GFCIs and their performance w/ and w/o EGCs

[MechEdetour]

I got into a little back-and-forth recently and it would help me to better understand some of the nuances with GFCIs. It shouldn't come as a surprise that the whole conversation kicked off when I said a GFCI does NOT need an EGC to operate. I've become obsessed with grounding the last few years. Nothing excites me more than when I here someone say that installing a GFCI in an ungrounded (no EGC) outlet solves the issue of not having an EGC. Mainly DIY'ers that take that stance, but there are professionals that agree.

GFCI= Ground-fault circuit interrupter
EGC = Equipment ground conductor

I am going to make some general comments (feel free to correct me if I'm wrong) and then I'll ask my questions about the nuances afterwards.

• GFCIs do not need an EGC to operate - they measure current imbalance between L and N and trip when the disparity >= 6mA
• EGCs provide a low-impedance path to ground to protect user's from shock when conductive "stuff" is inadvertently energized (ie. ground-faults)

This is where I am going --> Is there a situation where the presence of an EGC in a receptacle would make the operation of a GFCI "better" (ie. a situation where a GFCI w/ EGC would trip but a GFCI w/o an EGC would not)?

To help with the thought experiment:

Scenario 1: GFCI WITH EGC wired to ground screw. The case of a device (let the device be a fridge throughout) becomes energized @ 120V. The device has a L, N, G plug.

• EGC will provide a low-impedance path, breaker will trip.
• Also, current is flowing from L to ground, and the GFCI may also trip.
  • If breaker trips first, fault cleared. If GFCI trips first, fault is also cleared. If GFCI trips and breaker doesn't... Who cares? We're safe.

Scenario 2: GFCI WITHOUT EGC wired to ground screw (or present for that matter). The case of the device becomes energized @ 120V. The device has a L, N, G plug.

• EGC is absent, therefore no return path. Shock hazard - case is @ 120V.
• But, a GFCI is in the circuit! Is it possible a GFCI would ever trip in this situation? I would like to think that there is a possibility of leakage current from the case to ground (to ground, not the EGC specifically) to lead to a GFCI trip. @120V, we're talking 20,000ohm of impedance for 6mA. I know the impedance of the human body varies by A LOT, but if a person can have enough current flow through them to trip a GFCI, why couldn't some other arbitrary device? The physics are the same... If there is enough leakage to ground, the device will trip. Right?

For these two scenarios, it is obvious to me that a ground-fault with a properly installed EGC protects 100% of the time, whereas a GFCI protects ONLY if the leakage current is high enough to trip it. It does also seem that installing a GFCI in an ungrounded outlet may result in a situation where there may be enough leakage current from an unintentionally energized device. So use it where you can?

 

[don_resqcapt19]

It does also seem that installing a GFCI in an ungrounded outlet may result in a situation where there may be enough leakage current from an unintentionally energized device.

The studies done related to the ~5mA tripping point, is based on the protection of 95% of population...that is 95% of the population will not be harmed by ~5mA of current flowing through their body.

EGC will provide a low-impedance path, breaker will trip.

In the case of low level leakage faults, the breaker does not trip, but the only voltage available to drive a shock is the voltage drop on the EGC caused by the leakage current. This will be so low that there is not a hazard.

 

[suemarkp]

• EGCs provide a low-impedance path to ground to protect user's from shock when conductive "stuff" is inadvertently energized (ie. ground-faults)

I would change "ground" to "the source". You don't usually have low impedance paths to earth, but you have a low impedance path to the power source via the EGC.

In general, grounding is better than not having it. Some items need a path to earth to dissipate static electricity. Ungrounded wiring doesn't have this and doesn't get it via a GFCI. NEC 250.114 requires some things have an EGC, so you need to add one if you don't have it. GFCI isn't sufficient.

For your ungrounded GFCI scenario, leakage happens when someone touches it while standing on the earth. The impedance may still be too high to trigger 5ma of current depending on shoes and floor type, so the GFCI may not trip. But I would expect more leakage via touch than by a metal frame sitting on a finished floor (unless it is a bare concrete slab on grade).

