Safety question about 2/3-prong power cords and gfci outlets

[PaperclipGod]

Retailer near me is having a pretty good sale on 2-prong, 14 gauge outdoor extension cords. 90 feet for $10. But before I pick up a couple, I was hoping you smarty-pants might be able to help me out with some safety questions!

This is my understanding of how this stuff works:
3-prong cords are used to ground the enclosure of the powered device to prevent any potential electrocution hazard. Any errant current that manages to charge the enclosure is just grounded back down the cord instead of through the person touching the appliance. GFCI outlets are like "smart" three prongs, in that they monitor the amount of current flowing out of the wall on the hot wire and the amount of current returning via the neutral/ground wire. If there's a discrepancy (indicating that some of the current is being grounded elsewhere, e.g. through a person), it cuts the power.

Ok, so assuming that's a correct understanding, am I decreasing my safety margin by using a 2-prong extension cord instead of a 3-prong cord with a GFCI outlet? The only difference I can imagine is that with a 3-prong, I'd theoretically never be shocked, because the moment an errant wire grounds to the enclosure, it'd just be grounded back to the earth ground anyway. With a GFCI, I might get shocked, as there'd be a delay between when the current grounds through me and when the discrepancy is picked up by the GFCI outlet. It'd be such a small period of time, though, that I can't imagine it'd make a tangible difference.

And m8's -- feel free to call me a retard if my understanding is totally ignorant, but at least explain why. Cheers!

 

[ktehmok]

I can't believe someone sells a 90' 2 wire extension cord any more. Spend the extra few buck for a 3 wire.

Your assessment of GFCI devices is pretty accurate. One thing that is almost always neglected though, is that no one ever tests them monthly like they are supposed to. They are not a "install & forget" device. And they do fail, regularly. In the 15 years I've done electrical, I have come across around 30+ GFCI receptacles that simply stopped working. Not shorted out or damaged, they just stopped working. And they still had current going through them.

You will likely receive an electrical shock even with GFCI protection, enough to stop your heart. It all depends on your situation when the shock occurs. A GFCI is never a replacement for safe electrical practices. Your first safe practice is to not buy a 2 wire extension cord because it's cheap.

 

[DrPizza]

$10 for 90 feet of 14 gauge? That price sounds too good to be true.

 

[Vette73]

$10 for 90 feet of 14 gauge? That price sounds too good to be true.

Yep a lot of cheap ones ARE over rated and even have fake UL stickers on them.

Some local stations cut open the cheap ones and they do not have anywhere near the amount of copper they say they do. Like a 16ga cord only had like 22ga copper in it.

 

[PaperclipGod]

I can't believe someone sells a 90' 2 wire extension cord any more. Spend the extra few buck for a 3 wire.

Your assessment of GFCI devices is pretty accurate. One thing that is almost always neglected though, is that no one ever tests them monthly like they are supposed to. They are not a "install & forget" device. And they do fail, regularly. In the 15 years I've done electrical, I have come across around 30+ GFCI receptacles that simply stopped working. Not shorted out or damaged, they just stopped working. And they still had current going through them.

You will likely receive an electrical shock even with GFCI protection, enough to stop your heart. It all depends on your situation when the shock occurs. A GFCI is never a replacement for safe electrical practices. Your first safe practice is to not buy a 2 wire extension cord because it's cheap.

Wow, I didn't know GFCI's could just fail like that. How would I go about testing if they're still working properly, other than shorting them out?

I guess I'll stick to a 3-prong cord... although I could probably splice one together from two of the $10/90ft cords for cheaper than I could buy one.

BTW, I googled and found this, I think it's the same type of cord:
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200401169_200401169

90 ft for $14.

 

[ktehmok]

Wow, I didn't know GFCI's could just fail like that. How would I go about testing if they're still working properly, other than shorting them out?

I guess I'll stick to a 3-prong cord... although I could probably splice one together from two of the $10/90ft cords for cheaper than I could buy one.

BTW, I googled and found this, I think it's the same type of cord:
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200401169_200401169

90 ft for $14.

They have a "Test" button.

 

[disappoint]

The only difference I can imagine is that with a 3-prong, I'd theoretically never be shocked, because the moment an errant wire grounds to the enclosure, it'd just be grounded back to the earth ground anyway.

It is possible to touch a hot lead without touching the enclosure which is grounded however. Also possible for the whole unit to land in the tub with you. Want to theorize what happens in that case?

 

[PaperclipGod]

They have a "Test" button.

Ah, ok, I thought you meant those buttons were part of what would malfunction.

 

[PaperclipGod]

It is possible to touch a hot lead without touching the enclosure which is grounded however. Also possible for the whole unit to land in the tub with you. Want to theorize what happens in that case?

Yeah... i was using "theoretically" as in "all other variables excluded."

