revisiting the gfci

[DLB]

https://www.youtube.com/watch?v=DyeijLvh-yI

One thing I learned from this video is that the GFCI circuitry is monitoring for downstream "Grounded Neutral" (GN) in addition to Ground Fault (GF). One thing I did not learn, however, is exactly how that GN circuitry works. I have one GFCI circuit in my house that suffers from nuisance trips, aka ghost trips, maybe a rate of 10X per year. I see no straightforward way to distinguish whether the cause is relating to GF or GN, or what sort of transient would cause one or the other on such as infrequent basis.

- David

[RFGuy]

https://www.youtube.com/watch?v=DyeijLvh-yI

One thing I learned from this video is that the GFCI circuitry is monitoring for downstream "Grounded Neutral" (GN) in addition to Ground Fault (GF). One thing I did not learn, however, is exactly how that GN circuitry works. I have one GFCI circuit in my house that suffers from nuisance trips, aka ghost trips, maybe a rate of 10X per year. I see no straightforward way to distinguish whether the cause is relating to GF or GN, or what sort of transient would cause one or the other on such as infrequent basis.

- David

I am probably going to regret coming out of retirement to post this, but here goes. JPG had some good insights over on this thread (viewtopic.php?p=286821#p286821). As JPG pointed out he found a high frequency sensitivity of the comparator circuit used in GFCI's used where he worked. In watching the YouTube video posted on this thread about the circuit used in GFCI's, I can now better understand why this nuisance tripping "might" occur. From this video, they state "the fault current through the ground connection creates positive feedback that causes the sense amplifier to oscillate." When the oscillations last longer than the time delay circuit then the SCR fires to disconnect the load from the line. IF there are sufficient harmonics or noise from line connected equipment then it could easily trigger an oscillation in the sense amplifier that gets mis-interpreted as a ground fault (positive feedback triggers it). Also, if you listen to the YouTube video, he points out that the UL test circuit and Fairchild's application note has quite low resistance on the line conductors, so I wonder if less than ideal ground/neutral connections could exacerbate the nuisance trips? Looking into this and/or pursuing line filtering for a GFCI that nuisance trips is where I would start looking if I had this problem. The video also talks about some GFCI's having a 2nd toroid (sense inductor) that senses the current between ground and neutral so it might be nuisance trips are through this path so another reason to take a hard look at neutral and ground wiring and bonding if nuisance trips are common IMHO. Just my 1 cents (further reduced due to inflation at the moment ).

It seems like there is a myth on this thread that GFCI's are all protecting which is being busted. I agree that GFCI's don't protect against everything and that was never their intent. Even with a properly installed and functioning GFCI it is still possible to be shocked. There is a phenomenon when someone gets shocked by AC electricity where the muscles contract and can't respond fast enough between AC cycles to release and separate from a live powerline. GFCI's were only intended to break that connection should a ground fault happen, thus giving the person a chance to remove their hands from the live wire and prevent being continually shocked. The specs for GFCI's are designed around this intent based on preventing muscles from seizing during a shocking event. I don't know how they did these tests, but there were human tests performed that showed muscles tend to seize at around 10mA of fault current so it is believed that at 10mA and above that a person can't let go of a live AC wire. This is why the GFCI limit is typically set at 6mA (well below the believed point of no return). So, in a normal application with a properly functioning GFCI with a plugged in device that faults, can a person still be shocked? I would say it is possible, but not likely. Even if they are shocked, the GFCI should respond before the fault current gets too large that they can't remove their hand. Can they be electrocuted? I highly doubt it, but then again freak accidents are exactly that...freak accidents. Bottomline is GFCI's were NOT intended to prevent any and all shocks, but rather were designed to prevent muscle latching whereby a shock leads to electrocution because you can't let go of the live wire.

What I don't understand are the schematic drawings on this thread. As has already been pointed out, there are direct connections between line and load in the drawing and a GFCI would NEVER be connected this way so why are there direct connections there? This removes the GFCI from the circuit because it bypasses it, so I don't understand the intention behind this schematic drawing. Also confusing for me is why the human was drawn in a series connection in the other schematic. Clearly this wouldn't be the intended user (human) of the GFCI, so it would have to be an electrician working on the circuit in order to put themselves into the circuit path, but then why are they working on the circuit live to begin with?

