bad main?

[sabercatt]

quick question. had 2 light switches loose in a 2 gang box as the screws got stripped out. while I was getting new screws, my buddy came in and turned on the lights - without issue; but when he went to turn them off he pushed switch 1 into switch 2. these 2 switches are on separate phases, each on a 20 amp breaker and 12 wire. they caught on fire or just arc flashed real bad as the wall is scorched and the switches are black. so both breakers are tripped, but here is where my question comes in. it also tripped the main breaker - 200amp. is my main bad? I can't see how they could draw enough amperage to trip the main. I would think the 12 gauge wire would burn to nothing before that would happen. also, do I trust the wiring to the switches now or should I open up the wall and check/replace?
thanks for any advice

 

[Aceman]

How old is the panel? What brand, type?

It's not uncommon to trip a main or feeder breaker with a hard enough short. Since you said it scorched the wall, I'd say that was a pretty good one.

 

[Richard Cranium]

I think it would be normal for that type of short.

 

[1/2 Cup]

I think it would be normal for that type of short.

Spot on, it reads like a dead short and the fault co ordination my not be correct.

 

[Fierljeppen]

Your main circuit breaker saw considerably more than 200 amps and would have melted the copper wire if it hadn't tripped so quickly. I'll let you do the math, but your formula will look something like I=V/R, where I=current, V=240 (L1 short-to-L2) and R=0.002 (approximate resistance for 1' of 12 gauge copper wire).

Safety first! I'm glad no one was hurt and you didn't burn the house down, but you should buy and use a circuit breaker lockout whenever your working on a circuit.

 

[Jim greengo]

Reset the main.

 

[sabercatt]

Your main circuit breaker saw considerably more than 200 amps and would have melted the copper wire if it hadn't tripped so quickly. I'll let you do the math, but your formula will look something like I=V/R, where I=current, V=240 (L1 short-to-L2) and R=0.002 (approximate resistance for 1' of 12 gauge copper wire).

Safety first! I'm glad no one was hurt and you didn't burn the house down, but you should buy and use a circuit breaker lockout whenever your working on a circuit.

this is in my detached garage and the switch is about 1 foot from the panel.
it was just unlucky as I ran into the house to grab replacement screws and in that 5-10 minutes my buddy just walked in. i'll keep that in mind for the future that even though I deemed it safe to work on while hot there is no telling who or what might happen when my back is turned.

thanks everyone for the input

 

[rlitman]

Your main circuit breaker saw considerably more than 200 amps and would have melted the copper wire if it hadn't tripped so quickly. I'll let you do the math, but your formula will look something like I=V/R, where I=current, V=240 (L1 short-to-L2) and R=0.002 (approximate resistance for 1' of 12 gauge copper wire)...

The dead short current had to be WAY more than 200A to trip the main on the magnetic ("instantaneous"). How much more depends on the breaker model, but using a Square D QO 200A breaker's published trip curve, it takes between 4.5x and 11.25x the rating to trip the breaker on the magnetic.
i.e. you need a MINIMUM of 900-2250A to trip the main "instantaneously", so the main breaker actually saw at least that much current.

Your formula is off however, as the clearing time would be roughly 1 cycle, so you would need to know the circuit impedance (not resistance), as well as the transformer's sub-transient reactance to determine the short circuit fault current. Suffice it to say that breaker coordination in the case where the fault is so close to the panel bus is very difficult, as the short circuit currents can be extreme. In an industrial setting, one solution would be to have a microprocessor controlled main breaker that holds off tripping for an extra few cycles, giving the branch breaker time to clear the fault.

Remember, current flows through the entire circuit. So, the main will see the same current as each branch breaker. And breakers do not (as many people assume) actually prevent a short circuit from drawing too much current. They simply shut off the current AFTER the limit is reached. But for that first cycle (1/60 of a second) the current may be limited only by the wire and the power source, and during that time, it is possible for the trip current to be exceeded on several breakers on the same circuit at once (say on a power strip, on the branch circuit, on the main for a sub-panel, and on the main panel, etc.).

 

[alfredeneuman]

i'll keep that in mind for the future that even though I deemed it safe to work on while hot

Never ever deem anything to be safe working with the power turned on.

 

[walta]

How old is the panel and what brand? There are a few brands of breaker that are known as never trips.

Used breaker testing
https://inspectapedia.com/electric/Used-Circuit-Breaker-No-Trip-Failures-2017-10-13-JA.jpg

New breaker testing
https://inspectapedia.com/electric/New-Circuit-Breaker-No-Trip-Failures-2017-10-13-JA.jpg

After seeing this chart how are UBI a Siemens panels are still on the market.


Walta

 

[sabercatt]

new square d qo

 

[checkthisout]

new square d qo

Breakers will trip too soon if they have been tripped a bunch of times (in my experience).

In your case the tripping of the main wouldn't be out of the question. It reacted before the breakers protecting the circuit.

That's awesome and says something about QO..it QO'd.

 

[rlitman]

How old is the panel and what brand? There are a few brands of breaker that are known as never trips.

