ARC Fault tracing

[ssdave]

I'm wiring a new house. Have an arc fault circuit that holds until ANY load is put on it, anywhere in the circuit. It then instantly trips. It will hold indefinitely without a load on the circuit.

I split the circuit, then split it at the first receptacle trying to isolate the problem. Same results. Initially, the problem started when I put on a CFL ceiling light and turned on the circuit to test. It held, until I turned on the light. I presumed it was the light, and changed to LED. Still the same problem. If I use an extension cord to the light switch to power the light, the light doesn't trip that other circuit arc fault breaker. So, then, I started in testing the receptacles on the circuit and found that any load, anywhere in the circuit, trips the AFCI. I had tested the circuit in my normal manner, prior to the light, by testing the last leg of the circuit for resistance between load, neutral, and ground and then by turning on the breaker and testing each receptacle with my standard plug in tester to verify correct polarity and ground continuity.

The circuit shows infinite resistance load to neutral, load to ground, any point in the circuit that I've tried. Zero resistance ground to neutral. Test instrument is a known good Fluke clone that I've used for nearly 20 years.

Is there any logical way to test an AFCI, other than to switch it out with another known good one?

The device is a QO square D 20 amp.

 

[ddawg16]

Re: ARC Fault tracving

I'm assuming you wired it using the included white wire?

I have several. The only time I had a problem was when I had a broken wire. It was the hot in the back of the box. It was wired to a GFCI outlet for outside. Hooked up, it would trip the AFCI either instantly or after a few minutes.

If I disconnected the GFCI, everything worked fine.

And I have ran every power took I have off the GFCI's.

Start looking for broken wires or bad connections.

 

[ddawg16]

Re: ARC Fault tracving

Oh, forgot to add....

Disconnect the wire at the AFCI. If it still trips, you have a bad AFCI.

Another option, replace it with a normal breaker and start doing load tests

 

[marklc]

If your ground and neutral are touching anywhere along the line that will trip it also. Maybe check for that too.

 

[Bert_]

If your ground and neutral are touching anywhere along the line that will trip it also. Maybe check for that too.

^This, an afci will trip with something like 30ma of imbalance. And any current going to ground will cause imbalance.

 

[ddawg16]

If your ground and neutral are touching anywhere along the line that will trip it also. Maybe check for that too.

^This, an afci will trip with something like 30ma of imbalance. And any current going to ground will cause imbalance.

That is not how an AFCI works....that is more like a GFCI, and I think it's more like 5 ma....(but don't quote me on it)

But, you got me thinking....maybe the OP and combination AFCI/GFCI's?

SSDave....can you post a pic of your panel...with a close up of the breaker?

And if my assumption is right....how about a pic of your Chevy SS (small or big block?)

 

[teamextreme]

I believe you guys are referring to a GFCI, (which is around 3ma imbalance). I don't think arc faults are detecting imbalance, but rather, high frequency signal noise that indicates an arc is occuring. Sounds to me like a bad AFCI breaker. Try swapping it with one that is working fine.

Edit: too slow, DDawg beat me

 

[Bert_]

A GFCI id designed to trip between 4-6mA of current imbalance. An AFCI is not necessarily designed to do this but most of them use an internal ground fault ct to help detect arc's. That ct will also cause the breaker to trip if there is enough imbalance between hot and neutral. I don't know the exact amount of current it takes for this to happen but I think it is more than a gfci.

 

[ssdave]

I'm pretty sure not a ground/neutral problem. I'm going to have to trade out the breaker. It is definitely an AFCI, not GFCI or combination. New, in a new panel, with new wire. I took it out of the sealed package from Square D. I've got others to try, just was being lazy. Box is crowded, and I layer the wires neatly and sequentially, so it's a bit of a challenge to switch an AFCI and not disturb the box.

 

[Orionrising]

I'm pretty sure not a ground/neutral problem. I'm going to have to trade out the breaker. It is definitely an AFCI, not GFCI or combination. New, in a new panel, with new wire. I took it out of the sealed package from Square D. I've got others to try, just was being lazy. Box is crowded, and I layer the wires neatly and sequentially, so it's a bit of a challenge to switch an AFCI and not disturb the box.

test the afci. Wire an outlet or lamp to it in the panel.

Sent from my Moto G (5) Plus using Tapatalk

 

[checkthisout]

Why not just buy a new breaker or swap with another in the panel and re-test? 10 minutes worth of work.

 

[ssdave]

Why not just buy a new breaker or swap with another in the panel and re-test? 10 minutes worth of work.

Being lazy. That's what i WILL do.

 

[ssdave]

Took some more time tracing tonight. Wiring a direct receptacle to it in the panel works. Receptacle on first outlet box direct to the home run wire trips on any load.

