Help me trace a fault?

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ChuckGM

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Hi - just a DIY homeowner here... 1950's house, 20 amp lighting circuit, total of 10 bulbs and one kitchen exhaust fan on the circuit (all led or florescent bulbs). If I use a regular old 15A breaker everything seems to work fine. If I use a 20A GFCI/AFCI breaker one light fixture consistently trips it (with or without anything else on at the time.) I am not sure whether that fixture is first, last, or in between others on the circuit... What "assumptions" can I make about what is tripping the breaker? "ground fault"? "arc-fault"? - Can I assume that because no other combination of lights trips it, that the fault is either "within" the fixture itself? - or that the fixture is the last on the circuit and the fault is somewhere in that last run of the wire?... Thanks in advance for your patience with this novice!
 

Reach4

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Problem could be in that fixture. Less likely, but could be the way it is wired, such as if the unit is wired. For example, suppose the fixture was wired to the the ground wire rather than the neutral wire.
or that the fixture is the last on the circuit and the fault is somewhere in that last run of the wire?
Not that.
 

ChuckGM

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Problem could be in that fixture. Less likely, but could be the way it is wired, such as if the unit is wired. For example, suppose the fixture was wired to the the ground wire rather than the neutral wire.

Not that.
Ok- Thank you. - Any indication whether it's ground or arc fault? - and if it's not "in" the fixture wiring, why wouldn't other things on that circuit trip the breaker?
 

wwhitney

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The fixture in question is individually switched, and the multifunction breaker trips only when the switch is turned on? Or the fixture is switched with others, and if it has no lamp in it, the breaker holds, but with a lamp in it, the breaker trips?

Cheers, Wayne
 

Reach4

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and if it's not "in" the fixture wiring,
Clarify. Talking about the wiring to the fixture, or talking about wiring with the fixture? I suspect the former.

So suppose the green ground wire from the breaker box was connected to the white of the fixture?

Do you have ready access to the wire nuts or terminals at the fixture?
 

ChuckGM

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The fixture in question is individually switched, and the multifunction breaker trips only when the switch is turned on? Or the fixture is switched with others, and if it has no lamp in it, the breaker holds, but with a lamp in it, the breaker trips?

Cheers, Wayne
ahhh - good point. So, it only trips when it's own switch is turned on - that mean it's likely somewhere between the switch and the fixture?
 

wwhitney

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ahhh - good point. So, it only trips when it's own switch is turned on - that mean it's likely somewhere between the switch and the fixture?
Yes, although it's not so likely to be in the middle of a cable (unless you've been hanging a bunch of pictures with unusually long screws recently). So my first guess would be an incidental hot-ground contact in either the switch box or the fixture box.

Cheers, Wayne
 

Reach4

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ahhh - good point. So, it only trips when it's own switch is turned on - that mean it's likely somewhere between the switch and the fixture?
Is the switch external to the fixture, or built into the fixture?

With a built-in switch, suppose there is a 1 kilohm short from hot to ground inside the fixture. You get a ground fault trip only when the switch that switches the hot is on. If the switch is external, suppose there is a hard short from hot to ground. Turn the switch on, and the GFCI trips.
 

Reach4

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okay - gonna check into that "external" switch...
With the external switch, I would turn off the breaker. The measure the ohms to ground on both sides of the open switch. I suspect you will see a lower resistance on the light side. The resistance reading will tell you something. 100,000 vs 1000 ohms vs 2 ohms would tell different stories. Still not going to tell the whole story.
 

ChuckGM

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With the external switch, I would turn off the breaker. The measure the ohms to ground on both sides of the open switch. I suspect you will see a lower resistance on the light side. The resistance reading will tell you something. 100,000 vs 1000 ohms vs 2 ohms would tell different stories. Still not going to tell the whole story.
I'll report back... ;-)
 

ChuckGM

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Okay! - so..... I never got to checking the ohms. Tell me if the breaker was tripping "correctly..."? (I'm sure it was) but tell me if I may have further issues maybe? ;) The light fixture in question was wired into a neutral on another circuit (an outlet the gas stove is plugged into). That other circuit is on the main 200A panel in the basement, whereas the original light circuit is on the upstairs sub-panel (which is wired into that same 200A panel). I wired that neutral wire into the circuit that powers the light and it seems to work fine. I understand the presumption the guy made - I would also guess that it "should" be fine, and to be fair, it "worked" on a normal breaker for years. It just didn't like the gfci/afci - How come?
 

