GFCI basics...

madpenguin · Apr 9, 2009

Sorry guys, maybe this is better suited in the grounding forum. Bear with me, just want to understand better.

I realize that a GFCI doesn't need a grounding conductor to operate but there does need to exist an alternate path to ground, no?

Say my washer frame in the basement is energized and it is plugged into a GFCI, If I am completely isolated from ground and I touch that frame:

1.) Will I get shocked?
2.) Will the GFCI trip?

I'm under the impression you can touch a hot wire and not get shocked as long as you are not grounded. Correct or no?

Same deal with a bath tub that has all PVC plumbing... I'm as dead as a door nail if I jump in with a running hair dryer thats GFCI protected, correct? Or does the resistance of your body somehow leach current off of the circuit, thus preventing all of the current from returning back to the GFCI?

480sparky · Apr 9, 2009

madpenguin said:
..........
1.) Will I get shocked?
2.) Will the GFCI trip?

Given the conditions you state, the answer to 1.) is no, and 2.) is also no. If you do not provide a path to ground, no current flow is possible. You do not get shocked for the same reason birds can sit on high-voltage wires and not get shocked.

All you do in this case is raise your body's potential to the same voltage as the circuit.

madpenguin · Apr 9, 2009

That's what I thought. Thanks Sparky.

This is one reason why I don't fully understand why it's ok to throw a GFCI on a 2 conductor branch circuit.... Yes, it will protect you if you are actually grounded but conditions have to be right in order for that to happen.

Seems like that requirement is nothing more than crossing your fingers and hoping for the best....

don_resqcapt19 · Apr 9, 2009

madpenguin said:
That's what I thought. Thanks Sparky.

This is one reason why I don't fully understand why it's ok to throw a GFCI on a 2 conductor branch circuit.... Yes, it will protect you if you are actually grounded but conditions have to be right in order for that to happen.

Seems like that requirement is nothing more than crossing your fingers and hoping for the best....

If you are not grounded you are not getting shocked. If there is a path through your body that is flowing more than 5mA the GFCI will trip. If there is no path and no current flow there is no hazard.

don_resqcapt19 · Apr 9, 2009

madpenguin said:
...
Same deal with a bath tub that has all PVC plumbing... I'm as dead as a door nail if I jump in with a running hair dryer thats GFCI protected, correct? Or does the resistance of your body somehow leach current off of the circuit, thus preventing all of the current from returning back to the GFCI?

If the power cord is a two wire cord and there are no other current paths the GFCI will not trip. There will be voltage gradients in the water and it is possible that you could get a fatal shock without the GFCI tripping. I think that this is one of the reasons that these type of appliances are required to have immersion circuit interrupters...they open the circuit when the device is immersed in water.

220/221 · Apr 9, 2009

The way I understand it:

The GFCI measures amperage incoming (hot) and outgoing (neutral) and compares the two.

If there is less going back than coming in, that means it's going to ground somewhere and the GFCI interrupts the circuit due to the ground fault.

Say my washer frame in the basement is energized and it is plugged into a GFCI, If I am completely isolated from ground and I touch that frame:

I say that since the washer is hooked up to water, the GFCI will trip. I think that the water will provide enough of a ground path even if it was all plastic piping.

madpenguin · Apr 9, 2009

don_resqcapt19 said:
If there is a path through your body that is flowing more than 5mA the GFCI will trip.

Can you elaborate? Path to where? Between ungrounded and grounded or does there always have to be a grounding path?

GFCI bathroom receptacle using K&T conductors, no ground. I jump in a bath tub, hair dryer in hand, with PVC plumbing on the tub. I'm toast, right? Doesn't matter whether the cord is 3 conductor or not...

don_resqcapt19 said:
If there is no path and no current flow there is no hazard.

I assume you mean path to ground, just as above....

madpenguin · Apr 9, 2009

220/221 said:
I say that since the washer is hooked up to water, the GFCI will trip. I think that the water will provide enough of a ground path even if it was all plastic piping.

Hmmmm... Wouldn't that depend on the TDS of your tap water? I've seen some pretty low ppm tap water before. Especially, if the water wasn't tainted with body oil/salt and soap yet, I'd say that sucker wouldn't trip...

But then again, what do I know...

Wouldn't your assumption also rely upon having a metal water pipe coming into your house along with a high TDS? Water isn't a very good conductor until you start adding crap.

480sparky · Apr 9, 2009

madpenguin said:
That's what I thought. Thanks Sparky.

This is one reason why I don't fully understand why it's ok to throw a GFCI on a 2 conductor branch circuit.... Yes, it will protect you if you are actually grounded but conditions have to be right in order for that to happen.

Seems like that requirement is nothing more than crossing your fingers and hoping for the best....

