Why do arc-faults trip when the neutral & EG touch? - Electrician Talk - Professional Electrical Contractors Forum

Briancraig81 · 10-17-2009, 12:49 AM

Can anyone explain to me why an arc-fault will trip when the neutral and equipment ground touches? Also, we have had noticed newer GFI's (P&S) will trip when this happens also. Why is this?

steelersman · 10-17-2009, 01:07 AM

I believe ( I may be wrong ) that alot if not most AFCI's are also GFCI. And it's completely normal for a GFCI to trip if the neutral touches the EGC. I believe the reason is that the GFCI looks for the exact same amount of current to come back on the neutral. And since the neutral has touched the EGC some of the current is lost on the EGC and it senses that and trips. But in theory, a circuit with no load on it shouldn't trip the GFCI since there is no current going out or coming back on the neutral to begin with. But if I'm not mistaking it will still trip. I may be wrong though.

Someone else will chime in soon enough to either confirm what I've just said, or to totally school me.

JohnJ0906 · 10-17-2009, 05:30 AM

Quote:

Originally Posted by steelersman

I believe ( I may be wrong ) that alot if not most AFCI's are also GFCI.

Yes, but the GFCI setting is about 30 ma instead of 5 ma.

Quote:

And it's completely normal for a GFCI to trip if the neutral touches the EGC. I believe the reason is that the GFCI looks for the exact same amount of current to come back on the neutral. And since the neutral has touched the EGC some of the current is lost on the EGC and it senses that and trips.

Yup

Quote:

But in theory, a circuit with no load on it shouldn't trip the GFCI since there is no current going out or coming back on the neutral to begin with. But if I'm not mistaking it will still trip. I may be wrong though.

Someone else will chime in soon enough to either confirm what I've just said, or to totally school me.

I also think it is a bit odd that neutral/EGC contact will trip a GFCI without a load, but I've experienced it enough to accept it as is.

**480sparky** · 10-17-2009, 09:12 AM

This one is easy.

It's to keep DIYers from doing their own wiring. If they accidently create a N-G short, the AFCI opens. If they ain't smart 'nuff to figure it out, they call a pro.

Just P&S's way of helping us out.

seo · 10-17-2009, 09:13 AM

To simplify a GFCI has a toroidal coil that senses the unballanced current. If either of the conductors comes in contact with something grounded some of the current will return on a different path causing an unballanced current. The coil senses this and opens the circuit.

Briancraig81 · 10-17-2009, 09:29 AM

Quote:

Originally Posted by seo

To simplify a GFCI has a toroidal coil that senses the unballanced current. If either of the conductors comes in contact with something grounded some of the current will return on a different path causing an unballanced current. The coil senses this and opens the circuit.

I understand how GFI's work, about sensing current comming in and going back out. But what I don't understand is why there can be no load whatsoever and they will trip.

What baffles me a bit more is the arc-faults will do it on a 4-wire and 3-wire panel. We did a house a few months ago that had a back to back service, no need to separate the neutrals and grounds on the bar. That's real confusing to me about when N & EG touch because they are all on the same bar.

trevorwhopkins · 10-20-2009, 09:11 AM

i am by no means a master haha but for what its worth:

basics:

gfcis use that coil as a disconnect. it does not read load at all. it shouldnt be thought of as reading or testing. it just lets the neutral volts and phase volts bpass through. load is not a factor, hence tripping with no load. the neutral can bleed volts through any ground or grounded surface it touches. this causes a difference of voltage between neutral and phase. say 120v phase 120v neutral ideally, or 120v phase and 95 neutral 25 bleeding to ground. the difference activates a sort of relay, which in turn activates a disconnect in the gfi.

arc fault: af breakers have an extreme sensitivity to arcing in a circuit. even pulling a plug out while your vaccuum or whatever is on will cause the breaker to trip. if a neutral is feeding any load at all it can arc onto a ground if touchiing. hence the similarity to gfcis. i believe arc faults work on load though. amps as opposed to the volts that gfcis read

normel · 10-20-2009, 11:14 AM

Quote:

Originally Posted by trevorwhopkins

i am by no means a master haha but for what its worth:

basics:

gfcis use that coil as a disconnect. it does not read load at all. it shouldnt be thought of as reading or testing. it just lets the neutral volts and phase volts bpass through. load is not a factor, hence tripping with no load. the neutral can bleed volts through any ground or grounded surface it touches. this causes a difference of voltage between neutral and phase. say 120v phase 120v neutral ideally, or 120v phase and 95 neutral 25 bleeding to ground. the difference activates a sort of relay, which in turn activates a disconnect in the gfi.

arc fault: af breakers have an extreme sensitivity to arcing in a circuit. even pulling a plug out while your vaccuum or whatever is on will cause the breaker to trip. if a neutral is feeding any load at all it can arc onto a ground if touchiing. hence the similarity to gfcis. i believe arc faults work on load though. amps as opposed to the volts that gfcis read

Uhhh...no. GFCIs detect current flow/imbalance, not voltage.

The electronics in the GFCI applys a signal to a second coil (at 120Hz) that will interject current flow in the neutral wire when the load neutral touches a ground wire (completes the circuit back to the panel). This produces an unbalanced condition, causing the GFCI to trip. There does not need to be a load on the hot for this to occur.

Toronto Sparky · 10-21-2009, 12:26 AM

Never seen a GFI trip without a load even if the neutral is grounded.
They do trip instantly when any load is applied.
Even wiggys.

Briancraig81 · 10-21-2009, 05:12 PM

Quote:

The electronics in the GFCI applys a signal to a second coil (at 120Hz) that will interject current flow in the neutral wire when the load neutral touches a ground wire (completes the circuit back to the panel). This produces an unbalanced condition, causing the GFCI to trip.

So, I'm guessing arc-faults do the same thing and this is my answer?

Quote:

Never seen a GFI trip without a load even if the neutral is grounded.
They do trip instantly when any load is applied.
Even wiggys.

The new tamper resistant Pass & Seyemour ones do.