View Full Version : GFCI Warning

07-20-2002, 01:21 PM

Trap requested that I write in to make folks aware of some limitations of those magic GFCI outlets and breakers that protect us in homes and shops. Please understand that I am speaking in generalities and what I am saying may not be correct in all circumstances.

GFCI stands for Ground Fault Circuit Interrupter, meaning that the device will open the circuit (trip) if even a small amount of current flows to ground. The devices work by very accurately measuring the current that leaves the device on the “hot wire” as well as the current that returns on the “Neutral wire”. Theoretically, the two numbers should be equal – What goes out has to come back. If the two measurements are not equal within .006 Amps, the device will trip. The assumption is that if the currents don’t equal each other, some is most likely “leaking” to ground. Ok, so that’s how they work – now let’s consider some real world situations.

Your angle grinder’s cord has been bit by the wheel more than you would care to admit. As you try to keep the cord out of your way you slide your hand across a spot that has actually worn to the copper. Your other hand is on the steel welding bench that is grounded 8 ways to Sunday by the welder, the damp concrete floor, etc. The GFCI on the wall was seeing 5.211 (The grinder running) amps going out on the black wire and 5.211amps coming back on the white, it’s happy. Now your body has provided a path from the wire on the cord to ground, the path isn’t very good but it’s enough for 20 milliamps to flow through you. The GFCI sees 5.231 Amps going out, but only the 5.211 that is flowing though the grinder comes back. CLICK – maybe you felt and maybe you didn’t, but the GFCI tripped and saved you.

Now let’s try one that’s a bit different. You have decided to wire a new light fixture over your mill, you figure it’s safe enough to just flip off the wall switch. Your whole shop is wired with GFI breakers anyway. You set up your wood ladder on that nice new rubber mat in front of the mill, and get to wiring. As you hold both the black and white wire in your hands, stripping the ends, your kid walks in and helps dad by flipping on the light switch so you can see whatever you are doing up there. The GFCI sees 20 milliamps going out on black, through you, and the same 20 milliamps comes back on the white wire. The device cannot tell that this isn’t a digital clock, it’s happy and you are getting rapped.

In other words, if you aren’t grounded AT ALL, and you get between the hot and white wire the GFCI cannot tell that you are not a normal load and will NOT trip. You can develop your own scenarios, but consider a plastic bathtub with a plastic drain line and a radio with a two-wire cord. For a GFCI to work, there must be a tiny (6 milliamp) leak to ground. Plastic tub, plastic drain, two wire cord = no path to ground. If you are in the tub it might not be pleasant. Break the ground prong off the tile saw cord, have a problem with the pump and reach into the water pan – bare feet on concrete will leak to ground and trip the GFCI, Rubber soles on a wood floor and you are going to be hurting.

These devices are popular because MOST times some path to ground exists, but the point of my warning is that it is not ALWAYS there. Don’t rely on these things to make up for common sense. Ground things as the code requires, and consider what might go wrong on each and every job.


07-21-2002, 08:56 PM
thanks alan,

like i said most of my world is salty and wet and knowledge keeps you alive. Those ground fault critters have saved my butt several times along with wearing white rubber boots. Thanks for the reply and as we can see those things may not be perfect but they are a whole lot better than "sparky the electric chair"


07-22-2002, 01:44 PM
NSDesign--Thank you for the information, I didn't know about these limitations, I'll keep it in mind when I'm working around them.

Do you know anything about the new Arc Fault Circuit Interrupters?

I just remodeled an old farmhouse and 1/2 of the wiring was old 2 wire stuff, I replaced all 4 ( http://bbs.homeshopmachinist.net//biggrin.gif) 120V breakers with these in hopes of adding a little protection. Did I waste my $$$?

Thanks again for the info.


NRA Lifetime Member

07-22-2002, 06:02 PM

I'm afraid I don't have as much insight into how arc fault breakers work - but rather than prevent shocks, their intent is to detect sizzling, arcing connections. These types of faults are the most common cause of fires. When poor connections like loose wirenuts or terminal screws begin to arc 'n spark - the breaker trips. I have little experience with them, and as I said, I haven't read about their sensing technology, but I think it is money well spent.

P.S. - Article 210.12(B) of The National Electric code now requires that all new 15 and 20 amp circuits feeding 120 Volt receptacles in bedrooms be protected by arc-fault breakers. This took effect January 1, 2002.


07-22-2002, 06:21 PM
AFCI are supposed to help in situations like NSDesign describes where there isn't a path to ground (so GFCI won't work) and there's not an overload (so the breaker won't trip). Imagine when you were a kid and stuck a paper clip/bobby pin/whatever you had available into the receptacle. The AFCI will catch this.