 

[tortuga]

I got into a little back-and-forth recently .../...

Nothing excites me more than when I here someone say that installing a GFCI in an ungrounded (no EGC) outlet solves the issue of not having an EGC. Mainly DIY'ers that take that stance, but there are professionals that agree.

Years back a I was working for a realtor on a home sale of a 1950's home, all original two prong recepts, the new home owner wanted all the outlets 'grounded' so I quoted putting in GFCI's.
Well not so fast the new home owner was a savvy techie type and wanted his home office outlets actually grounded. Thats when I learned 250.114(3)(b) 'information technology equipment' covers lots of stuff like a typical home computer with a 3-prong cord. Even a laptop charger with a 3-prong cord is 'information technology equipment'

Scenario 2: GFCI WITHOUT EGC wired to ground screw (or present for that matter). The case of the device becomes energized @ 120V. The device has a L, N, G plug.

• EGC is absent, therefore no return path. Shock hazard - case is @ 120V.
• But, a GFCI is in the circuit! Is it possible a GFCI would ever trip in this situation? I would like to think that there is a possibility of leakage current from the case to ground (to ground, not the EGC specifically) to lead to a GFCI trip. @120V, we're talking 20,000ohm of impedance for 6mA. I know the impedance of the human body varies by A LOT, but if a person can have enough current flow through them to trip a GFCI, why couldn't some other arbitrary device? The physics are the same... If there is enough leakage to ground, the device will trip. Right?

For these two scenarios, it is obvious to me that a ground-fault with a properly installed EGC protects 100% of the time, whereas a GFCI protects ONLY if the leakage current is high enough to trip it. It does also seem that installing a GFCI in an ungrounded outlet may result in a situation where there may be enough leakage current from an unintentionally energized device. So use it where you can?

GFCI protection with no EGC is permitted if 250.114 and 406.4(D) or 410.44 Ex.1 (2023 NEC references) can be met, if something is UL listed with double insulation like the exceptions in 250.114 mention they have to be clearly marked and use a two prong cord not a 3-prong. There are probably zero three prong cords you can legally plug into a Scenario 2 receptacle in a non-residential occupancy.
Its kinda odd that 406.4(D) would let you replace a 2-prong receptacle for say a clothes washer with a 3-prong and not add a EGC but 250.114(3)(b) would not allow the homeowner to plug in the washer.
Most typical electrical inspectors that are going to inspect the work can't or don't enforce 250.114 since they stop enforcing the NEC at the receptacle, but if a accident were to occur and one of those TV accident attorneys gets involved the liability would be pinned on the electrician or electricians insurance for creating the hazard.

 

[don_resqcapt19]

Its kinda odd that 406.4(D) would let you replace a 2-prong receptacle for say a clothes washer with a 3-prong and not add a EGC but 250.114(3)(b) would not allow the homeowner to plug in the washer.

I tried to add and exception to 250.114, but CMP 5 rejected that with a panel statement that the UL product standards for any listed equipment that is provided with a cord having a ground pin on the plug requires an actual connection to earth ground.

 

[tortuga]

I'd change the wording to something like this:

406.4(D)(2) Non–Grounding-Type Receptacles.
Where attachment to an equipment grounding conductor does not exist in a 15 or 20 amp 250 volt or less receptacle enclosure, or a non-grounding type receptacle is installed the installation shall be GFCI protected at the origin of the branch circuit, receptacles or their cover plates shall be marked “No Equipment Ground”, and comply with (D)(2)(a), (D)(2)(b), or (D)(2)(c).
A wire type equipment grounding conductor shall not be connected to any outlet supplied from a Non–Grounding-Type circuit.
Metallic raceways, cables and enclosures used to extend non-grounded circuits shall be grounded in accordance with 250.130(C).

