Quick GFCI question

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sckby5

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If you installed a gfci in a ungrounded two wire situation and say plugged a three wire grounded refrigerator into it. And then the case or metal parts of the fridge became energized somehow would the gfci trip? Or would it only trip when someone touched the fridge who was grounded? Basically what im getting at is gfci basically do "sense" to ground in order to trip. I've heard people say you don't need a ground for them to work which is correct, but it still won't technically work/trip until it senses a fault to ground? So in essence it really does need a ground to work properly otherwise someone is going to have to get shocked for the gfci to trip in a two wire system as opposed to a three wire system with a ground in that situation the gfci would have already tripped prior to someone touching it? Am i correct in this? I would really love some exact clairification on situations like this with gfci's. So basically the only difference between a gfci and a circuit breaker in a two wire system is that the gfci will trip faster than a circuit breaker would? I understand the difference, breaker to protect the branch circuits and such and gfci to protect personnel. So in essence you would want a gfci on every circuit in a two wire system?
 
A Quick GFCI diagram

A Quick GFCI diagram

410ecm17fig2.jpg


And if you follow all of the road map... note how the GFCI is working...

But by present code your not allowed to install a new circuit without a grounding wire, but I think I understand where U been!

Note that your allowed to protect down line from a GFCI.
 
like his picture shows, the gfci would sense an 'imbalance' of current (5 milliamps) between the hot and neutral from its load side, and then trip.

So you can miswire circuits and get neutrals mixed up and the gfci would trip. So it really doesnt need the 'ground' to trip, just an imbalance.
 
Mr. 5, welcome to the forum! :)

Yes, the shock needs to occur for the GFCI to trip, unless there's a similar current leaking from the frame to earth. The "ground" that is required is the bonding (grounding) of one of the circuit conductors back at the service.

The only contribution having a grounding conductor could have is if the accidental personnel contact was between the energized conductor and the grounded appliace frame. Any grounded surface will suffice for the shock to occur.

If the supply system is not bonded, meaning no conductors are grounded, accidental personnel contact between a cicruit conductor and a grounded surface won't produce enough current for the GFCI device to trip.

The functional difference between a regular breaker and a GFCI is the current flow required to trip the device. The current that can injure or kill is not enough to trip a standard breaker (unless it happens to be a 5ma breaker).
 
sckby5 said:
If you installed a gfci in a ungrounded two wire situation and say plugged a three wire grounded refrigerator into it. And then the case or metal parts of the fridge became energized somehow would the gfci trip?
Click to expand...

Not if the case or metal parts where isolated from 'ground'


Or would it only trip when someone touched the fridge who was grounded?
Click to expand...

Yes.
 
Thanks for the welcome and thats what i was getting at, what Iwire replied.
As far as I've ever seen any three wire,three plug appliance and such is isolated from one another on the ground plug and the neutral blade. I was curious so i tested a bunch of things in my house with three "pronged" plugs with my continuity tester and all of them were always isolated, never having continuity between one another.

So if the gfci had been grounded in that fridge situation i stated, it would have immediately tripped when the case/metal parts of the fridge had become energized. Not causing a shock hazard to anyone, but in the ungrounded situation someone would have to get shocked first before it tripped. So basically gfci are still an upgrade in a two wire situation, but still are sub standard without a proper ground system to properly 100% protect personnel?
 
sckby5 said:
Thanks for the welcome and thats what i was getting at, what Iwire replied.
Click to expand...

Great :)


As far as I've ever seen any three wire,three plug appliance and such is isolated from one another on the ground plug and the neutral blade. I was curious so i tested a bunch of things in my house with three "pronged" plugs with my continuity tester and all of them were always isolated, never having continuity between one another.
Click to expand...

The EGC should always be isolated from the neutral in any appliance.

When I said isolated I was thinking more about the refrigerator contacting something else grounded like say the water line for ice maker etc.

So if the gfci had been grounded in that fridge situation i stated, it would have immediately tripped when the case/metal parts of the fridge had become energized. Not causing a shock hazard to anyone, but in the ungrounded situation someone would have to get shocked first before it tripped. So basically gfci are still an upgrade in a two wire situation, but still are sub standard without a proper ground system to properly 100% protect personnel?
Click to expand...


I agree 100% with all your points. I will add that GFCIs have had a high failure rate so in my opinion the EGC should be run if at all possible.
 
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I remember a thread from a while back about testing a GFI by touching the protected hot and putting your other hand in the sink full of water. I just can't seem to find it.
 
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