I priced AFCI's for my shop but decided with GFCI receptacles for now. I certainly don't think AFCI's (or anything that provides extra safety) are a waste, but I was concerned that certain motors might create a condition that trips them, so I decided to wait until I learn more about them.

If you have a problem with a particular piece of equipment, you MAY need to put it on a non-AFCI breaker. But then again, my fear could be unfounded. I haven't found much info on them myself.

Good explanation of GFCI, btw.

In the scenario where you're working on the light fixture, as always, it's always a good idea to keep the "path to ground" in mind. If you have one hand on the white wire and one hand on the black wire, then the path is through your heart! Never a good idea to touch those two at the same time, no matter how sure you are that the circuit is off.

07-22-2002, 06:25 PM
BTW, when the prices drop a bit for AFCI breakers, I'll retrofit them in the bedrooms. We're not required to, because the wiring was there before the code went into effect, but we'll be having kids soon, so I'll want to be extra careful. I remember the STUPID things I did when I was a kid. http://bbs.homeshopmachinist.net//wink.gif

mike thomas
07-22-2002, 07:23 PM
I do not know how my mother survived my childhood. Once while playing with electricity, I stretched a zip cord across my bed over to an outlet. Somehow, I did something wrong, and all of the insulation burned off of the zip cord. The next to go were the blankets on the bed. I had a hell of a time trying to explain why all of the blankets were cut in half, except where they hung over the edge of the bed. I saved the day by bashing the plug out of the wall with a wooden chair, and had to explain the hole in the wall too. I must have been in the sixth grade by then.
I probably would not have done this had she caught me a couple of years earlier when I ran a bunch of pieced together wire out of my bedroom outlet, through the window, and then through the bushes to my club house. Damn near fried myself that time. Kids and electricity do not go well together. Mike

07-22-2002, 10:56 PM
My dad told me about the time grandpa strung lamp cord across the basement and out to the garage for a heater or some such thing. Lamp cord in those days was thin and flexible, with cloth insulation.

When the heater was turned on, the lamp cord shorted near the heater, and the arc burned right across the yard and back into the house like a lit fuse, until it stopped at the knob-and-tube wiring where he had spliced it in. The wires were separated too much for the arc to continue, which was part of the idea with knob and tube wiring.

Grandma had been delegated to throw the switch, so she got a goodly surprise. Being a Swede, she lit into grandpa about that, and didn't stop hollering at him for about 15 minutes.

Their fuses must have been pretty stout.....

[This message has been edited by Oso (edited 07-22-2002).]

07-23-2002, 11:24 AM
About 7/8 th grade, I became interested in electricity. Dad took a transformer from a AC/DC radio- which as I learned more, don't have a 60 cycle transformer in them. The tranformer must have been the audio out put transformer. It made a small spark, I loved sparks (still do), and no heat. Dad Tells me- Boy you dont just put those wires together like that- you just made a short circuit. Just as I sparked two wires, a pole transformer popped. Lights out every where. Old man thought fast, grabbed my stuff threw it out. Would not let me go out side to watch the power company change the transformer. Dad died at 81, still convienced, despite all my assurances that I could not have possibly been at fault. He claimed we would have been paying for a transformer if "they ever found out" what I did.

He found a Six volt auto battery with one bad cell and let me experiement with that. Big Sparks, could almost weld, but he "KNEW" it was safe. Melted several extention cords, big chunk out of mom's kitchen knife blade. Dad excused it all on the grounds that it was "educational"! Old ignition coils for making magnets and the little wire, with a switch fused explosively.
Grandsons mom says she wishes she had played more with me when she was a girl http://bbs.homeshopmachinist.net//smile.gif!

07-23-2002, 12:16 PM
When I was about 12, I awoke to a burning smell, combined with what smelled like a jar full of pennies. I got up to turn the light on, but it didn't work. I awoke my dad, and he checked the breaker for my bedroom, which had, of course, tripped.

As I stood and watched, he turned on the breaker and a huge spray of sparks emerged from under the bed. There was an extension cord under the bed that I had run to connect a TV, and it had shorted out. It wasn't overloaded, and it had appeared to be in good condition.

When we unplugged the cord and got the lights back on, there was a big charred spot on the hardwood floor and small burned holes in the blanket that had been covering me. The circuit breaker saved my life.

To this day, I hate extension cords, and avoid them whenever possible. I'd rather install a new receptacle than use one. A few weeks ago, I read about a family that perished because the extension cord they were using for a window A/C shorted out and started a fire. It reminded me how lucky I am to be here.