GCFI vs grounded

mrbb008February 5, 2013

I'm trying to understand the difference between the above. So I read on another forum the following: "A GFCI and a grounded outlet do two different things and protect against two different scenarios, and one is not a replacement for the other. A grounded outlet protects against a short in the WIRING, specifically, a wire coming loose and contacting a metal electrical box or housing. If the box or housing is grounded, the fuse or breaker will blow and presumably you'll realize you have a problem. If this DIDN'T occur, anyone who touched the box or housing would get a shock - whether you've got a GFCI outlet is irrelevant. A GFCI outlet protects against a short (or more commonly a current leakage) involving YOU and an electrical device plugged into the outlet. If an appreciable amount of current flows from the appliance through you to an earth ground (presumably because your hands are wet - typically GFCI outlets are installed near sinks), the outlet trips. So, in the first scenario, the problem usually is a frayed or broken wire, whereas in the second, the problem is carelessness on your part, because you didn't dry your hands."

BUT, if the appliance is plugged in a GCFI and the wires get lose (like in the grounded example above) as the person touches it wouldn't that trip the GCFI because then the current going out of the outlet would be different than the one going back in?

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If the GFCI outlet trips there is still power in the lines feeding the outlet. If the wires back there are loose and in contact with something the person touches, they'll get shocked.

    Bookmark   February 5, 2013 at 2:45PM
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The original reason for each is safety from shocks.

Ground were not added to provide a fault path, but to make sure exposed conductive material could not support a dangerous voltage.

This is why you are allowed to use a GFCI on a two wire branch circuit, and then install 3-hole outlets (with the stickers that indicate "GFCI PROTECTED" AND 'NO GROUND'.

The only shock a GFCI would not protect against is if you got across the hot and neutral and had NO other path to ground (like standing on a non-conductive rubber mat).

Your skin in most cases is high enough impedance to provide a decent amount of protection at lower voltages.
Hand to hand is often in the 10,000 ohm range.

Get wet (especially with sweat or soaking in a lot of water) and it gets down to the danger level pretty easily.

All the salt in our tissue makes us a decent conductor once you get past the dry layer of skin.

Around 40 V is considered easily detected.
Below that it gets more variable, but by about 15 V it is very hard to detect unless you are very wet.

Just as an example, while an external defibrillator uses hundreds of volts to drive current through the skin at each paddle and reach the heart with enough current density, in open heart surgery it only take a very few volts to start the heart up with cardiac paddles touching the heart itself.

    Bookmark   February 5, 2013 at 3:42PM
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A GFCI device compares the electrical current in the Hot leg to the current in the Neutral leg. If there is a difference, even a very small one, it means that the current is going somewhere it shouldn't, like through a person, and the device trips. GFCIs are required in 'wet' locations since water is the major cause of the current going astray, and does not need a 'loose wire' to create the hazardous condition.
A ground connection provides a 'path of least resistance' for current or a charge to be dissipated in the event of an equipment failure or mishap. The benefit is that it keeps the exposed surfaces of the equipment from being electrified and waiting for you to touch it and provide a low resistance path to ground.

    Bookmark   February 5, 2013 at 3:46PM
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An electrical ground and a circuit breaker work together to provide a means to protect from the buildup of excess heat. A breaker operates because of excess current flow/heat. This is why someone can be electrocuted and a breaker never trips. The breaker doesn't work unless there is excess heat. Simply put, a breaker is designed to prevent fire. A GFCI does exactly as described, constantly monitors the in/out electrical current flow. When the two don't match, the GFCI trips.
Most men will get this simple explination. A GFCI is like a woman. She measures the amount of effort she is putting into a relationship against the amount of effort her significant other is putting into it. When her efforts exceed her significant other, bam... she cuts them off. Crude, but an effective explination when trying to teach a room full of firefighters. I apologize if I've offended anyone with more cultivated sensabilities.

Most important to the conversation is to realize when a GFCI is tripping, it's working. There is something wrong with your situation. Do not bypass the GFCI and run a cord from a normal outlet. This is asking for trouble. The biggest danger to folks are the soft sided swimming pools they sell at Wally World. Folks buy them and then run a cord from the garage outside. If the outlet is not protected by a GFCI then no personal protection is available. A regular breaker is not designed for personal protection. If you ever see this, please correct it immediately. This is how young children die every year.


    Bookmark   February 9, 2013 at 12:10AM
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Ron Natalie

One more thing. It's still possible to kill someone on a GFCI circuit if the person touches both sides of the protected circuit and not some external ground.

    Bookmark   February 9, 2013 at 8:51AM
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The difference is between being shocked and electricuted. Our home was built in 1959 and was not a grounded system. Over the years we gradualy rewired the home to a grounded system.

    Bookmark   February 10, 2013 at 10:12AM
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"The breaker doesn't work unless there is excess heat."

Not correct.

Residential breakers have been thermal-magnetic for many years.

They are designed for a 'slow blow' action on overloads up to a couple 100s of percent based on a thermal element.

At very high fault currents or shorts (1000%) they have a magnetic trip tat opens them nearly instantly.

Magnetic only breakers are occasionally used in commercial applications wen the circuit is unlikely to have overloads to provide the fastest protection.

    Bookmark   February 10, 2013 at 12:55PM
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