(a) A non–grounding-type receptacle(s) shall be permitted to be replaced with another non–grounding-type receptacle(s).
(b) A non–grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interrupter-type of receptacle(s).
(c) A non–grounding-type receptacle(s) shall be permitted to be replaced with a grounding-type receptacle(s)

 

[MechEdetour]

I would change "ground" to "the source". You don't usually have low impedance paths to earth, but you have a low impedance path to the power source via the EGC.

In general, grounding is better than not having it. Some items need a path to earth to dissipate static electricity. Ungrounded wiring doesn't have this and doesn't get it via a GFCI. NEC 250.114 requires some things have an EGC, so you need to add one if you don't have it. GFCI isn't sufficient.

For your ungrounded GFCI scenario, leakage happens when someone touches it while standing on the earth. The impedance may still be too high to trigger 5ma of current depending on shoes and floor type, so the GFCI may not trip. But I would expect more leakage via touch than by a metal frame sitting on a finished floor (unless it is a bare concrete slab on grade).

All good points. I do my best to use "source" instead of "ground" when talking in this context but I think I defaulted to ground since the bonding screw and equipment bonding conductor are referenced as "ground" in the NEC (and other places).

I guess that's one of those things I don't have a sense of is how much current would flow in various leakage situations. Maybe nobody does.

 

[tortuga]

I would change "ground" to "the source". You don't usually have low impedance paths to earth, but you have a low impedance path to the power source via the EGC.

Yeah that is a good point in the UK and EU codes I think they even define the limits on what low impedance means a little better that we do.

I guess that's one of those things I don't have a sense of is how much current would flow in various leakage situations. Maybe nobody does.

There are a ton of academic papers on it, recently NASA did some research. Whats overlooked in the US standards is the exposure time in ms to the full fault current instead the NEC focused on the trip current setting, but not trip time.
If you look at a Residual Current Device (RCD) used in Germany for example are required (DIN VDE 0100-410)
they must trip in 300 milliseconds (ms) when tested at its rated residual operating current 30mA, thats General Non-Delayed RCDs.
(They also allow a few more grounding schemes or earthing systems than we do here but I am not up to date on the DIN's)
In part this 300ms limit is due to the likely interference of fault current with the hearts T-phase that occurs at around 400 ms which is more likely to cause fibrillation of the heart, you can find this in all kinds of documents.

When I look at UL 943 they use a formula that states the maximum permitted time to trip in seconds is equal to the quantity (20/fault current in milliamps) raised to the 1.43 power. If I did the math right this formula would permit about 7200 ms (7.2 second ) trip time for a 5 mA ground fault.
UL 943 does not get even close to 400ms until around 30mA of fault current.
At 30mA of fault current a UL 943 GFCI must trip no later than in 560ms or around 33 cycles of AC, slower than a RCD but close enough.
Between 5mA and 30mA there is a 'dead band' where trip time in UL 943 is in excess of 400ms.
Eaton has a paper on RCD's that covers this you can read here they discuss this trip time.

The kind of extremely hi resistance faults you'd see indoors on old ungrounded wiring *might* be in this range, that I dont know of any studies on. Where as a equipment grounding conductor intact would pull the full fault current in a few cycles triggering trip times below the 400ms.

 

[don_resqcapt19]

Where as a equipment grounding conductor intact would pull the full fault current in a few cycles triggering trip times below the 400ms.

Would it if the fault path is through the human to the EGC or other conductive grounded thing?

 

[tortuga]

Would it if the fault path is through the human to the EGC or other conductive grounded thing?

I was not thinking human, just stating the obvious when I commented.
A 'bolted fault' in a 120V appliance with a 3-prong cord and a proper EGC will facilitate a very fast (~100ms) GFCI trip probably 6 cycles or less with up to 1200 ohms.
I am not aware of any academic research into the 406.4(D) scenarios it would be interesting to see.

 

[don_resqcapt19]

A 'bolted fault' in a 120V appliance with a 3-prong cord and a proper EGC will facilitate a very fast (~100ms) GFCI trip probably 6 cycles or less with up to 1200 ohms.

The very reason I see no need for GFCIs on hard wired equipment.