I'm pretty good at troubleshooting, do it sometimes at work when no-one else can find the answer. Did a lot of commercial wiring for 4 years, was on code review committee for 7, and did electrical inspection for 3. So, have a lot of confidence in my skills. This one is tough.

When I disconnected the neutral at the breaker, I tested it to ground and infinite resistance. Tested neutral to hot, infinite resistance. Ground to hot, same result. So concluded it wasn't a screw in the wiring. Double checked the panel to make sure my neutrals were all on the isolated bus and my grounds on the panel bonded bus. Good.
So, got under the floor, traced the wire from where it goes down from the first outlet box to where it goes up into the panel. It's only about a 10 foot run. No apparent problems. Then, thought to check the outlet end. With neutral removed at the panel, the other end shows that neutral and ground are connected. Checked at panel again, no connection. So, between the panel and first outlet, the neutral is broken, and connected to ground. I can't think of any defect that can cause that to happen. A drywall screw or nail in the wire wouldn't do that.

Now, it's easy to fix, but will take some demo. I'll cut a hole in the sheetrock at the outlet to see that 18" of wire that comes up from below to the box. If no apparent problem there, I'll cut a hole and see the 3 feet between the floor and panel at the other end. At that point, I can replace the wire.

Strange problem that I can't figure out. Will be interesting to see what it is when I cut the holes and expose the wire ends that I can't currently see.

The circuit works normally on a non-arc fault breaker, as the hot is isolated from ground/neutral, and the neutral carries current back correctly, except that it goes to the panel ground in the last leg. Without an arc-fault breaker, would never have seen this problem, except that a plug in tester shows it up at the first outlet as a hot/ground reverse wiring problem for an instant before the arc-fault trips.

I wired this myself and it's new construction, so no strange hidden unknown stuff. Simple home run from outlet box to service panel. Hard to fathom what the problem could be.

 

[vtsoundman]

Took some more time tracing tonight. Wiring a direct receptacle to it in the panel works. Receptacle on first outlet box direct to the home run wire trips on any load.

I'm pretty good at troubleshooting, do it sometimes at work when no-one else can find the answer. Did a lot of commercial wiring for 4 years, was on code review committee for 7, and did electrical inspection for 3. So, have a lot of confidence in my skills. This one is tough.

When I disconnected the neutral at the breaker, I tested it to ground and infinite resistance. Tested neutral to hot, infinite resistance. Ground to hot, same result. So concluded it wasn't a screw in the wiring. Double checked the panel to make sure my neutrals were all on the isolated bus and my grounds on the panel bonded bus. Good.
So, got under the floor, traced the wire from where it goes down from the first outlet box to where it goes up into the panel. It's only about a 10 foot run. No apparent problems. Then, thought to check the outlet end. With neutral removed at the panel, the other end shows that neutral and ground are connected. Checked at panel again, no connection. So, between the panel and first outlet, the neutral is broken, and connected to ground. I can't think of any defect that can cause that to happen. A drywall screw or nail in the wire wouldn't do that.

Now, it's easy to fix, but will take some demo. I'll cut a hole in the sheetrock at the outlet to see that 18" of wire that comes up from below to the box. If no apparent problem there, I'll cut a hole and see the 3 feet between the floor and panel at the other end. At that point, I can replace the wire.

Strange problem that I can't figure out. Will be interesting to see what it is when I cut the holes and expose the wire ends that I can't currently see.

The circuit works normally on a non-arc fault breaker, as the hot is isolated from ground/neutral, and the neutral carries current back correctly, except that it goes to the panel ground in the last leg. Without an arc-fault breaker, would never have seen this problem, except that a plug in tester shows it up at the first outlet as a hot/ground reverse wiring problem for an instant before the arc-fault trips.

I wired this myself and it's new construction, so no strange hidden unknown stuff. Simple home run from outlet box to service panel. Hard to fathom what the problem could be.

Doing a simple continuity check or ohmmeter/resistance check is not sufficient to detect a wiring fault - you'd need a meggar. Sounds like you have pinched romex under a staple or kinked/damaged the insulation somewhere along the way. Happens to the best of us.

 

[Radix2]

To have both an open and a short in a single wire seems quite unlikely. ...


Just to make sure it is not being confused by your test method - did you disconnect both ends of the wire at the same time to verify?

I.e. At panel - ground, neutral and hot sticking straight out, same at first outlet

With this configuration you see open ground to neutral at panel and short at outlet??

If so, that is a severe and unusual puncture of the cable.


But check this way before you tear up your drywall the if this is not what you did already
Hate to make more work only to find out it is a defective outlet or breaker.

 

[ddawg16]

Alright....you found the problem. And the AFCI was doing it's job. It detected the broken neutral.

I wouldn't be surprised if the broken neutral is at the back of your box where the outlet is.

How are you wiring your outlets? Do you use the outlet as the junction? i.e., connect the incoming romex to one set of screws and the outgoing to the other set?

If so....stop doing that. Look up pigtails.

