

I plotted trip curves for several 15A 120V breakers from different manufacturers. At 70A the resultant trip times are:

Manufacturer A = maybe at .86sec definitely at 3.6sec
Manufacturer B = maybe at 1.8sec definitely at 4.9sec
Manufacturer C = maybe at 1.59sec definitely at 3.9sec

In all cases the 70A point is not in the magnetic (instantaneous) range of the breakers.
 
If you haven't tried disconnecting light fixture, try that and if it still trips, replace wiring. I have seen a lot of older light fixtures where people have put in larger lamps than fixture rating which causes over heating and problems with shorts and/or grounds.
 
Jim,
SC-4559-96 for the 1p BR series breaker, a copy of which I have as a PDF file.
When I "blow up" a copy of the curve at 470% the rating it looks like it the trip time could be as short as 1.8 sec to as long as about 6.5 sec on the high side of the calibration curve if I followed my line up accurately.
It may appear that it in the thermal part of the curve but it also may be splitting hairs as it's so close to the instantaneous that one could also consider if the meter is capable of measuring the actual peak current at the time of trip as I believe one of the other posters related to.
All I know is that turning 70a loose of a couple of seconds is still one heck of a lot of heating watts.
Dave
 
The problem lies somewhere along the section of AC that ran from the switch to the lighting outlet. Instead of running new cable the property owner requested that I just pull the switch and light and blank the outlet boxes.

The AC from the switch to the outlet is run behind a lathe and plaster interior, and there is no easy access above the ceiling to run new cable - i.e., finished room above. So, since the ceiling fixture was rarely used, they opted to save money (surprise, surprise :rolleyes: ) and not run new cable at this time.
 
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