GFCI basics...

[Pierre C Belarge]

I posted this link on another thread., check it out

 

[hurk27]

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 importance 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.

The statement in red is not true, I think what your missing is how a GFCI work's, It does not use the EGC of a circuit to detect a ground fault, this is why a ground to a GFCI is not needed to function, as was said, it measures the current balance between the hot and the neutral, if if this current balance is off one way or another more then the preset point (5ma) the device will open the circuit to the load side of the GFCI.
This balance detection uses a simple system by utilizing a current transformer coil, that the hot and neutral passes through to the load side of the GFCI, if the current that flows to the load on the hot is equal to the current that flows from the load on the neutral then all is well, since each will cancel each other being 180deg out of phase, the current transformer will not produce any voltage, but if the current flowing to the load is not equal to the current returning from the load then this will allow a differential current to produce a voltage in the current transformer. when the set point is reached the GFCI opens the load circuit.

So it only needs the hot and neutral to accomplish this.

Now the understanding of a complete circuit. As you know you must have a complete circuit for current to flow, this also includes your body if it is the load even if it is unintended for this, for a shock to happen that will open a GFCI a person must be in contact with "two" conductors to form a complete circuit.
this could be the washer frame to the dryer frame, or the hair dryer to the sink, toaster to the stove, but in any case it must be a complete circuit.
if the other point is not available then a complete circuit is not possible and there exist no shock hazard, but if the hazard does ever form then the GFCI is there and will preform as design with a EGC or not.

Remember this, A GFCI does not protect against the shock hazard between the hot and neutral on the load side of a GFCI device, it can not do this even with a ground, or the load its self would trip it. but they function perfectly with a ground or without.

 

[don_resqcapt19]

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

No, I am not talking about a path to ground because the path has to be back to the source. The earth or other conductive object may be that path, but that is not a path to ground.

 

[LarryFine]

So it only needs the hot and neutral to accomplish this.

As I've explained before (and mildly challenged on), the GFCI device does not require an EGC to function, but it does require that the supply system has a grounded conductor.

Without that bond (and excluding capacitive coupling, etc.), the GFCI would not trip, nor would there be a need for it. If you follow the shock-hazard pathway, the MBJ is in the loop.

 

[iaov]

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.

Yhe post previous to this one makes sense. This does not. GFIC is is looking at in and out or hot and neutral. Whatever you want to call them. If they are not within 5ma of equal the circuit opens. The GFIC does not care where that 5ma goes. Ground, the moon, whatever. If you are connected to the output of the GFIC (Plug in stabs) you will be protected. I'm sure there is something I'm missing here Don. Are youtalking about throwing a GFIC in the water. I'll buy that that could kill you as a path exists that is not through the device.

 

[ericsherman37]

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.

Hmm yes. Part of the ground-fault current path, but certainly not the effective variety

 

[mivey]

Please note what 480 said in #15.

You can trip the GFI without a ground. Supposed the hot of a 2-wire BC was connected to the washer past a GFI receptacle. Supposed the neutral of the same BC was shorted to the receptacle box. If you touch the box and the washer, you can trip the GFI and get shocked. No ground involved.

 

[don_resqcapt19]

Yhe post previous to this one makes sense. This does not. GFIC is is looking at in and out or hot and neutral. Whatever you want to call them. If they are not within 5ma of equal the circuit opens. The GFIC does not care where that 5ma goes. Ground, the moon, whatever. If you are connected to the output of the GFIC (Plug in stabs) you will be protected. I'm sure there is something I'm missing here Don. Are youtalking about throwing a GFIC in the water. I'll buy that that could kill you as a path exists that is not through the device.

The question was about a two wire GFCI protected device in the tub with no metallic paths...tub and plumbing all non-metallic. It is very likely that the GFCI device will not trip as the only current path is between the two circuit conductors....there is no leakage, but it still is not safe as there is current flow in the water and the voltage gradient may be great enough to cause a fatal shock.

 

[madpenguin]

No, I am not talking about a path to ground because the path has to be back to the source. The earth or other conductive object may be that path, but that is not a path to ground.

Maybe I'm using my terminology wrong then. When I say "path to ground", I mean a grounding or grounded conductor. This is inaccurate? I realize all power instinctively seeks it's source, being the center tap at the XFMR. My reasoning is, that on a properly grounded system, the neutral buss is a "path to ground".

Someone mentioned that an alternative path could be another phase or leg of the system. Sparky maybe. Is that possible? Wouldn't you create a short if you grabbed the load ungrounded of a GFCI and the load ungrounded of another branch circuit?

On a side note, Is it "proper' to bond your bathtub if you live in a house where you have all pvc plumbing and K&T wiring? Seems like a pretty smart move for the GFCI in the bathroom, else what's the point in having a GFCI receptacle?

 

[480sparky]

........Someone mentioned that an alternative path could be another phase or leg of the system. Sparky maybe. Is that possible? Wouldn't you create a short if you grabbed the load ungrounded of a GFCI and the load ungrounded of another branch circuit?..........

Not a short, as I don't think your body has a low enough resistance to consider the current path a short circuit. But it would be another path for current to flow, and the GFI should detect that and open the circuit.

Possible? Not likely, at least statistically, but mathematically possible. Imagine someone working on a light fixt...... uh..... sorry....... luminiaire.... in a bathroom, so they plug in a portable lamp into the GFI. If the luminaire, or vent fan, were on another circuit (which is usually the case), then both the hot and neutral of that luminaire/fan could provide an undesirable current path if there was a fault in the portable lamp.

 

[madpenguin]

Imagine someone working on a light fixt...... uh..... sorry....... luminiaire.... in a bathroom

Hah. My instructor thanks you... :wink: I still say "light bulb" and "outlet" instead of 'lamp' and 'receptacle' tho I'm working on it.

I tend to be a bit idealistic and overly anal in the field (this gets me into trouble as you might imagine), but I'm really curious about this:

Is it "proper' to bond your bathtub if you live in a house where you have all pvc plumbing and K&T wiring? Seems like a pretty smart move for the GFCI in the bathroom, else what's the point in having a GFCI receptacle?

I'm good on "what-ifs?"... Say your in a clients house, was built in 1923. Your there to trouble shoot a bathroom luminiaire :wink: that quit working. You see funked up/chared K&T wiring connected to the fixture. It's tapped off the line side of a GFCI a couple feet away, which is also fed with K&T. You remember seeing all PVC plumbing in the basement. Some kids run down the hallway as your working. For whatever reason, you recall the incident of those 2 girls in the bathtub that wanted to make bubbles while taking a bath by putting a hairdryer in the tub (this happened and happens).

Do you leave without a suggestion to the homeowner of bonding the tub or running a new BC to the GFCI receptacle? I say most people would.

 

[LarryFine]

On a side note, Is it "proper' to bond your bathtub if you live in a house where you have all pvc plumbing and K&T wiring? Seems like a pretty smart move for the GFCI in the bathroom, else what's the point in having a GFCI receptacle?

If the plumbing is not bonded, on the other hand, is the need for the GFCI still there?