 

[ssdave]

To have both an open and a short in a single wire seems quite unlikely. ...


Just to make sure it is not being confused by your test method - did you disconnect both ends of the wire at the same time to verify?

I.e. At panel - ground, neutral and hot sticking straight out, same at first outlet

With this configuration you see open ground to neutral at panel and short at outlet??

If so, that is a severe and unusual puncture of the cable.


But check this way before you tear up your drywall the if this is not what you did already
Hate to make more work only to find out it is a defective outlet or breaker.

I agree. Can't think of a single way that that combination of short and open could occur in a romex cable.

I checked with all wire ends disconnected, and verified by repeating tests at both ends. Makes me think that there's another box, covered up by the drywall that I forgot about. But, I traced the wires in the crawl space to see that isn't occurring; the wires go up through the floor at the right spots. And, then the hot shouldn't be continuous. And, I don't strip wire ends before drywall, so there shouldn't be connections between ground and neutral. Even with another hidden box it would be an unusual short/open combination.

Not a lot of work to redo the drywall, it's unfinished on the back side of the wall in a closet, so can do it and patch it easily.

I should have time to cut it out and check tonight. Just ran out of daylight yesterday.

 

[ddawg16]

Dave....it's possible a nail went through breaking the neutral and shorting it to ground...with wire flexing, the load center side is not touching.

Given you have a 'solid' failure, you really don't have much choice but to fix it....after all, it is the first leg of that ckt.

Would it be easier to just run a second one and just cut off the ends of the existing cable?

 

[ssdave]

Dave....it's possible a nail went through breaking the neutral and shorting it to ground...with wire flexing, the load center side is not touching.

Given you have a 'solid' failure, you really don't have much choice but to fix it....after all, it is the first leg of that ckt.

Would it be easier to just run a second one and just cut off the ends of the existing cable?

I'll see what it is when I cut it open. Unlikely to be a nail/screw, as both ends come up through the floor into the wall cavity, and don't go through a stud. Drywallers didn't attach the bottom of the drywall to the plates much, and I haven't nailed down trim yet, so shouldn't be nails there. Still possible, just not likely.

Just as easy to replace the wire as run another one, either way would have to mess up the drywall a bit. It's not a lot of work either way, it's an easy run to replace, and goes through the floor joists in the crawl space. If I find that the fault is on the outlet end, I may not open up the other end. I may put an outlet box in the crawl space. I need one there anyway and neglected to do it as I wired. So, could cut the wire back and splice in the crawl space box, then run a new one up to the outlet.

I'll post out what I find as I work through it further.

 

[ddawg16]

It's handy have an outlet in the crawl space.....I have one....along with lights.

Make sure you make it a GFCI under there.

 

[purplezr2]

Had the same issue, had two neutrals mixed up in the panel. Make sure all your neutrals are correct in the box and panel. Also make sure they are separate in the boxes.

 

[ssdave]

Okay, got it fixed. Somethings been bothering me all day. First rule of troubleshooting is measure and check; and if something doesn't measure out or check out how it should, figure out what could be causing that.

The shorted neutral on one end, and open neutral on the other was bugging me. There's no probable defect that can cause that. Especially since in this particular installation, there's no nails or screws that could have cut a wire.

So, rechecked both ends. Same result on outlet end, neutral shorted to ground. Started to check the other end, and decided to retrace the wire.

Purplezr2 had it right; when I retraced the neutral, I had switched up two neutrals. Disconnected the correct one, rechecked at outlet and neutral to ground is open. Wired neutral and hot together, and at other end measure continuous. Bingo. Right answer. Rewired breaker, other breaker that had the mismatch pair, and first outlet, turn on circuit, load it, and it holds and tests correctly. Try lights, they work. Problem solved. No sheetrock harmed in the solution. I'll have to pull my crawl space light and receptacle from somewhere else.

It's hell getting old and not seeing well, and there's a lot of wires in a 40 circuit box. Makes it hard to trace the wires. I had checked it at least once already. Just glad to be back on track. I probably had 6 hours in messing with this circuit.

While I was at it today and had the power off, I traced the rest of the Arc Fault breakers; all of the rest are wired correctly to the right neutral/hot pairs. So, should be uneventfull from here out.

 

[ddawg16]

Glad you found it.

I'm guessing you stripped back the jacket quite a ways? Which is why it was hard to tell?

Early on when I was doing my panel I left plenty of jacket so I could keep track of the wires.

Here is an early pic....it's been cleaned up since.

One of the tricks I did to keep me from getting associated neutrals misplaced, I landed the neutral in a position relative to the breaker. If we assume 20 ckt's per side...I have potentially 20 neutrals. Hence, the neutral for the last breaker is in the last screw. So all you have to do is count the breakers up...and the neut will be the same number of screws up.

Some of my wires are stranded THWN...so I used a heat shrink label on them.