[Hobbyman2]

my intent was not to show how a GFCI is wired , or how they work but to show the dangers implied in the previous conversation . however ,, even if the wiring is done right or wrong as long as a { ground fault } anomaly passes through the GFCI device it would still trip . according to my teaching {even if the wiring diagram was changed to show its internal working } if that light bulb was a motor running on 3 amps that is the current that will pass through the stick person in that serries , the GFCI will not react as the person is now seen as a additional normal power using anomaly, not a ground fault , I may be wrong however if 1/2 amp of noncontinuous voltage passes through a person it can stop a beating heart , the person does not have to hang on to the wire to die from electrical shock . if the current passes through the body from one arm and exits the other the current passes through the heart , that is all it takes . years ago we didn't have plastic water lines , they were mainly copper and they were a direct ground , in this case { if } the light bulb was a motor ,, { even a window fan or bathroom exhaust fan on a GFCI protected breaker } , the amps in the circuit pass through the stick person , as long as the amp increase doesn't exceed the amp rating of the circuit the breaker it will not trip either . if you happen to be in a puddle of water and can not get away ?????? the GFCI only works in a direct ground fault, drop a hair dryer into a sink or tub and it will trip, that is a direct ground fault .

[RFGuy]

my intent was not to show how a GFCI is wired , or how they work but to show the dangers implied in the previous conversation . however ,, even if the wiring is done right or wrong as long as a { ground fault } anomaly passes through the GFCI device it would still trip . according to my teaching {even if the wiring diagram was changed to show its internal working } if that light bulb was a motor running on 3 amps that is the current that will pass through the stick person in that serries , the GFCI will not react as the person is now seen as a additional normal power using anomaly, not a ground fault , I may be wrong however if 1/2 amp of noncontinuous voltage passes through a person it can stop a beating heart , the person does not have to hang on to the wire to die from electrical shock . if the current passes through the body from one arm and exits the other the current passes through the heart , that is all it takes . years ago we didn't have plastic water lines , they were mainly copper and they were a direct ground , in this case { if } the light bulb was a motor ,, { even a window fan or bathroom exhaust fan on a GFCI protected breaker } , the amps in the circuit pass through the stick person , as long as the amp increase doesn't exceed the amp rating of the circuit the breaker it will not trip either . if you happen to be in a puddle of water and can not get away ?????? the GFCI only works in a direct ground fault, drop a hair dryer into a sink or tub and it will trip, that is a direct ground fault .

WHY do you keep trying to put the human in series with the circuit??? Please give me an expected application where a normal human inserts themselves into the circuit in a series connection. Without doing something very "wrong", I fail to understand how any piece of modern equipment fails and causes a human user to be put in series in an AC circuit, especially with double insulated equipment. GFCI's are exactly that, ground fault circuit interrupters. They were NEVER intended to be human in series circuit interrupters (HSCI ???). Hence, I don't understand where this thread is going...haven't we beat the proverbial GFCI horse to death already on this forum?!?!? By the way, below is a useful table on the amount of current through the human body and the expected effects (300mA is typically considered sufficient to cause cardiac arrest). Note however that where the current is flowing in the body matters as well.

human_body_current_table.jpg (31.17 KiB) Viewed 762 times

[DLB]

my intent was not to show how a GFCI is wired , or how they work but to show the dangers implied in the previous conversation . however ,, even if the wiring is done right or wrong as long as a { ground fault } anomaly passes through the GFCI device it would still trip . according to my teaching {even if the wiring diagram was changed to show its internal working } if that light bulb was a motor running on 3 amps that is the current that will pass through the stick person in that serries , the GFCI will not react as the person is now seen as a additional normal power using anomaly, not a ground fault , I may be wrong however if 1/2 amp of noncontinuous voltage passes through a person it can stop a beating heart , the person does not have to hang on to the wire to die from electrical shock . if the current passes through the body from one arm and exits the other the current passes through the heart , that is all it takes . years ago we didn't have plastic water lines , they were mainly copper and they were a direct ground , in this case { if } the light bulb was a motor ,, { even a window fan or bathroom exhaust fan on a GFCI protected breaker } , the amps in the circuit pass through the stick person , as long as the amp increase doesn't exceed the amp rating of the circuit the breaker it will not trip either . if you happen to be in a puddle of water and can not get away ?????? the GFCI only works in a direct ground fault, drop a hair dryer into a sink or tub and it will trip, that is a direct ground fault .

