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Old 08-26-2010, 10:24 AM   #16
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Help Needed with Open Ground


Quote:
The GFCI looks like it is fed from a light fixture that sits over top of a medicine cabinet, about 2 feet above it
.

Good place to check if it is on the same Circuit.

I understand better now what you have going on. I thought you were getting 120 volts to ground at the gfci. Think of the ground and neutral as parallel paths back to the panel neutral bar where they are then joined. The neutral is a current carrying wire for completing the circuit for 120 volt loads. The ground wire (EGC) is a bonding path to connect all metal together so in the event of a ground fault it will conduct fault current back to the transformer and cause a breaker to trip out. Like the diagrams I have at the end of this post.

It does sound like you have an open ground. But a gfci will protect you from electrocution but not shock. Which means if you have a ground fault and no equipment ground or an open equipment ground the leakage current out to your body if you come in contact with the fault will cause the gfci to trip...you may feel a slight shock before it trips. If the ground is present then the fault current travels the ground back to the panel and then to the transformer causing the circuit breaker to trip and clearing the fault. until that is corrected the breaker will not reset.

Anyway you are just going to have to divide and conquer till you find the open.
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Help Needed with Open Ground-bonding-diagram.jpg   Help Needed with Open Ground-120-volt-branch-circuit.jpg  

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Old 08-26-2010, 10:52 AM   #17
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Help Needed with Open Ground


Thanks Stubby.

I'll keep at it. Now that I've gone through all the reachable spots (receptacles. light fixtures, switches) and checked that they were all wired correctly, it's time to just as you said - divide/isolate.

I'm gonna start tonight with pulling the gfci out from the wall and measuring the voltage from the ground screw to hot (that way I'll know for sure it's not just a bad gfci (if it is, I'll be p*ssed!!)). While that's out, I'm going to turn the circuit breaker off, pull the light fixture that I *think* feeds the gfci, and measure the resistance from ground there to the ground on the gfci. That will tell me if the ground is connected between them. Then go from there.

I had given up, but now that all of the likely/probable situations are confirmed fine, it's just a matter of isolating with my meter using the resistance measurements, correct? Just measuring from one ground point to the next, right?

I never had any electrical training but back when I was in the Navy I went through some electronics schools. I did very poorly in them - just enough to fake my way through (I was young and didn't have much interest). So I know enough not to get myself killed/injured, and how to use test equipment safely - that's about it : ) Working with electronic circuits I always used an oscope to trace things out. Much easier when you can see the entire circuit (soldered to a board/boards) than trying ti visualize things through walls! : (

I'll post my results.
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Old 08-26-2010, 11:32 AM   #18
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Help Needed with Open Ground


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Originally Posted by mark2741 View Post
I never had any electrical training but back when I was in the Navy I went through some electronics schools. I did very poorly in them - just enough to fake my way through (I was young and didn't have much interest). So I know enough not to get myself killed/injured, and how to use test equipment safely - that's about it : ) Working with electronic circuits I always used an oscope to trace things out. Much easier when you can see the entire circuit (soldered to a board/boards) than trying ti visualize things through walls! : (

I'll post my results.
Oh, another Navy guy What school/rating did you get your electronics in?
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Old 08-26-2010, 11:37 AM   #19
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Help Needed with Open Ground


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Oh, another Navy guy What school/rating did you get your electronics in?
I'm ashamed to say that I spent approximately 1.5 years in electronics classes while in the Navy. Unfortunately I didn't pay much attention : (

I was a Fire Controlman. Worked on the Aegis operational readiness testing system, as well as Harpoon (that thing never broke so no real work there).

Most of my old shipmates that didn't re-enlist stayed in that field and now work for contractors (Lockheed, Northrop, GE, etc.) doing the same type of work.
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Old 08-26-2010, 07:30 PM   #20
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Help Needed with Open Ground


Update:

I turned off the circuit breaker and pulled the GFCI again. Measured voltage between the ground screw and the hot and got 0 volts, so I know it's not the GFCI itself at fault. Also did some measurements between the ground wire connected to the GFCI and some grounds in the two fixtures in that bathroom (a light fixture and a rocker switch). Infinite ohms. Those two fixtures are not connected directly to the GFCI (though they are on the same circuit).

I am pretty certain, based on the romex connected to the gfci coming from the direction of the ceiling, that the open ground is either hidden behind the bathroom wall and in the romex wire sheathing itself (very doubtful), or is in a junction box in the attic which is only about 5 feet above where the GFCI is located (in a 2nd floor center-hall bathroom). As I said before - that attic has blown in insulation and no flooring. I looked up there with my flashlight and I don't see anything - no electrical wiring at all, so my guess is that the junction box is buried somewhere under that insulation, above the bathroom.

I'm not optimistic. I may just wait until the fall/winter, when it's not boiling hot in that attic, and see about trying to get a narrowed piece of plywood up there. The two attic accesses are too small to fit two large sheets of plywood up there in order to alternate 'sliding' them one at a time towards where the junction might be.

I'm guessing there is no other way to isolate this than to physically try to slide my fat a** across the joists of that hot attic?

Last edited by mark2741; 08-26-2010 at 07:35 PM.
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Old 08-27-2010, 07:00 AM   #21
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Help Needed with Open Ground


when checking upstream junctions for your ground integrity, don't assume that wires in a wire nut are connected electrically. they need to be checked. fyi there should not be any junction boxes behind wall coverings.

the gfci functions by watching for current to take a path other than the standard neutral return path.

