GFCI comments and questions

[John Gilmer]

In another NG (all.home.repair) someone mentioned a TV plot in which the bad
guy had changed the wiring to make a GFCI circuit no longer "idiot proof."

That got me to thinking.

Seems to me that the electronics in the GFCI are powered from the line input
leads.

Let us say we have properly installed a GFCI and it is working properly and
at the start of our experiment the GFCI is "on" (not tripped) and there is a
moderate load with little or no "leakage" to ground connected.

In our "experiment" we disconnect the NEUTRAL from the line side (not the
"load" side) of the GFCI.

This would (duh!) cause the load to lose power BUT the GFCI would not "trip"
because there hasn't been a ground fault. But with the neutral gone, the
electronics of the GFCI go cold.

At this point both NEUTRAL & HOT on the "load" side of the device are at 120
volts. The HOT is still hot because the GFCI hasn't tripped. NEUTRAL is
also hot because of the load.

Seems to me that at this point in time, a hapless individual could get a
nasty shock if he is grounded and touches either the "neutral" or the "hot"
of the GFCI "protected" circuit. Since the lights are "off" he may be more
likely to touch things than if things are working.

IOW: Anything that would cause the neutral supply to a GFCI to be opened
can create a dangerous situation.

Any comments?

 

[Andrew Gabriel]

In Europe, some RCDs include monitoring for loss of neutral, loss
of earth connection, or live/neutral reversal, and will trip on
any of these fault conditions.

Any of the newer GFCIs would trip open on loss of power so opening the
neutral would open the hot. With the older GFCIs your scenario might
work.

We have "fail-safe" RCD's which trip on loss of power, but they
aren't commonly used or found. The wiring fault conditions checked
for above are treated quite separately from loss of power -- power
loss does not normally trip fault monitoring RCDs.

 

[[email protected]]

Any of the newer GFCIs would trip open on loss of power so opening the
neutral would open the hot. With the older GFCIs your scenario might
work.
--

I just had a bad GCFI breaker today, old style. It tripped with both
load conductors off and the pigtail connected.
Yes if you disconnect the pigtail from the neutral; bus and connect
the circuit neutral to the bus it becomes a non-GFCI breaker.
I doubt the new style would ever reset without the neutral pigtail
connected but if it was already "on" I am not sure what would make it
trip. They don't trip with a power failure.

 

[[email protected]]

I thought I knew what "pigtail" meant until I read this post. I am not sure
now what "off" means. Does it mean disconnected or open? How would you write
this up without using the pigtail and off?

Bill

The GFCI breaker has a white wire permanently attached to the breaker
(I called a pigtail)
This connects to the neutral bus. You then connect both white and
black from the load side circuit to the breaker. The power rail and
the pigtail provide power for the GFCI circuitry along with the load.
This one worked fine when I removed both white wires from the GFCI and
only used it to source the hot leg. (circuit "white" directly on the
bus) This was the old style that would reset without power to it. I
have not tried the new ones in this configuration but I can't imagine
how anything could trip without both sides of the power attached..
I do have the bad one here and I will open it up later tonight to see
what is inside.

 

[John Gilmer]

Any of the newer GFCIs would trip open on loss of power so opening the
neutral would open the hot. With the older GFCIs your scenario might
work.

Really?

I have some 12 or so GFCIs in the house. We lose power on the average of
once a week (semi rural location).

Trust me: GFCIs don't "trip open" on loss of power.

If they did, I would be running all over the house resetting the gadgets all
the time.

 

[John Gilmer]

I sort of remember taking a receptacle GFI apart some years ago and
being impressed that it looked like a simple, purely magnetic, trip
mechanism.

Do a Google search and find a diagram. There is a "chip", a diode bridge,
a few caps, and the magnetic "trip."

Among other things the "electronics" injects a common mode 120 Hz signal on
the load lines so that the device can detect NEUTRAL/GROUND faults.

 

[[email protected]]

Do a Google search and find a diagram. There is a "chip", a diode bridge,
a few caps, and the magnetic "trip."

Among other things the "electronics" injects a common mode 120 Hz signal on
the load lines so that the device can detect NEUTRAL/GROUND faults.