The underlined part is not correct. For a series circuit, the stick person becomes a second (high value) series resistor. Therefore decreasing total current. Current will be very low compared to 3 Amps. But still a path for current flow. Assuming dry skin and a healthy stick person, I doubt this would be a lethal shock.

- David

[Hobbyman2]

Skin resistance protects the body from electricity
The body has resistance to current flow. More than 99% of the body's resistance to electric current flow is at the skin. Resistance is measured in ohms.Oct 12, 2009

Conduction of Electrical Current to and Through the Human ..

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Why does wet skin have less resistance?
When you are wet, water makes a surface on your skin. This is equivalent to connect your body with this water surface in parallel. ... If it is wet, you add water in parallel and thus lower resistance of arm. So shoulder has higher potential than it would had in case of dry arm.

[Hobbyman2]

I assume you are talking about something entirely different than the potential for a shock or electrocution ?
A GFCI breaker would shut down power to the entire circuit , a GFCI plug will only shut down power to the ground fault receptacle only , current will still be present in the line voltage and at the line terminal on the receptacle . you description of skin effect is only one scenario and electrocution or electrical shock is a very broad brush.

[RFGuy]

Skin resistance protects the body from electricity
The body has resistance to current flow. More than 99% of the body's resistance to electric current flow is at the skin. Resistance is measured in ohms.Oct 12, 2009

Conduction of Electrical Current to and Through the Human ..

--------------
Why does wet skin have less resistance?
When you are wet, water makes a surface on your skin. This is equivalent to connect your body with this water surface in parallel. ... If it is wet, you add water in parallel and thus lower resistance of arm. So shoulder has higher potential than it would had in case of dry arm.

Well, I think I have seen it all now! I never thought I would see a quote from a plastic surgery journal medical article in the Shopsmith forum, but here we are (see Hobbyman2 post above). Below is a more complete excerpt from the medical journal article in case anyone wants more context.
The GFCI fear mongering on this thread is reminiscent of the AC fear mongering that happened a century ago. I certainly have a greater appreciation for what Tesla endured at the hands of Edison now. I am going back into retirement. Have fun debating theoretical ways to electrocute someone that will never happen in practice...I am out.

Excerpt that Hobbyman2 referenced:
"The body has resistance to current flow. More than 99% of the body's resistance to electric current flow is at the skin. Resistance is measured in ohms. A calloused, dry hand may have more than 100,000 Ω because of a thick outer layer of dead cells in the stratum corneum. The internal body resistance is about 300 Ω, being related to the wet, relatively salty tissues beneath the skin. The skin resistance can be effectively bypassed if there is skin breakdown from high voltage, a cut, a deep abrasion, or immersion in water."
https://www.ncbi.nlm.nih.gov/pmc/articl ... 20corneum.

[Hobbyman2]

WHY do you keep trying to put the human in series with the circuit??? Please give me an expected application where a normal human inserts themselves into the circuit in a series connection.

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Not sure what you are talking about ?
the dangers of electricity have not changed in my life time .
be care full .
LOL how brave are you ? and is your life insurance up to date ?? a person can touch one line at a time with out a shock , turn something on in that circuit and touch one line, I promise you will understand pretty quickly . the current will pass through your skin in that small area of your finger or body part , that part of your skin / finger is now in series in that circuit , touch it with both hands and electricity travels through your body , high voltage electricity can jump 6 ft and when it leaves your body it normally leaves a bad smell and a nasty burn , stand in a water puddle or happen to touch anything grounded ? now you entire body is in the direct path of ground out side of any protection ,your body is not much different than a resistor , its in some cases the breaker or slow blow fuse will not pop for a limited time , a split second or so.
I dont know what else I can say about this except it is a dangerous situation and GFI will only protect against a ground fault passing through the device .

[Hobbyman2]

If you are familiar with how electricity travels through a wire , for the most part it travels along the outside of the wire , we use to be call it the skin effect , the more resistance the deeper into the wire the current goes until it eventually burns in to ,its why we have the AWG , same with your dead skin argument. your skin can only repel so many OHMS so much power until the current travels deeper through the water in your body .