Reach4

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I would also guess that it "should" be fine, and to be fair, it "worked" on a normal breaker for years. It just didn't like the gfci/afci - How come?
GFCI works by making sure the current through the hot is the same current through the matching neutral, but in the other direction. If the difference is more than 5 mA (milliamps), the GFCI trips.

When you ran the current through a different neutral, the currents through the hot going thru the GFCI and the neutral going through that GFCI did not cancel. GFCI tripped.

If you touch a hot line on a working circuit, and 10 mA goes through your finger to ground, the GFCI will detect the difference in currents, and will trip. That is because there will be 10 mA more current though the hot than the neutral.
 

ChuckGM

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GFCI works by making sure the current through the hot is the same current through the matching neutral, but in the other direction. If the difference is more than 5 mA (milliamps), the GFCI trips.

When you ran the current through a different neutral, the currents through the hot going thru the GFCI and the neutral going through that GFCI did not cancel. GFCI tripped.

If you touch a hot line on a working circuit, and 10 mA goes through your finger to ground, the GFCI will detect the difference in currents, and will trip. That is because there will be 10 mA more current though the hot than the neutral.
So - should a neutral wire "never" be shared between circuits?
 

jadnashua

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So - should a neutral wire "never" be shared between circuits?
You should only connect neutrals from the same circuit together, and if using a GFCI, you can only connect neutrals from the same side together (i.e., you can't connect the line and load neutrals together, only those on the same side of the GFCI). Neutral is a current carrying conductor. If you were to share it with another circuit, you might overload it, and a GFCI won't work as the power on the 'hot' and 'neutral' won't be even, so it will trip for that safety reason.

People think of neutral as a ground...it isn't. In a properly setup system, the ground connection should NEVER have current on it...it's there as a safety backup to give power a chance to trip the breaker or blow the fuse.
 

wwhitney

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So - should a neutral wire "never" be shared between circuits?
Basically yes.

[The qualification is that there are 3 wire circuits (120V/240V) with two ungrounded conductors on different legs of the service, and one neutral conductor. I would call that one circuit with one neutral. But it can be thought of or used as (2) 120V circuits with a shared neutral. Either way, that neutral is not to be shared with any other circuits.]

Cheers, Wayne
 

Reach4

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So - should a neutral wire "never" be shared between circuits?
The neutral should not be shared between things downstream of your GFCI and anything else.

Shared neutrals are fine before the input to the GFCI, such as when every outlet has its own GFCI.

If your GFCI is in the breaker panel, which yours may be, then those should never be shared with loads on an another breaker...
 

wwhitney

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Shared neutrals are fine before the input to the GFCI, such as when every outlet has its own GFCI.
Not shared across branch circuits. Each neutral needs to serve only one branch circuit. So any 2 wire branch circuit should be wired so that if you replaced a regular breaker with an GFCI breaker, the GFCI would still hold. [And as most AFCI breakers implement ground fault detection, in new construction compliance with this is now forced for the majority of circuits that require AFCI protection.]

Cheers, Wayne
 

Reach4

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Not shared across branch circuits.
You could have a MWBC with a 2-pole breaker with independent overload trip, and that can all share a neutral. So semantics-wise, is that all one branch circuit, or two? :D
 

wwhitney

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You could have a MWBC with a 2-pole breaker with independent overload trip, and that can all share a neutral. So semantics-wise, is that all one branch circuit, or two? :D
Hence the qualification I included 2 posts ago, and my use of the modifier "2-wire" in my last post (thereby excluding MWBCs). MWBCs are dualistic in nature and may be considered one circuit or two circuits depending on context.

To keep my statement "not shared across branch circuits" simple, an MWBC would need to be considered one circuit. I carefully formulated my statements to both be correct and sidestep this issue, but now it is explicit.

Cheers, Wayne
 
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