ECN Forum Forums General Topics of the Trade General Discussion Area Breakers not tripping

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Skipr, re-read iwire's post. Especially this:


I think that's the best explanation of what happened, similar situations have happened to me. (An arcing fault I though would have taken out a breaker, but other problems or a long homerun prevented the trip.)


As others have given other possible explanations here, I will also join the bandwagon of disagreeing with your test method. First, It didn't prove or disprove a bad breaker. Second, various circuit factors could have prevented the tripping of the breaker (for instance, what size was the jumper you used? Anything smaller than the branch circuit conductors would just act as a heater element anyway.) Third, you could have easily started a fire elsewhere if there were any bad/questionable connections or as mentioned, conduit fittings if the EGC was involved.

Finally, it reminded me of the worst possible troubleshooting technique I saw while in the Air Force. One of the avionics techs couldn't figure out why a system kept popping the circuit breaker in the cockpit. His solution? HOLD the breaker in until something smoked. (Aircraft breakers are NOT trip-free.)

That genius caused over $170,000 worth of damage to the aircraft and required the replacement of a wiring harness from the cockpit back to the tail. And yes, he was discharged and had to pay back the damage. (It wasn't his first time using the "smoke test.")

So you learned that a teacher was dumb enough to get her jewlery into a hot outlet. The thing to have done was replace the outlet and move on.

Even a simple bad plug on a cord can arc enough to look really bad, and those who don't know electricity tend to get overly dramatic when describing arc faults.

Nothing personal, just my 25c. (Used to be 2c, inflation, you know.)

Edit to add smiley....

I'm not going to try to be overly technical - because this is just the way I see it. Others may clarify or obfiscate at will....

Time, in the grand sceme of things AC is very short, and exceadingly long... The amount of time it takes to do a while lot of arching, and blasting of metal away opens and closes the circuit really quickly. That buzzing is about the frequencey of it... Not un-like welding.... The smaller points of contact just get expelled away - like a fuse, and then reconnects.... What you were doing was blowing away the portions of wire faster than the CB could gather the energy to trip, which takes some time.... Upwards of a second or two, but blowing away metal in micro-seconds in flashes of white light.

Common HACR breakers will hold much more than thier rating for some time - each are different - but relitively simular. About 175 for a minute or two, but will hold hundreds - thousands for a shorter time. Since the GE example was illegible, try this one: http://www.eatonelectrical.com/unsecure/cms1/TC00302001E.PDF

Note that this breaker simular to what you are using, can hold at minimum 600 - 1200% (PERCENT) of its current rating for 1 second! ~2400 amps In that amount of time you could have opened and closed the circuit 60 times - every spark of the arching.... Each not enough time to trip the breaker. Collectively, they may heat the elemants of the breaker to the trip point, but over a longer time. Look at the graphs agian and notice that the breaker can hold many thousands of times it's rating for milliseconds. Which is what the breaker saw, a whole bunch of millisecond shorts at several thousand amps a piece. Or, whatever the transormer feeding the neighborhood could supply at that distance from it.

Now if you had bolted, or wirenuted two wires together as a direct short you would be looking at a shorter reaction time, but then again you might be going to the hospital for arch-flash burns from the breaker exploding after it superheated any gas and metal in the breaker to the flash point. Because even though you may have had AIC calcs done, they are still only calculations... If that particular breaker can take it is another story. So don't do that either.

"So you learned that a teacher was dumb enough to get her jewlery into a hot outlet. The thing to have done was replace the outlet and move on. " Mxslick - thats priceless.....

I'm gonna go out on a limb here, but if there was an issue of a breaker not tripping - an EXTREMELY DANGEROUS situation - I would rather find out by testing live with a dead short then simply replacing a breaker or testing for high impedence and "assuming" or "hoping" the problem is corrected. In a situation like this, you really need to test live to make sure the electrical system functions as it was intended to. I mean really if a circuit breaker doesn't clear the fault, then what's the point?

If you're called in for this type of problem and you don't test live and a fault occurs that results in the building burning down or someone getting killed, guess who's assets are on the line? YOURS!

Of course, there are far more "elegant" ways of creating a dead short, like using a higher amperage breaker to create the short. For example, if you're testing a 20A circuit, you could use a 30A or even a 40A breaker for the dead short. If the test breaker trips and the panel breaker doesn't, obviously you've got a SERIOUS issue that absolutely needs to be corrected.

And if you damage a branch circuit as a result of a dead short all the better! A bad splice finally giving out, a nick in Romex now staying open, all better outcomes in the hands of someone who knows what they're doing and what to look for then failing in catastrophic way, arcing, heat buildup, open neutrals, etc.

Simply replacing an outlet that was damaged as a result of a breaker not tripping, in my opinion, would be negligence in the worst way in my opinion. That breaker should've tripped, what's the sense of a breaker panel then?

Liability is a REAL issue here.

Joe

Joe, after you have shorted this circuit how do you know what insulation or conductor damage is now concealed inside the walls?

It is very easy to use heating elements to make an adjustable load bank that can be monitored as you increase the load to a set point.

Roger

Was his name Norm?

