Hot to hot short (1∅) and testing a breaker...

[marmathsen]

I feel like I should know this but at the risk of sounding naive I need to tap the knowledge of those smarter than me.

We just went out and repaired a damaged feeder at a residential home. Back in December we had installed a new 200A overhead meter/main and ran a 200A feeder to a panel in the basement. A couple days ago someone took a sawzall to the 200A feeder (on accident of course) and cut partway through both hot conductors. The main 200A breaker upstream didn't trip, but there was lots of evidence of arcing and heating.

It raises questions for me:

1. Why didn't the breaker trip? I'm assuming because the sawzall blade only hit the two hots rather than the neutral or ground, right?
2. Given enough time would the breaker ultimately have tripped by reaching a thermal limit?
3. Is there a way to test a circuit breaker's ability to trip under it's designed load limit other than connecting a high wattage load?

Rob, Seattle

 

[jim dungar]

1. The saw blade likely caused an arcing fault, which contained a relatively high resistance.
2. Eventually the breaker would have tripped based on its thermal function. The lack of tripping is one reason 'locating' breakers by causing short circuits is not suggested.
3. It is almost never cost effective to test breakers smaller than 400A. Replacement is typically cheaper and faster.

 

[LarryFine]

1. Why didn't the breaker trip? I'm assuming because the sawzall blade only hit the two hots rather than the neutral or ground, right?

Definitely not. L-L creates a greater fault current than L-N.

It didn't trip because there was not enough solid contact for long enough.

The energy was dissipated making fireworks.

2. Given enough time would the breaker ultimately have tripped by reaching a thermal limit?

See answer 1.

3. Is there a way to test a circuit breaker's ability to trip under it's designed load limit other than connecting a high wattage load?

Yes, breakers can be tested.