GFCI

[pkalis]

I have a GFCI at the end of an extension cord, I was told to put it at the source, what difference does this make.?

[terri]

Protects you personally from faults in the extension... like after you've left the extension run under your garage door for five months and the neutral wire is broken. For example.

[Bob Scott]

The GFCI can only protect you if a fault occurs downstream from the GFCI. If a dog chews on your extension, it would be better for the dog if the GFCI were at the source.<p>A GFCI works like this: A GFCI compares the current through the black wire (hot wire) with the current going through the white wire (neutral wire). They should be equal and going in opposite directions. The sum of the two currents should be zero. If there is an imbalance and the currents are not equal, the circuit will trip. eg: Man touches black wire and bathroom sink with the other hand causing current flow through his body. Black wire then has more current than returning current in the neutral. GFCI senses imbalance and the circuit trips. Curiously, a GFCI does not require attachment of the ground wire to its sensor.<p>Which brings up a couple of technical mistakes they made in one episode of the TV show "CSI". A man was electrocuted because the perpetrator had allegedly disabled a GFCI circuit by cutting off the ground terminal of the an electric drill's plug. Bull-spit!<p>In the same CSI episode the same victim also fell of a building after being shocked and "hit the ground with a velocity of 9.8 metres per second squared" according the character Grissom. Someone forgot their high school physics. "Metres per second squared" is a unit of acceleration, not velocity.<p>I assume that anything technical is suspect in this TV show. If I hear any biological or any technical info from CSI I don't trust it anymmore.<p>BTW, in a 3 phase sytem, all 3 hot wires and the neutral wire (if present) go through the toroidal core of the GFCI sensor. Again, the ground wire does not connect to the GFI's sensor core even though there may be a ground all the way from the source to the destination. There, that covers 3 phase Delta and Wye types.<p>Bob<p>[ September 22, 2005: Message edited by: Bob Scott ]</p>

[Robert Reed]

One added bit of trivia to Bobs post. The trip test shunts a 22K resistor from black to ground, there by prodcing an imbalance of approx. 5 ma to the circuit. This 5ma is the current that would be flowing thru you upon accidental contact, so apparently this is below the lethal current to a human being. Probably determined in a worst case scenario like from rt. hand to lt. hand where the current could pass thru the heart. Just remember that these devices do not protect you from shocks if you bridge black to neutral (White).

[Crowbar]

Off topic sorry, but glad to see I'm not the only one who noticed the error...<p>'Which brings up a couple of technical mistakes they made in one episode of the TV show "CSI". A man was electrocuted because the perpetrator had allegedly disabled a GFCI circuit by cutting off the ground terminal of the an electric drill's plug. Bull-spit!<p>In the same CSI episode the same victim also fell of a building after being shocked and "hit the ground with a velocity of 9.8 metres per second squared" according the character Grissom. Someone forgot their high school physics. "Metres per second squared" is a unit of acceleration, not velocity.'

[jollyrgr]

The GFCI needs to be at the source to protect along the full length of the cord, just as Terri and Bob stated. I want to throw my two cents in as I've had first hand experience with failed GFCI outlets.<p>I had put up a bunch of Christmas lights and had extension cords run everywhere. Most ends were taped/sealed and hanging out of the grass but the cords themselves ran through the grass to the various structures, decorations, and trees. It rained and certain decorations went out due to the water infiltrating the connections. While trying to correct the problems I picked up a wet extension cord and felt a definite tingle through my arm and feet. It turns out the GFCI failed and was not tripping. The extension cords were soaked and were "live" as the GFCI never tripped. If things were working correctly the GFCI would have tripped and I'd not felt anything; which is the way it should have been. If there were no GFCI (as what had happened in my case) the potential for shock exists.<p>Simple rule? Put the GFCI as far upstream as possible. In fact in my current house the outside outlets are controlled by a GFCI outlet inside the garage and inside the kitchen. This is to prevent someone from resetting the GFCI while outside in the rainy conditions.<p>[ October 02, 2005: Message edited by: Jolly Roger ]</p>