Used breaker testing
https://inspectapedia.com/electric/Used-Circuit-Breaker-No-Trip-Failures-2017-10-13-JA.jpg

New breaker testing
https://inspectapedia.com/electric/New-Circuit-Breaker-No-Trip-Failures-2017-10-13-JA.jpg

After seeing this chart how are UBI a Siemens panels are still on the market.


Walta

That 135% line refers to the thermal portion of the trip curve. As I pointed out above, the magnetic portion requires significantly more current. Since two breakers tripped simultaneously, this was clearly a magnetic trip. At merely 135%, it make take 3 hours before the breaker trips on the thermal.

new square d qo

Ok, so the numbers I mentioned above are exactly for your breaker, since I referenced Square D's documents.

Breakers will trip too soon if they have been tripped a bunch of times (in my experience).

In your case the tripping of the main wouldn't be out of the question. It reacted before the breakers protecting the circuit.

That's awesome and says something about QO..it QO'd.

I don't think anything tripped too soon. And no, the main did not react before the branch breakers. As I pointed out above, the same current flowed through both, so both reacted simultaneously.

In the case of a residential panel, there's not much the OP can do to coordinate breakers. The manufacturers just assume that the impedance of all of the branch circuits is high enough to prevent a main instantaneous trip, but as we've seen, it is still possible, both on very short wire runs, and on larger branch circuits.

 

[larry_g]

Do not discount all the other loads going through the main. If an all electric house then the main could have had many amps flowing through it already so do not assume that all the current flowing that tripped the main to have flowed through the short.

lg
no neat sig line

 

[sberry]

Tripped up a 100 with a 12 ground wire. It got hot fast. I also put an old replacement end on a 16 cord, was for a test for a lamp. One little strand of wire frayed out under the cardboad gard, so small barely see this sucker and it grazed the steel cover plate. There is still an arc strike. Flash from hell, knock off the 20 and the 60 double pole back at the feed.

 

[dscheidt]

In the case of a residential panel, there's not much the OP can do to coordinate breakers. The manufacturers just assume that the impedance of all of the branch circuits is high enough to prevent a main instantaneous trip, but as we've seen, it is still possible, both on very short wire runs, and on larger branch circuits.

I was working in a barn once, up on a ladder drilling a hole in a beam. Pulled the trigger on the drill, and the lights went out. I'd tripped the main breaker, several hundred feet away. We had to find someone with a key to get it reset. I got back up on the ladder, and did it again. After that, they took the drill away from me, so I never got to find out what was wrong with it....

 

[checkthisout]

I don't think anything tripped too soon. And no, the main did not react before the branch breakers. As I pointed out above, the same current flowed through both, so both reacted simultaneously.

In the case of a residential panel, there's not much the OP can do to coordinate breakers. The manufacturers just assume that the impedance of all of the branch circuits is high enough to prevent a main instantaneous trip, but as we've seen, it is still possible, both on very short wire runs, and on larger branch circuits.

Didn't see that both had tripped. No big deal.

QO is a commercial, not a residential panel.

 

[rlitman]

Do not discount all the other loads going through the main. If an all electric house then the main could have had many amps flowing through it already so do not assume that all the current flowing that tripped the main to have flowed through the short.

lg
no neat sig line

Let's pretend that the 200A main already had 190A of current on it at the time of the short. How much does that matter, when at an absolute minimum, 900A of fault current are needed to trip that main on the magnetic? Not all that much.

...

QO is a commercial, not a residential panel.

QO is residential. NQ is a commercial panelboard that accepts both QO and QO bolt-on breakers. But neither NQ, nor it's higher voltage NF cousin offer adjustable magnetic breakers (that I am aware of). For that, you'd need to step up to Square D's I-Line bus, and even then, not all breakers feature this.

 

[sberry]

Yes, a while back I was working in a small panel, it had a backfed 100 main and I am packing stuff in it. I pushed a 12 ground against a man lug. It was about 8 inches long or so, it wend dim and couldn't really tell if I shut it off or if it tripped. My helper says it tripped about the same time but the wire got real hot like instant and it tripped rather slowly for sure. I mention to helper,,,, always wondered what would happen if I did that,,, ha. Would have been interesting to see the current. I am sure it would have been higher with a bigger wire.

 

[dave*99]

I drilled into a 40A 240V circuit for a central AC unit. Hit the Romex with an auger bit about 12" from the panel. Tripped the branch and 150A main. Big arc flash too. When a fault occurs that close to the main, the fault current is very high.

 

[CoogarXR]

The way my old service was wired, my shop had a dedicated 100a drop from the street into a panel with only 3 branch breakers. The lights, the opener, and ALL receptacles. The window AC compressor would occasionally "bog down" (I don't know really what it was doing, but it would "groan" and the lights would dim quite a bit, and then it would recover). Anyway, sometimes when this would happen, the main would pop. Not the branch breaker though, only the main. I'd just reset it and go about my day.

Well, when I had my new underground service installed (connected to the house), the electrician showed me that the old main lugs were stripped and there was some arc damage. But from the front it looked fine. I would have never known just by looking in the box.

So, long story short, in my case it WAS a bad main. My AC still "groans" from time to time, but it never blows a breaker anymore.