ECN Forum Forums General Topics of the Trade General Discussion Area Electrical-related motion picture trivia

I saw a rerun of an old CSI episode this past weekend. In this episode, a construction worker fell to his death. In the scene investigation, they found the dead worker's power tool had a scorch mark on it.
They immediately suspect an electric shock made him fall. The foreman says the GFCI would have protected him from a shock.

Grissom follows the cord back to the spider box and unplugs the tool. He says something to the effect "It won't protect him if the third prong is not connected" and shows where the ground prong has been cut off of the cord.

I'm like "WHAT?!"

I tried to point this out to my better half, that GFCI's are specifically used for protection when there is no ground, but she doesn't even look at me.

Seems to be some confusion on the part of the writers between "ground falt protection" and a "certified grounding system". Studio electrician should have been able to point this out.

I can't go along with you there Kale. The safety is severely compromised. Consider that the same problem that would've caused a path from hot, through you to ground, may have also created a connection between tool neutral and tool ground. If you are isolated in space from ground (fiberglass ladder and haven't wet yourself enough yet), you don't get the benefit of a ground fault current flowing through you to trip the GFCI. The current that will help the fall to kill you is flowing from hot, through you, to neutral & floating tool ground. I wouldn't bet my life on that one.
Joe

[This message has been edited by JoeTestingEngr (edited 01-18-2006).]

Sorry, Joe, but I have to agree with Kale.

First of all, the EGC has no bearing on GFCI operation. A GFCI detects only one thing: a current difference between the two circuit conductors. If a person is truely isolated from earth, and from any grounded surfaces, and they grasp a tool with a hot-to-case fault, they won't trip the GFCI.

Since no current is being diverted from the intended pathway, they also won't receive a shock; there's no current path through their body parts. Now, the hard-wired EGC is a different story. That is there to operate a standard breaker, or a fuse, as a result of a direct-to-ground, full-current fault.

Losing the EGC pin leaves the tool case floating, and if there's an internal hot-to-case fault, only a GFCI can sense the shock current level, and not require an over-current device to open. The difference is the current required for operation; breakers and fuses require amps; GFCI's require only shock-level current.

In other words, standard 3-pin grounding methods operate to prevent the shock hazard, while GFCI's operate by sensing the shock.

(L)Sorry, Joe, but I have to agree with Kale.

(J)OK, let's work through it.

(L)First of all, the EGC has no bearing on GFCI operation. A GFCI detects only one thing: a current difference between the two circuit conductors.

(J)Absolutely, but the only way to have a difference is for some of what should be going back through the sensing coil, to find an alternate path. That would usually be established through the grounding prong and perhaps other path ways.

(L)If a person is truely isolated from earth, and from any grounded surfaces, and they grasp a tool with a hot-to-case fault, they won't trip the GFCI.
(J)We agree there but look at what else I mentioned, the possibility that this defective tool is faulted between neutral and tool ground. Once a tool is broken enough to let the hot contact me, who am I to presume that the neutral isn't faulted to ground?

(L)Since no current is being diverted from the intended pathway, they also won't receive a shock; there's no current path through their body parts.
(J) Sure it is. We have parallel paths. Current is flowing from (L) through the drill to (N) which is common to floating (G). We also have current flowing from (L) through me to floating (G) because it is common to (N). The drill is turning and I am burning! Had the grounding pin been intact and the leakage from hot was after the switch, the GFCI would trip as soon as the trigger was pulled. The difference current is flowing through the grounding conductor. If the leakage was before the switch, it would trip as soon as you plugged it in.

(L)Now, the hard-wired EGC is a different story. That is there to operate a standard breaker, or a fuse, as a result of a direct-to-ground, full-current fault.
(J)No chance of that saving us. Our only hope is for the tool to short out badly enough for an overcurrent trip of the feeder breaker before we're dead. It's been nice knowin' you.

(L)Losing the EGC pin leaves the tool case floating, and if there's an internal hot-to-case fault, only a GFCI can sense the shock current level, and not require an over-current device to open.
(J) I think we already agreed that GFCIs don't understand high falutin' things like "shock current level" It's sitting there going, Difference/TRIP, No difference/No Trip.

(L)The difference is the current required for operation; breakers and fuses require amps; GFCI's require only shock-level current.
(J)Which is a few mA difference current pre-determined to be less than lethal.

(L)In other words, standard 3-pin grounding methods operate to prevent the shock hazard,
(J)And providing a possible path for a difference current.
(L)while GFCI's operate by sensing the shock.
(J) I know "shock" has less letters but a GFCI still doesn't understand anything but Difference/Trip, No Difference/No Trip.

(J) But since we're pals now and playing with our bad tools and bad cords on good ladders, I'll meet you half way. If you promise to stay insulated from ground and tie the green wire from the GFCI cord to the neutral wire between the wall & GFCI module, I'll agree that it will function as a GFCI. We'll just need to call it an Alternate return current around the sensing coil circuit interruptor.
Joe

PS: Of course, I would hope that both of us would just reterminate the cord with a new 3-prong plug.

[This message has been edited by JoeTestingEngr (edited 01-19-2006).]

i have a pic of the plug at a different angle.

<IMG SRC=\"http://img359.imageshack.us/img359/1420/1955docc4ep.th.jpg\">
Click to enlarge

notice the cable clamps and cable used. doesn't look like it could handle 1.21 jigawatts does it. lol

[This message has been edited by Wizzie Electric (edited 01-19-2006).]

But it's the Doc of the 1950s who uses that connector, and at that point in time he had not yet done any time traveling. Of course, the Doc could have seen that connector in the 1980s or later, thought "Hey, that was just what I needed 30 years ago" and popped back in time to leave it in his own 1950s' workshop, thereby providing a neat closed time loop explaining the connector's presence in the 1950s. Of course, he would have recognized that connector when he found it in the 1980s, because he had already used it in the 1950s!

Boy, don't these time paradoxes make your brain work hard?




[This message has been edited by pauluk (edited 01-19-2006).]

I agree that a ground is important. The tool shown had a metal case and was not double insulated. My argument was with Grissoms explanation, mainly that the GFCI would not work without a ground prong.

As has been discussed on these pages before, you are permitted to use a GFCI to replace a two wire receptacle.

As the story progressed, the sabatour had 'reversed the wiring' in the tool. I gave Grissom the benefit of the doubt on this one, assuming he meant that the tool frame was connected to the line so that the ground was hot.
(but wouldn't that still trip the GFCI?)

For an easy explanation of how GFCI's work see http://www.leviton.com/pdfs/gfci/GFCI_Brochure.pdf