40 amp 208v circuit to HWH had a dead short to Ground- Breaker did not trip. Why?

[Scottybars2288]

So plumber was finalizing his end, filled heater with water and went to turn breaker on, upon doing this he said there was an arc where the wires splice to the heater wires. I opened up, one of the legs had been pushed in hard and insulation was nicked by the head of the ground terminal screw. fixed it, turned it on, checked for power, all good. I asked if the breaker had tripped and he said no. it is a standard 2 pole 40, Homeline, 10,000 AIR. What could this be besides bad ground connections, or a faulty breaker?

 

[wwhitney]

Was it really a dead short, like bolted? Or was it a nick in the insulation and the conductor too close to the ground terminal screw? In the latter case, you'll get arcing and possibly even sustained current. But apparently the plumber noticed the problem before there had been a sufficiently large sustained current for sufficiently long to trip the breaker.

Looking at one random 40A breaker trip curve, to guarantee a trip within 60 seconds, the current would have to be about 55A sustained if the breaker was at 40C ambient, but it would have to be 140A at 25C ambient.

Cheers, Wayne

 

[roger]

I would advise taking what others say with a grain of salt be it customers or other tradesmen.

 

[Scottybars2288]

Was it really a dead short, like bolted? Or was it a nick in the insulation and the conductor too close to the ground terminal screw? In the latter case, you'll get arcing and possibly even sustained current. But apparently the plumber noticed the problem before there had been a sufficiently large sustained current for sufficiently long to trip the breaker.

Looking at one random 40A breaker trip curve, to guarantee a trip within 60 seconds, the current would have to be about 55A sustained if the breaker was at 40C ambient, but it would have to be 140A at 25C ambient.

Cheers, Wayne

I guess technically you’re right a dead short would be 0 resistance, and i’d assume that would trip the breaker, i’m also assuming the arc blowout probably stopped the two from touching anymore- assuming that’s the case it probably wouldn’t trip correct?

 

[roger]

i’m also assuming the arc blowout probably stopped the two from touching anymore- assuming that’s the case it probably wouldn’t trip correct?

That is probably what happened.

 

[infinity]

If the hot leg hit the ground screw it would be a ground fault. It probably just arced and blew clear happens all of the time.

 

[jim dungar]

I agree the fault probably blew itself apart.
But, it is possible for a breaker not to open when our experience says it should. It all depends on how much current was flowing through the breaker and for how long.
You might be amazed at how the resistance of branch circuit wiring can impact breaker opening times. You need the actual utility/supply fault current in order to make the calculation.

 

[hillbilly1]

If it’s actually 208, and the service is 480, neutral is not bonded in the step down transformer.