Last edited by TimPa; 08-27-2010 at 07:02 AM.
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Old 08-27-2010, 08:33 AM   #22
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Thanks Tim.

I just wish I could get to the next point upstream : (

That's the problem at this point. I am pretty certain that that's where the problem is, but I can't get to it. And due to it likely being buried under lots of blown-in insulation, on the opposite side of where the small attic access is, I am not even positive it's up there. I'm just assuming it is.
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Old 08-27-2010, 09:39 AM   #23
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Help Needed with Open Ground


If you could get access to a toner, commonly called a fox and hound, you could place a tone over the ground and trace it behind the drywall. When the tone stops you have your open.

Gardner-Bender has a wire tracer for about $40 that I have used. IIRC it was called the lan tracker.
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Old 08-27-2010, 09:44 AM   #24
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Thanks Jim - I was wondering about this. I used tone/signal generators when I was in those Naval schools : ) In fact, I thought about how useful that would be in this application, so I looked at some tone generators at HD the other day and wondered why they couldn't be used in electrical wiring. They were all labeled for use with telephone and cat 5 wire and they made no mention of being used for residential wiring so I was hesitant to plunk down the cash for one. They had one for $30 to $40. I think I might give it a try and return it if it doesn't work.

If I recall correctly, it's simply a matter of cutting the power to the circuit, inserting the signal (if I recall correctly, the ones I saw had alligator clip probes, so I guess I'd have to pull the gfci yet again!), then trace out with the sniffer/listener device?

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If you could get access to a toner, commonly called a fox and hound, you could place a tone over the ground and trace it behind the drywall. When the tone stops you have your open.

Gardner-Bender has a wire tracer for about $40 that I have used. IIRC it was called the lan tracker.
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Old 08-27-2010, 10:01 AM   #25
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Help Needed with Open Ground


Yes, the power needs to be off when testing. You can clip the alligator clips to the ends of the wires. Since I do this more often I made up a cord cap with short lengths of wire sticking out to make it easy to clip on to.

Funny that you said you used to work around the Aegis system. In another life I used to draw the illustrations in the tech manuals.
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Old 08-28-2010, 03:24 PM   #26
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Help Needed with Open Ground


An update....no good news....

Yesterday I bought a tone and probe kit (a Fluke Pro3000 for $69 from HD). Cut power, hooked up the tone generator to the hot/neutral that were previously hooked up to the gfci, did a quick test to trace where the wire went behind the wall once it leaves the gfci box. It goes up then to the wall to the left, with the signal still strong at the ceiling. So I figure it's definitely in the attic.

Fortunately, after thinking before that there was no way back there to that spot in the attic, I remembered that there are 2 attic accesses (there was an extension added to this house many years ago). The extension attic is floored and lighted, and a previous owner broke through the drywall to get to the main attic. So today I was able to rig an extension light through, and there were a number of thin boards for me to crawl/slide over to the vicinity of where the wires would have come through the ceiling and presumably up to the attic. I placed the tone probe everywhere I thought and couldn't get a signal anywhere. I dug around a lot but there's so much blown-in insulation, which is then over top of regular bat insulation, and a lot of the wiring is under that I think. I eventually found a junction box that was under insulation, waved my tone probe over top of the wires going into it and got a *very* slight signal. But by that time the walls were closing in on me and, with it being at least 120 degrees up there, and no one home other than me (in case I wind up killing myself with electricity), I gave up. The signal was SO much slighter than the signal I was getting while probing through the wall down in the bathroom where the GFCI is, that I doubt that's it. But it's the only signal I could get.

I may try again tomorrow morning, when it's not so hot up in the attic. But I'm not hopeful. I'm seriously considering just leaving the open ground.
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Old 08-28-2010, 03:46 PM   #27
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Help Needed with Open Ground


first off,you have a open ground.get on neutral and ground.since you said open ground.when tone goes away you have found open ground.
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Old 08-28-2010, 03:55 PM   #28
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Good grounds are called upon to carry current.
You can expect the voltage on ground to rise up to 3 vac or so when passing 10 A through it. Measuring voltage is half the story.

You can check this ground impedance with a hair dryer or toaster or
buy Ideal's 65-165 tester. Just reading the instruction manual for this thing is an education unto itself.

I don't have a financial interest in Ideal's products.

Last edited by Yoyizit; 08-28-2010 at 03:58 PM.
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Old 08-28-2010, 07:43 PM   #29
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Help Needed with Open Ground


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Originally Posted by Yoyizit View Post
Good grounds are called upon to carry current.
You can expect the voltage on ground to rise up to 3 vac or so when passing 10 A through it. Measuring voltage is half the story.

You can check this ground impedance with a hair dryer or toaster or
buy Ideal's 65-165 tester. Just reading the instruction manual for this thing is an education unto itself.

I don't have a financial interest in Ideal's products.
??

I'm not sure I'm following you. I know I definitely have an open ground in the circuit. I am 99% sure it is in the junction/connection that is just before this GFCI. I am 99% sure that this junction/connection is somewhere in my attic, underneath insulation.

How would measuring impedance help me find the open ground?
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Old 08-28-2010, 07:45 PM   #30
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first off,you have a open ground.get on neutral and ground.since you said open ground.when tone goes away you have found open ground.
Thanks oleguy. That makes sense, but I just assumed that since the hot and neutral would be in the same cable/sheathing as the ground, it would work just as well. I was tracing it back to the junction before the GFCI, which is up in the attic I believe, underneath insulation.

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