This is the bad one I replaced today. Manufacture date is mid 90s
Square D QO 30a GFCI

http://members.aol.com/gfretwell/gfci.jpg

 

[Bud--]

Really?

I have some 12 or so GFCIs in the house. We lose power on the average of
once a week (semi rural location).

Trust me: GFCIs don't "trip open" on loss of power.

If they did, I would be running all over the house resetting the gadgets all
the time.

I read it as the load terminals will disconnect if the line-neutral is
removed (for the new GFCIs). The load-hot (and load-neutral) will be
disconnected, but the GFCI will not be tripped. That is my understanding
of how the new ones work (but I havn't made the simple test to see if it
is true). My guess is that a 2 pole GFCI breaker would be powered by the
240 circuit and would work normally without the breaker line-neutral
pigtail connected.

The new standard also prevents reverse wired GFCIs (line to GFCI load
terminals) from powering the local receptacle or GFCI line terminals.

bud--

 

[John Gilmer]

This is the bad one I replaced today. Manufacture date is mid 90s
Square D QO 30a GFCI

http://members.aol.com/gfretwell/gfci.jpg

Neat!

That, of course, is a combination CB and GFCI used in the CB box. All I
have are the ones that place a duplex outlet.

Isn't taking stuff apart FUN?

 

[John Gilmer]

I read it as the load terminals will disconnect if the line-neutral is
removed (for the new GFCIs).

I haven't done that particular experiment. I just removed power and
observed that there was still continuity. Since I still have my
"experimental setup" nearby (tangled in misc. lamp cords, computer cords,
etc), I will do some additional research this weekend.

The load-hot (and load-neutral) will be
disconnected, but the GFCI will not be tripped.

But how can they be "disconnected" if the device isn't "tripped?"

That is my understanding
of how the new ones work (but I havn't made the simple test to see if it
is true). My guess is that a 2 pole GFCI breaker would be powered by the
240 circuit and would work normally without the breaker line-neutral
pigtail connected.

It seems that the 120 volt jobs are ALSO two pole. Since the insert says
that you don't need a ground for the device to function, to a good
approximation they just don't care which is HOT and which is NEUTRAL.

 

[ehsjr]

This is the bad one I replaced today. Manufacture date is mid 90s
Square D QO 30a GFCI

http://members.aol.com/gfretwell/gfci.jpg

*Don't* throw it out. You may be able to make a nice
remote sense switch from it. I made one from a GFCI
receptacle. Shoehorned into a power strip, it turns
on all the remaining receptacles when the first one
senses that the connected load has been switched on.

You *must* modify the existing circuit, but the mod
is simple - or at least it was for the receptacle
circuit I used. In principle, it should be the
same: limit the current through the chip and use the
scr output to drive an opto which drives a triac.

Ed

 

[[email protected]]

*Don't* throw it out. You may be able to make a nice
remote sense switch from it. I made one from a GFCI
receptacle. Shoehorned into a power strip, it turns
on all the remaining receptacles when the first one
senses that the connected load has been switched on.

You *must* modify the existing circuit, but the mod
is simple - or at least it was for the receptacle
circuit I used. In principle, it should be the
same: limit the current through the chip and use the
scr output to drive an opto which drives a triac.

Ed

This one is bad. Trips immediately when the white pigtail is
connected. I would use a good one if I was building something

 

[Bud--]

I haven't done that particular experiment. I just removed power and
observed that there was still continuity. Since I still have my
"experimental setup" nearby (tangled in misc. lamp cords, computer cords,
etc), I will do some additional research this weekend.




But how can they be "disconnected" if the device isn't "tripped?"

You can manufacture them with a DPST NO relay connecting the line to the
receptacle. No line-neutral, no control power, no relay pull-in,
"disconnected".

Another of the changes for new GFCIs was to be more likely to fail OFF
(commonly failed ON previously). Using a NO relay may aid in that.

It seems that the 120 volt jobs are ALSO two pole. Since the insert says
that you don't need a ground for the device to function, to a good
approximation they just don't care which is HOT and which is NEUTRAL.