 

[Red Squirrel]

Those cords would maybe do fine in summer time if it's not raining. Great for an electric lawnmower or weed whacker. It would almost be tempting to use those for Christmas decorations too. Really they look half decent heavy duty, if you're careful you could maybe get away with it. Make sure they are plugged all the way and if you really want to play it safe duct tape the ends together (ex: end of christmas lights + that cord).

Now I wonder, if a GFCI can detect that current is going to another ground. So if you touch the hot at the end of the 2 prong cord and it goes through you then to the ground where you're standing, if it detects.

 

[WHAMPOM]

Retailer near me is having a pretty good sale on 2-prong, 14 gauge outdoor extension cords. 90 feet for $10. But before I pick up a couple, I was hoping you smarty-pants might be able to help me out with some safety questions!

This is my understanding of how this stuff works:
3-prong cords are used to ground the enclosure of the powered device to prevent any potential electrocution hazard. Any errant current that manages to charge the enclosure is just grounded back down the cord instead of through the person touching the appliance. GFCI outlets are like "smart" three prongs, in that they monitor the amount of current flowing out of the wall on the hot wire and the amount of current returning via the neutral/ground wire. If there's a discrepancy (indicating that some of the current is being grounded elsewhere, e.g. through a person), it cuts the power.

Ok, so assuming that's a correct understanding, am I decreasing my safety margin by using a 2-prong extension cord instead of a 3-prong cord with a GFCI outlet? The only difference I can imagine is that with a 3-prong, I'd theoretically never be shocked, because the moment an errant wire grounds to the enclosure, it'd just be grounded back to the earth ground anyway. With a GFCI, I might get shocked, as there'd be a delay between when the current grounds through me and when the discrepancy is picked up by the GFCI outlet. It'd be such a small period of time, though, that I can't imagine it'd make a tangible difference.

And m8's -- feel free to call me a retard if my understanding is totally ignorant, but at least explain why. Cheers!

Unless you are using three prong tools a GFCI will do nothing. Most home-owner grade tools are polarized two prong self insulated. Table saws, air compressors and contractor grade tools (three prong)pull enough amps to need twelve gauge wire for a ninety foot run.
Hmmm? A ninety foot cord seems a little odd in lenght, is it green or white and meant for christmas lights?

 

[geno]

It is possible to touch a hot lead without touching the enclosure which is grounded however. Also possible for the whole unit to land in the tub with you. Want to theorize what happens in that case?

GFCI kicks in and nothing bad at all happens? :biggrin:

 

[Colt45]

Now I wonder, if a GFCI can detect that current is going to another ground. So if you touch the hot at the end of the 2 prong cord and it goes through you then to the ground where you're standing, if it detects.

That's what it's for! :awe:

 

[Red Squirrel]

That's what it's for! :awe:

I know it works that way if current goes to it's own ground, but was not sure if it did if it goes to another ground. If that's the case then 2 prong vs 3 prong extension is pretty much a non issue if using it for items that don't require 3 prong (like a car block heater). They'd work fine for stuff like Christmas lights.

 

[Colt45]

If the current through neutral and hot are UNequal, it assumes it's going to *some* ground *somewhere* (through you, perhaps) and trips.

 

[Red Squirrel]

If the current through neutral and hot are UNequal, it assumes it's going to *some* ground *somewhere* (through you, perhaps) and trips.

Yep just confirmed it and it works. I was not sure if GFCIs only checked it's own ground but it seems to check the neutral and hot, which makes sense.

I took a 2 wire light socket, put both wires into the proper terminals and it lit up as expected. I moved the wire from neutral to ground and it tripped. I then took a 3 prong extension cord and plugged it into a non GFCI circuit, and put the wire to the ground of that cord, and it also still tripped.

The test button is nice and all but a real life scenario test is always nice.

 

[DrPizza]

Unless you are using three prong tools a GFCI will do nothing.

This is incorrect. How the GFCI works has nothing to do with that ground wire going to tools. Quite simply: if the hot and neutral don't have the same current, the GFCI trips.

It is worth pointing out that a GFCI will do nothing to help you if you're on the other side of a transformer.

 

[Red Squirrel]

This is incorrect. How the GFCI works has nothing to do with that ground wire going to tools. Quite simply: if the hot and neutral don't have the same current, the GFCI trips.

It is worth pointing out that a GFCI will do nothing to help you if you're on the other side of a transformer.

That's good to know, so basically put if you plug an isolation transformer in a GFCI it wont do nothing if you have an arc fault on the other side?

 

[disappoint]

That's good to know, so basically put if you plug an isolation transformer in a GFCI it wont do nothing if you have an arc fault on the other side?

Right because neutral and hot are running through the primary of the transformer and the current will be equal, even if you touch the hot on the secondary. So in essence to protect you, you'd need a GFCI on the secondary circuit of the transformer. Unless the transformer is a robot that turns into an 18 wheeler. In which case, there is no escape.

Well except "arc" fault, I think you mean ground fault?