Dead short, Which should trip first, GFCI or CB ?

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newservice

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Friday service call, resi kitchen counter, found NM cable pinched in the old metal 1g box at the bushing/clamp in back, replaced, worked fine and left the customer home. The wife called me at 8 am, tripped the CB when she plugged in micro, went and found a wire in the box (behind the GFCI receptacle) had gotten up top and snagged a protruding screw. (Don't we love the old metal boxes on kitchen counters when putting in gfi's) The #12 hot that grounded to the box was on the load side out of the GFCI.

Question: Why did the GFCI not trip out first? Brand new P&S gfi, works correctly both with the test button and a tester. Just seemed to me the electronics in the gfi would react before it got to 20A to trip the new 20A cutler hammer single pole breaker I put in . Thought this was interesting topic for discussion.
 
newservice said:
Friday service call, resi kitchen counter, found NM cable pinched in the old metal 1g box at the bushing/clamp in back, replaced, worked fine and left the customer home. The wife called me at 8 am, tripped the CB when she plugged in micro, went and found a wire in the box (behind the GFCI receptacle) had gotten up top and snagged a protruding screw. (Don't we love the old metal boxes on kitchen counters when putting in gfi's) The #12 hot that grounded to the box was on the load side out of the GFCI.

Question: Why did the GFCI not trip out first? Brand new P&S gfi, works correctly both with the test button and a tester. Just seemed to me the electronics in the gfi would react before it got to 20A to trip the new 20A cutler hammer single pole breaker I put in . Thought this was interesting topic for discussion.
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Breakers can be faster than GFCIs when the fault current is above the instantaneous
trip level.
 
newservice said:
Friday service call, resi kitchen counter, found NM cable pinched in the old metal 1g box at the bushing/clamp in back, replaced, worked fine and left the customer home. The wife called me at 8 am, tripped the CB when she plugged in micro, went and found a wire in the box (behind the GFCI receptacle) had gotten up top and snagged a protruding screw. (Don't we love the old metal boxes on kitchen counters when putting in gfi's) The #12 hot that grounded to the box was on the load side out of the GFCI.

Question: Why did the GFCI not trip out first? Brand new P&S gfi, works correctly both with the test button and a tester. Just seemed to me the electronics in the gfi would react before it got to 20A to trip the new 20A cutler hammer single pole breaker I put in . Thought this was interesting topic for discussion.
Click to expand...

This was sort of covered pretty recently in another thread- here are a couple of posts from the mods:

iwire said:
Ok in the case of a hot to ground short, yes the GFCI will see a current imbalance but it will be seeing this at exactly the same time as the breaker sees an overload condition.

There is no reason to think a GFCI can physically open the circuit faster than the breaker, that is not the purpose of GFCI.
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jim dungar said:
Many people are amazed at how fast circuit breakers actually are, when tripping in their instantaneous region, like with a direct short.
Likewise, many are surprised that GFCI's actually have a time delay in their operation.
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Knuckle Dragger said:
I agree and have found that statement to be true.
I just don't technically know why cicuit breakers trip faster on a dead short.:?
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Typical branch circuit breakers react to a dead short directly with a magnetic trip. No electronics or sensors involved.

On the other hand a GFCI trip is accomplished with sensors, electronics and firmware that has to all react and tell a relay to open.
 
user 100 said:
This was sort of covered pretty recently in another thread- here are a couple of posts from the mods:
Click to expand...

Cool beans, makes sense...On thinking about it, by the time the first half cycle of the rms voltage makes it to the peak, you're already at 170vpeak, ..more than enough to deliver 20A..with no resistance, or say less than 1 ohm, the current would be at 20A as soon as it hit 20V....so time wise that seems at least enough time for the CB to trip...thats a apretty fast sequence of events. Thanks.

The bad part was, it was Friday night and while the couple was thrilled to have it fixed as I was the 3rd electrician they had to try, it was their 38th anniversary, and although I had identified the problem, I really wanted to come back but decided to hustle it up to get out of there and likely overlooked the wire dressing I did in the box. But they were the nicest people about it anyway.
 
Here are a few more posts for the op from that other thread ("testing ungrounded receptacles on GFCI circuits" in the grounding vs. bonding forum of this site):

MD84 said:
The GFCI should trip within 25ms on a 500ohm fault.
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don_resqcapt19 said:
The UL standard would permit the trip time to be 0.78 seconds for that load.
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don_resqcapt19 said:
I don't think the GFCI is faster. The trip time for a GFCI is often stated at 1/4 second. That is 15 cycles and a breaker can clear a short much faster than that.
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newservice said:
Cool beans, makes sense...On thinking about it, by the time the first half cycle of the rms voltage makes it to the peak, you're already at 170vpeak, ..more than enough to deliver 20A..with no resistance, or say less than 1 ohm, the current would be at 20A as soon as it hit 20V....so time wise that seems at least enough time for the CB to trip...thats a apretty fast sequence of events. Thanks.

The bad part was, it was Friday night and while the couple was thrilled to have it fixed as I was the 3rd electrician they had to try, it was their 38th anniversary, and although I had identified the problem, I really wanted to come back but decided to hustle it up to get out of there and likely overlooked the wire dressing I did in the box. But they were the nicest people about it anyway.
Click to expand...
A 20 amp breaker will never trip at 20 amps. In fact the standard would permit a 20 amp breaker or fuse to carry 26.99 amps forever without tripping.
To get into the instantaneous trip range you would need a fault current of 120-200 amps depending on the breaker.
 
IMHO,
I was lead to believe that the intent of the three way switch was the convenience of the user to turn the lights on and off( without walking in the dark) and not walk around in the dark to make sure that the switch levers are always in the same position?? :)

I once had a customer ask me to do this for her so I did(place them in the same position when they were off) then I told her have fun with that and no guarantees after you turn them on.:D
 
takelly said:
A GFCI will not trip on a hot to ground short because the same amount current will be on both and a gfci looks for a difference.
Click to expand...

Hot to ground is a ground fault and exactly what the GFCI is looking for.

I think you mean a hot to neutral short.
 
Outside of those metal boxes just being a pain and fill up too fast, one of the biggest issues I seem to have encountered over the years is the new guy that thinks the cable clamp needs 100 inch pounds of torque on the screw and ends up piercing the cable.
 
Knuckle Dragger said:
IMHO,
I was lead to believe that the intent of the three way switch was the convenience of the user to turn the lights on and off( without walking in the dark) and not walk around in the dark to make sure that the switch levers are always in the same position?? :)

I once had a customer ask me to do this for her so I did(place them in the same position when they were off) then I told her have fun with that and no guarantees after you turn them on.:D
Click to expand...

Umm, I think you have the wrong number (thread)!:p

This thread has nothing to do with 3-way switches!:happyno:
 
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