Two wire gfci

Threets

New User
Location
Virginia Beach
Occupation
Electrician
Question for you, I have been asked to install a GFCI receptacle on a two wire circuit as an approved “fix” to the ungrounded outlets.
My question is what if any would the gfci provide since it’s still ungounded?
 

480sparky

Senior Member
Location
Iowegia
You don't need a ground for the GFCI to provide protection. GFCIs work on the ability to measure any current difference between the ungrounded (hot) and grounded (neutral). If they aren't equal, the GFCI opens.

You only need an EGC for a plug-in tester to check it. Using the devices' test button, however, is the only approved testing method.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
The GFCI provides some protection against what an EGC normally provides: losing power if metallic equipment becomes energized.

You should also never connect any EGC in newer cable to any receptacles, so one malfunction does not energize other equipment
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Yes, that.

Suppose you interconnected the grounding terminals of twp receptacles on an extension from a non-grounded circuit.

A hot-to-case fault in device A would energize the case of device B, which would still trip a GFCI device upon a shock.

But, why now have two shock risks instead of one? Whether there's a GFCI device or not, it's a bad [practice to do it.
 
Yes, that.

Suppose you interconnected the grounding terminals of twp receptacles on an extension from a non-grounded circuit.

A hot-to-case fault in device A would energize the case of device B, which would still trip a GFCI device upon a shock.

But, why now have two shock risks instead of one? Whether there's a GFCI device or not, it's a bad [practice to do it.
But not code required?
 

DerekCa

New User
Location
Incline Village, NV
Occupation
Retired Motion Picture Engineer
You don't need a ground for the GFCI to provide protection. GFCIs work on the ability to measure any current difference between the ungrounded (hot) and grounded (neutral). If they aren't equal, the GFCI opens.

You only need an EGC for a plug-in tester to check it. Using the devices' test button, however, is the only approved testing method.
@480sparky the "they" (aren't equal) in the sentence caught my eye. Is it hot to neutral and/or neutral to hot leakage & if it exceeds "x" amperes the GFI trips? What is the value of current that causes a trip? Could you elaborate on possible scenarios that could present as a leakage situation? This forum is awesome. Thank you!
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
@480sparky the "they" (aren't equal) in the sentence caught my eye. Is it hot to neutral and/or neutral to hot leakage & if it exceeds "x" amperes the GFI trips? What is the value of current that causes a trip? Could you elaborate on possible scenarios that could present as a leakage situation? This forum is awesome. Thank you!
The current coming and going in the circuit conductors must add to zero, if there is any leakage (imbalance) between 4 to 6 ma in the conductors the GFCI will trip.

Roger
 

GoldDigger

Moderator
Staff member
So any leakage between hot and neutral (ungrounded to grounded) will simply look like a tiny load in parallel with the real load. There will be no current imbalance and hence no trip.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
A GFCI works by comparing the currents in the two circuit conductors, which are normally identical. When someone gets a shock, the current causing the shock is current that is flowing on the hot wire, but not on the neutral wire. The difference is detected, amplified, and used to trip the GFCI.

Note that an EGC is not part of the GFCI's function, but the power source does need to have a grounded conductor (the grounded neutral) for the GFCI to function. Otherwise, there would be no tendency for the accidental human contact to create a current to shock the person to begin with.
 

480sparky

Senior Member
Location
Iowegia
So any leakage between hot and neutral (ungrounded to grounded) will simply look like a tiny load in parallel with the real load. There will be no current imbalance and hence no trip.
Any 'leakage between the hot and neutral' will not be picked up by the GFCI. It will simply be another load.

Current that does not return on the neutral must be going somewhere else, usually to ground (called a ground fault, hence the name Ground Fault Circuit Interruptor). This 'missing' current is detected by the GFCI circuitry as will open the circuit.
 
Top