GFCI test oddity

[K8MHZ]

Scenario:

Old two wire in workshop with concrete floor.

Mission:

Replace old two wire receptacle with new GFCI. Old receptacle in a metal 'handy box'.

Of course, I had to go find one of the new slim line types to even fit in the box. (I HATE it when people use handy boxes for receptacles!!)

So, I get done and my friend walks over to it with a plug in tester. I told him that his tester wouldn't work and that only the test button would as there was no ground wire.

Wrong!!!!

Not only did his tester show that there was a ground, but when he pushed the button on his tester, the GFCI tripped!

The only thing I can think of that provided a path to ground was the fact that the box was in a real rock plaster wall that went all the way to the floor. I could see the old NM in the box and it definitely was only two wire.

I just can't imagine that the plaster would be of low enough impedance to trick a tester into acting like there was a hard wired ground at the receptacle.

I just thought I would pass this along. Do not trust these testers 100 percent. (I don't trust ANY tester 100 percent, but that is just me!)

 

[kwired]

Scenario:

Old two wire in workshop with concrete floor.

Mission:

Replace old two wire receptacle with new GFCI. Old receptacle in a metal 'handy box'.

Of course, I had to go find one of the new slim line types to even fit in the box. (I HATE it when people use handy boxes for receptacles!!)

So, I get done and my friend walks over to it with a plug in tester. I told him that his tester wouldn't work and that only the test button would as there was no ground wire.

Wrong!!!!

Not only did his tester show that there was a ground, but when he pushed the button on his tester, the GFCI tripped!

The only thing I can think of that provided a path to ground was the fact that the box was in a real rock plaster wall that went all the way to the floor. I could see the old NM in the box and it definitely was only two wire.

I just can't imagine that the plaster would be of low enough impedance to trick a tester into acting like there was a hard wired ground at the receptacle.

I just thought I would pass this along. Do not trust these testers 100 percent. (I don't trust ANY tester 100 percent, but that is just me!)

Why do you not trust these testers? If you press the test button and the GFCI trips it is working. (chance of any failure with that particular result is pretty low)

If you press the test button and the GFCI does not trip you need to figure out why. Could be defective tester, no equipment ground, miswired receptacle, etc.

These testers do place load from the hot to the EGC when you press the test button. It is wise to check the lights on the tester before pressing the test button. If you have no EGC and press the test button it is possible for you to become part of the "test circuit" if you are in contact with any metal object that is bonded to the EGC terminal of the receptacle being tested. Seen this happen on an outdoor receptacle with metal WP cover. Plug in tester says no ground, cover is connected to EGC of the receptacle - when pressing the test button current flows from tester to receptacle ground to cover to hand that is touching the cover through person performing test to whatever else they are touching that actually is grounded.

 

[roger]

The only thing I can think of that provided a path to ground was the fact that the box was in a real rock plaster wall that went all the way to the floor.

That is your answer and to test this remove the device and try the analysers trip function with the receptacle floating.

Roger

 

[kwired]

That is your answer and to test this remove the device and try the analysers trip function with the receptacle floating.

Roger

And if you want a big suprise hold on to the receptacle yoke while pressing the test button. If you don't get a big suprise take your shoes off and try it again.

 

[K8MHZ]

Why do you not trust these testers? If you press the test button and the GFCI trips it is working. (chance of any failure with that particular result is pretty low)

If you press the test button and the GFCI does not trip you need to figure out why. Could be defective tester, no equipment ground, miswired receptacle, etc.

These testers do place load from the hot to the EGC when you press the test button. It is wise to check the lights on the tester before pressing the test button. If you have no EGC and press the test button it is possible for you to become part of the "test circuit" if you are in contact with any metal object that is bonded to the EGC terminal of the receptacle being tested. Seen this happen on an outdoor receptacle with metal WP cover. Plug in tester says no ground, cover is connected to EGC of the receptacle - when pressing the test button current flows from tester to receptacle ground to cover to hand that is touching the cover through person performing test to whatever else they are touching that actually is grounded.

I know exactly how they work.

Again, there was no equipment ground and the tester said there was. Without an equipment ground, pushing the button on a plug in tester shold not trip a GFCI. Pushing the button on the plug in tester did trip the GFCI.

The tester should have indicated that there was no equipment ground.

 

[kwired]

I know exactly how they work.

Again, there was no equipment ground and the tester said there was. Without an equipment ground, pushing the button on a plug in tester shold not trip a GFCI. Pushing the button on the plug in tester did trip the GFCI.

The tester should have indicated that there was no equipment ground.

Well you apparently had enough of a path to carry 4-6mA minimum or it would not have tripped. What kind of reading do you get with a wiggy or low impedance meter to the ground terminal of this receptacle?

 

[roger]

I know exactly how they work.

The tester should have indicated that there was no equipment ground.

Once again, you had an equipment ground through the yoke to the metallic box to the plaster. It may not have been enough to clear a fault but it was enough for the analyser to see and enough to create an imballance through the GFCI sensor.

Roger

 

[don_resqcapt19]

Old plaster installations often used metal lath and that could easily provide a metallic path to something that is connected to the electrical grounding system. This path may not be able to clear a fault, but it could very likely make the tester show that the EGC is present and to flow enough current to trip the GFCI.

 

[K8MHZ]

Once again, you had an equipment ground through the yoke to the metallic box to the plaster. It may not have been enough to clear a fault but it was enough for the analyser to see and enough to create an imballance through the GFCI sensor.

Roger

When I said I didn't have an equipment ground, I meant one as defined by the NEC.