My intent was that a 2 pole breaker with neutral could have control
power from line-to-neutral or line-to-line. My guess is it is
line-to-line so it doesn't matter if the line-neutral pigtail is
connected or not.

I thought GFCIs triped on reverse line-hot-neutral although it is not
obvious how that would work unless they referenced the ground wire
connection.

Yep!

If a GFCI is tripped, is the control power dead or live?


bud--

 

[John Gilmer]

You can manufacture them with a DPST NO relay connecting the line to the
receptacle. No line-neutral, no control power, no relay pull-in,
"disconnected".

I suppose one could make them that way. Easy to determine: just listen
for the "click" when power comes back on.

Another of the changes for new GFCIs was to be more likely to fail OFF
(commonly failed ON previously). Using a NO relay may aid in that.

Well, my OP was to question how the devices operate as NOW made.

Having a relay at the outlet is "Interesting." It reminds me of something
that was proposed YEARS ago: "Smart" outlets. The idea was that each
"smart" outlet would "test" the connected load for safety BEFORE permitting
power to flow.

My intent was that a 2 pole breaker with neutral could have control
power from line-to-neutral or line-to-line. My guess is it is
line-to-line so it doesn't matter if the line-neutral pigtail is
connected or not.

Again, my OP concerned 120 volt US standard stuff in which the neutral is
bonded to ground at or near the meter or main panel.

I thought GFCIs triped on reverse line-hot-neutral although it is not
obvious how that would work unless they referenced the ground wire
connection.

Since they have two pole "relays" that just isn't a safety concern.

If a GFCI is tripped, is the control power dead or live?

I "think" it's live. Some years ago I had a GFCI with a "pilot" light.
Now they have a light that goes on if the device is "tripped" but (of
course) the power is still coming in.

 

[Bud--]

I suppose one could make them that way. Easy to determine: just listen
for the "click" when power comes back on.

Has anyone taken a "new" GFCI outlet apart?

bud--

 

[Guest]

The GFCI would not lose "power" in this scenario. There is no direct
connection to the gfci circuit from the line side wiring. A GFCI compares
the currents on the hot and neutral. this comparison is made using coils
that convert the line currents to voltages and currents utilized by the GFCI
circuit. In your scenario, if someone were to come in contact with either
the hot or neutral on the load side of the GFCI that creates a ground fault
you would have current flow through the hot conductor and none through the
neutral, the GFCI would "trip" if this current were above the set point of
the GFCI, approximately 5-6 milliamps for a Class A GFCI. For a good
discussion on how a GFCI works go to www.codecheck.com/gfci_principal.htm

 

[John Gilmer]

The GFCI would not lose "power" in this scenario. There is no direct
connection to the gfci circuit from the line side wiring.

There isn't?

Actually, there just about has to be.

Most "new" GFCIs have a LED that only lights when the device has "tripped."
The power for that LED has to come from someplace.

A GFCI compares
the currents on the hot and neutral. this comparison is made using coils
that convert the line currents to voltages and currents utilized by the GFCI
circuit.

The analog of the "differential current" is a very weak signal. It is
"amplified" to provide the power needed to "trip" the contacts. There is a
tiny electro-magnetic which is activated.

In utility applications the difference current is measured in amps or
hundreds of amps and this can provide plenty of energy to "trip" something.
But a home GFCI is expected to "trip" of a few ma of net ground current.
It just needs "amplification!"

In your scenario, if someone were to come in contact with either
the hot or neutral on the load side of the GFCI that creates a ground fault
you would have current flow through the hot conductor and none through the
neutral, the GFCI would "trip" if this current were above the set point of
the GFCI, approximately 5-6 milliamps for a Class A GFCI.

For a good
discussion on how a GFCI works go to www.codecheck.com/gfci_principal.htm

Funny thing you post this link.

The last picture is a schematic of a "typical" GFCI. It clearly shows the
electronics being powered from the "line" side. No "line" power and no
electronics power.

The GFCI is a good device; you can't force it to power a fault by holding
down the "reset".

It protects against a lot. But it doesn't protect (I think) from a
disconnection of the neutral while the GFCI is in a "un-tripped" state.