I worked with a guy once whose troubleshooting procedure was to put bigger fuse elements in a fuse, and see what smoked. And then he would know what was causing the fuse to blow.

Thank goodness they did away with those type fuses.

Well, that's what meggars are for. Wouldn't leave after that kind of stress testing without meg testing the circuit.

Load testing with a heating element and a rheostat will often require going over the CB rating, which also has the potential of damaging wire too. DaveT also made some excellent points about magnetic versus thermal tripping. The heating element test might cause the breker to trip thermally but not magnetically, which still leaves potential danger.

Seriously, can you imagine a lawyer having you on the stand, asking you if you actually TESTED to see if the circuit breaker tripped like it should, and then you go on about how electrocution, fire or explosion could result? Guaranteed that lawyer gets a big paycheck, and you joinh the guy on the corner with the tin cup.

Breakers trip all the time. How many times have you troubleshooted a live circuit and accidentally caused a breaker to trip? Did an fire or explosion occur? Of course not! You saw and heard a little spark for a moment, and that was it. What about job sites where part of a building is energized and someone cuts into something? We're not talking testing a 1000A main breaker, we're talking about 15, 20 and 30A branch circuits.

Sure some nasty things have happened with small branch circuits, but those are exceedingly rare.

Over the weekend, I was working in the garage, when an extension cord similar to the one below made that short circuit "pop" after being pulled too tightly around an obstruction:



The circuit breaker tripped uneventfully, and since my cord was pretty well beat, it went in the trash. Branch circuit wiring wasn't even a consideration, despite the fact the extension cord clearly shorted.

How many times do you go on a troubleshooting call, reset a tripped breaker, and then it snaps right back? No explosion, fire, extensive wire damage, right? Ok, if you're gonna bring up FPE you got me, call the fire dept, otherwise it's essentially a non-event.

I'm not saying this should be a standard troubleshooting "technique" and totally discounting the potential dangers involved, but if you're on a call for a CB that didn't trip, you'd BETTER make absolutely sure it trips next time a fault occurs others.

Joe


[This message has been edited by JJM (edited 10-31-2006).]

Joe, a megger is far from a reliable method of checking non metalic cable. A megger is not fool proof even when checking conductors inside metallic conduit .

Roger

[This message has been edited by Roger (edited 10-31-2006).]

If both faults were L-G then I would suspect high impedance ground fault path. I wouldn't worry too much about conductor damage if that was the case because the conductor probably never saw enough amperage. Did the jumper melt because along the whole conductor, or from the contact point back?


skipr, I thought at least 3 explained it pretty well. Lose the defensiveness and reread.

[This message has been edited by Jps1006 (edited 10-31-2006).]

JJM, if a lawyer got you on the stand and asks how you tested the breaker - take the fifth... Testing a breaker in the field like that will cause more damage than good if they have the industry expert from the factory on right after you. Not to mention dangerous. There are places to take breakers for testing, there is one in my area (SF Switchgear - I sell my larger breakers to them... They test and re-sell.), and Randy knows of some in the So. Cal area that are closer to the OP's area that will test in a controled enviornment against UL standard 489. Load banking for thermal, and bolted short for magnetic against the parameters of the breaker with a limited AIC control board. It is not expensive, and if friendly enough, they may not even charge you for a single breaker.

Quite possible these to papers were written by the same person????? http://www.e-t-a.com/fileadmin/user_upload/USA/PDF-files/White_Papers/wp_standards.pdf
http://www.maintenanceresources.com/referencelibrary/ezine/eleccircbreak2.html

OK, reality check here. Assume a short run from the box- 25' of 12-2 cable, 50' electrical round-trip to the box. That #12 cable has a resistance of about 2.0 Ohms/1000' and a reactance of about 0.177 Ohms/1000'. Since the PF is close to 1, the overall impedance rounds to 2.0 Ohms/1000'.

50' of #12 wire would have an impedance of 0.1 Ohms. At 120V, that's 1200A (6000% breaker rating) for a dead-short. 1200A is going to cause significant voltage drop, though! If the poco ran 100' of 4/0 Al to the pole pig, voltage drop would be about 40V, and voltage/current through the #12 wires would actually be 80V and 800A (4000% breaker rating).

Approximately 64kW will be dissipated over the length of the wire. The specific heat of copper is 385 J/kg/K. 50' of #12 contains about of copper. It would take 2 tenths of a second for 64kW of power to heat up 1.06kg of copper from 25C to 60C. If the breaker trips within those 2 tenths of a second, there will be absolutely no cable damage whatsoever. And this is conservative- if the cables in the house were longer, current would be lower and they would heat up slower. If a 50 Amp load was connected (2.4 Ohms), and that breaker trips within 5 seconds, there would be no chance of cable damage. Cross referencing the breaker trip curves- surprise! All the numbers you can run very closely match the upper curves for residential breakers.

...but if the breaker doesn't function correctly, any fault capable of tripping the thermal overload of the breaker MAY begin to damage the cable if the temperature rises much past 60C before the breaker finally trips.

[This message has been edited by SteveFehr (edited 11-01-2006).]