It is apparent that the box has continuity to ground. That (to me) does not qualify as an EGC. Maybe I wasn't clear about that, sorry.

The only other thing I could think of would be a neutral touching the box. Although not impossible, it's not likely because there were no splices in the box and the receptacle had clamp type terminals with plastic ridges that kept the terminals from touching the box.

It's kind of surprising that the plaster is conductive enough to trick the tester.

Another possibility is that someone grounded the box externally and then covered it. I'm not curious enough to grind a hole in the wall to see for sure.

 

[K8MHZ]

Old plaster installations often used metal lath and that could easily provide a metallic path to something that is connected to the electrical grounding system. This path may not be able to clear a fault, but it could very likely make the tester show that the EGC is present and to flow enough current to trip the GFCI.

Hmm......I did not mention that the receptacle was only a few feet from an outside faucet with lot's of underground metal pipe feeding it.

I forgot about the metal lath, but I know exactly what you are talking about. Good observation!

 

[K8MHZ]

Well you apparently had enough of a path to carry 4-6mA minimum or it would not have tripped. What kind of reading do you get with a wiggy or low impedance meter to the ground terminal of this receptacle?

I didn't check it any further. I have one or two more receptacles to do there and if I think about it and am still curious I may check further. So long as a normal load doesn't trip the GFCI (thus indicating a neutral / ground fault) I really see no concrete need (pun intended) to check any further. I just thought it interesting that my eyes saw no EGC and the tester did, and wanted to illustrate the fact that the testers do not serve to verify the presence of a properly installed EGC. If there are any doubts about that, tie a neutral to a third prong of a three prong receptacle fed by a two wire cable and see what the plug in tester reads. It will indicate that you have a properly wired grounded receptacle when the reality is that there is no proper EGC to be seen.

 

[K8MHZ]

Oh, and the other reason for this thread was to get some ideas as to the reason, which I did. Thanks!

I am sure there may be other scenarios that cause the incorrect reading on the tester, so I'll be watching. I love discussing anomalies inherent to test equipment.

 

[kwired]

I didn't check it any further. I have one or two more receptacles to do there and if I think about it and am still curious I may check further. So long as a normal load doesn't trip the GFCI (thus indicating a neutral / ground fault) I really see no concrete need (pun intended) to check any further. I just thought it interesting that my eyes saw no EGC and the tester did, and wanted to illustrate the fact that the testers do not serve to verify the presence of a properly installed EGC. If there are any doubts about that, tie a neutral to a third prong of a three prong receptacle fed by a two wire cable and see what the plug in tester reads. It will indicate that you have a properly wired grounded receptacle when the reality is that there is no proper EGC to be seen.

Checking with a meter would not necessarily tell you if the EGC is installed properly either. It just tells you what voltage is present, not what current path is taken. With the plug in tester all you get is lights indicating whether or not each terminal is within a certain acceptable voltage range of the other terminals. If you would place a 10 amp load on the circuit and then plug the tester - it may indicate differently if there is compromised connections. Placing load on the EGC may change results also - but will trip GFCI's. These plug in testers are nothing more than a shortcut for using a conventional meter and making three different measurements. The GFCI test button is for testing operation of the GFCI with actual fault current flowing through EGC terminal, and not a test of how much current the EGC can carry.

 

[K8MHZ]

Checking with a meter would not necessarily tell you if the EGC is installed properly either. It just tells you what voltage is present, not what current path is taken. With the plug in tester all you get is lights indicating whether or not each terminal is within a certain acceptable voltage range of the other terminals. If you would place a 10 amp load on the circuit and then plug the tester - it may indicate differently if there is compromised connections. Placing load on the EGC may change results also - but will trip GFCI's. These plug in testers are nothing more than a shortcut for using a conventional meter and making three different measurements. The GFCI test button is for testing operation of the GFCI with actual fault current flowing through EGC terminal, and not a test of how much current the EGC can carry.

Oh, trust me, I know.

I actually own several of those plug in checkers. I also have several tick tracers. I also have several DVOMs, several analog meters, a solenoid tester, three megohmmeters, various circuit tracers, a see snake camera, an IR thermometer and a real honest to goodness Geiger counter, for a partial list. I also make my own test equipment (I think next will be an EMF detector or measuring device using a Hall Effect sensor) and have test equipment for radio and automotive purposes. When I was in the automotive electrical business I had to build many test devices since they weren't available to non-dealers.

When I was 18 I was an electrician's helper in a camshaft factory (1976). I remember one day when the electrician, who was REAL old school, was standing on a ladder, licking the back of his hand and placing it on bare copper bus to see if it was hot. I freaked! I yelled up to him that I had a meter right with me. (Nice Triplett or Simpson, I don't remember) He looked down at me and said, 'You would trust your life with that meter??'.

That's exactly what I had been doing. He told me that he wouldn't think of grabbing a metal wrench and putting it on a bus bar bolt without 'back handing' the bar first, and I certainly wasn't going to touch that bar until he back handed it and there was just no two ways about it.

Since then I have seen and used just about every type of meter and measuring device known to man and have witnessed many erroneous readings and outright failures.

As such, I do not trust any device to be 100 percent accurate 100 percent of the time. It's hard enough when they are working correctly! Think about how many times a DVOM will indicate voltage where it shouldn't be, but a solenoid tester shows no voltage.

I know that the plug in tester 'saw' a ground on its third prong, even though that ground was not coming from an EGC in the cable that fed the receptacle. What I am curious about is how that is happening. I think the best explanation was the metal mesh or screen in the plaster was acting as an EGC.