Whether or not the receptacle has a proper ground has no bearing on the conditions you are getting shocked with. You would get shocked with our without a ground at the receptacle.

madpenguin · Apr 9, 2009

Nevermind Don... You were talking about "path to ground"... If you wern't, please say so.

madpenguin · Apr 9, 2009

480sparky said:
Whether or not the receptacle has a proper ground has no bearing on the conditions you are getting shocked with. You would get shocked with our without a ground at the receptacle.

Yes, but as you stated in your first post, the potential to ground has to be there, right?

So.... Who cares whether it's the grounding conductor or the copper water pipe that does the job...

Sorry, I'm trying to not get confused here...

zog · Apr 9, 2009

See attachment. Any other path will cause an imbalnce tripping the GFCI your washer frame is grounded so it will trip the GFCI as soon as contact is made with the washer frame.

480sparky · Apr 9, 2009

madpenguin said:
Yes, but as you stated in your first post, the potential to ground has to be there, right?

So.... Who cares whether it's the grounding conductor or the copper water pipe that does the job...

Sorry, I'm trying to not get confused here...

That's correct. As long as there's an >5ma imbalance between what goes out on the ungrounded (hot) and coming back on the grounded (neutral), the GFI will trip. If you get between the hot and neutral, the GFI simply sees that as a load, and merrily lets you get shocked.

madpenguin · Apr 9, 2009

zog said:
Any other path will cause an imbalnce tripping the GFCI your washer frame is grounded so it will trip the GFCI as soon as contact is made with the washer frame.

Well.... If the frame was energized but yet grounded at the same time, wouldn't that be impossible? The BC breaker would throw, no? Or rather the GFCI would trip first.

My assumption in these hypotheticals was that I'm missing a ground path, thus trying to determine the impotance of a ground in a GFCI circuit. Without it, it doesn't work. Period. Think I can now be reasonably assured in that statement. Thus, the reason I don't like people saying "a GFCI doesn't require a grounding conductor". It's too misleading.

Thanks guys for the explanations.

480sparky · Apr 9, 2009

madpenguin said:
...........My assumption in these hypotheticals was that I'm missing a ground path, thus trying to determine the impotance of a ground in a GFCI circuit. Without it, it doesn't work. Period. Think I can now be reasonably assured in that statement.......

It's not the presence of a ground, it's the presence of an alternate path for current to flow. In most cases, that path is a ground, but it's not the only path. If conditions are right, the current could flow to another phase, or leg, in another circuit of the same system, or the grounded (neutral) of another circuit.

madpenguin · Apr 9, 2009

480sparky said:
It's not the presence of a ground, it's the presence of an alternate path for current to flow. In most cases, that path is a ground, but it's not the only path. If conditions are right, the current could flow to another phase, or leg, in another circuit of the same system, or the grounded (neutral) of another circuit.

Exactly.:wink: But that neutral of another BC is technically "grounded".

I guess I was never really confused in the first place, just wanted to make sure. So, any path to ground besides the grounded conductor of which the GFCI is monitoring will cause a trip.

220/221 · Apr 9, 2009

Can you elaborate? Path to where?

Anywhere. The GFCI doesn't know or care. All it knows it that there is less current going back that what is coming in (more than 5 MA). It has to be going somewhere it's not supposed to be.

any path to ground besides the grounded conductor of which the GFCI is monitoring will cause a trip.

Correct. What goes out, must come back. If it's not coming back on the neutral, it's coming "back" somewhere else...somewhere not good (in some cases)

480sparky · Apr 9, 2009

220/221 said:
Anywhere. The GFCI doesn't know or care. All it knows it that there is less current going back that what is coming in (more than 5 MA). It has to be going somewhere it's not supposed to be.

Correct. What goes out, must come back. If it's not coming back on the neutral, it's coming "back" somewhere else...somewhere not good (in some cases)

On the flip side is the scenario where there's more current on the neutral than the hot. If this circuit is monitored by a GFI, this is still considered an imbalance and cause it to trip.

ericsherman37 · Apr 9, 2009

I actually tore apart a GFCI receptacle to see what was in there (which was no easy task, they're definitely not designed to be torn apart). It appeared to be a little tiny CT encircling both the internal GFCI load-side hot and neutral conductors. There was also a little electronic gizmo in there. So under normal operating conditions, roughly the same amount of current should be coming in on the hot as is leaving on the neutral, resulting in a cancellation of the respective magnetic fields. The CT would measure nothing. So if an ungrounded dryer frame is energized and you touched it while grounded, then some of that circuit current would be going through you, and if that current exceeded 5 or 6 mA (whatever the number is) then the GFCI would trip. You became the EGC and facilitated operation of the GFCI device :smile:

480sparky · Apr 9, 2009

ericsherman37 said:
.....You became the EGC and facilitated operation of the GFCI device :smile:

Well, technically 'you' do not become the ECG, since the ECG is designed to provide as little resistance to current flow as possible to allow for proper OCD operation. 250.4(A)(1). In the case of your body carrying enough current to trip a GFI device, you're really just part of the ground-fault circuit path.

GFCI basics...

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