Question about portable GFI

Discussion in 'General Electronics Chat' started by someonesdad, May 10, 2011.

  1. someonesdad

    Thread Starter Senior Member

    Jul 7, 2009
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    I have a small GFI (ground fault interrupter) that is intended for portable use -- the tag says Tower Mfg. Co. model 30339 (similar to this). It's like a small wall wart with a standard 120 VAC 15 A plug on one side and a 15 A outlet on the other side.

    This GFI specifically states "Do not use extension cord between GFCI and power outlet".

    I was curious why they restrict such use. The only thing that comes to mind is that the extension cord wouldn't be protected and sometimes they have outlets in the middle of them. Are there other reasons?
     
  2. mbohuntr

    Active Member

    Apr 6, 2009
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    It might also be for the same reason not to use extension cords with high current loads (light guage wire), and to keep the GFI out of wet or destructive enviroments. I think my pressure washer is set up the same...
     
  3. steveb

    Senior Member

    Jul 3, 2008
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    Yes, exactly. The extension cord is (presumably) not protected. An unprotected power cord is danger by itself, whether or not a GFI is plugged into the end of it. It really doesn't matter whether there is other outlets in the middle. Imagine if you are standing in water and you drop the end of the extension cord (with the GFI attached there) into the water. The GFI can only protect other points down the line from it. It doesn't protect its own input connection.
     
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
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    I think the more likely reason is that there may be no ground wire in the extension cord and that would cause the GFI not to work. For instance if you use a two-wire extension or have one with the ground pin cut off. A GFI requires the ground wire in order to work.
     
  5. steveb

    Senior Member

    Jul 3, 2008
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    That is a good point, but why doesn't the warning say, " make sure you use a grounded power outlet and don't defeat the ground connection"? There are two wire outlets still around and one can easily plug into one with a 3 to 2 wire adaptor, or use a 3 to 2 wire adaptor in a grounded outlet and defeat the ground.

    Or, why doesn't it say, "if you use an extension cord, make sure you use a 3-wire extension cord with a ground".

    The main issue is that the GFI is to protect people from electrocution. Any extension cord before the GFI is a hazard, - a BIG one. It is one of those "say goodbye to your spouse and kids first" kinds of mistakes. And, only God knows if it will be you, your spouse, or one of your kids that pays the ultimate price. Basements and bathrooms are especially hazardous.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    A GFCI does NOT need a ground wire to function properly.

    The reason stated above about the extension cord not being protected if used before the GFCI is the reason for that label (and UL requires it)
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    100% correct.
     
  8. mbohuntr

    Active Member

    Apr 6, 2009
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    I think Steve's origional explanation said it best. The GFI is useless if your electrocuted while plugging it in. Mains are usually required to be installed in non hazardous enviroments. If there is access to water, they are required to be protected with a GFI at the panel box.
     
  9. someonesdad

    Thread Starter Senior Member

    Jul 7, 2009
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    Thanks, fellas -- I'm aware of safety aspects of GFIs and that an extension cord would be unprotected if the GFI was plugged in on the end. However, the message didn't indicate why it was there, so I thought that there might be a physical reason I was unaware of. It sounds like mcgyvr's statement that the UL requires it is the reason I was curious about.

    I consider the GFI one of the most important safety inventions. I remember in the 60's how it bugged the hell out of me when some idiot took a hack saw and cut off the grounding plug of an appliance cord because it wouldn't fit in the existing outlets (IIRC, the grounding plugs and receptacles started appearing in the early to mid-60's). These people didn't understand what protection they were giving up (and I'd assume 99.9% of the population still hasn't a clue).

    For those of you who are interested, you can look up the LM1851N datasheet and see a typical GFI design on page 6 (albeit an older one, as GFIs after 2002 are required to stay off if they trip and the power goes off and comes back on). It's a clever design -- especially coupling the 120 Hz back into the hot/neutral conductors to detect neutral-ground faults. They're clever, even if they're just a straightforward application of Ampere's Law. Oh, and don't throw those old busted GFIs away -- salvage those toroidal current transformers, as they could be useful in other projects. Be careful, as they're typically wound with small 40 gauge wires.

    I've made some measurements on a few of the GFIs installed in my house. The typical neutral to ground resistance that will cause a trip is a quite-repeatable 27 ohms. The hot-to-neutral trip currents I've measured are usually in the range of 4.5 mA to 5 mA.
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    As just about anyone knows when they open a manual for a product, UL requires LOTs of labels to cover any situation of misuse they can think of.. Even those that are impossible for some products but because you can't cover everything in 1 testing requirements sometimes you need to put labels that make no sense just to pass testing. However this warning we are talking about on the GFCI makes perfect sense.

    It only protects something upstream of the device.. So a extension cord before the device is not protected..hence the warning.

    One of my favorites is that the FDA requires a warning label on Peanut M&M's stating "This product contains peanuts"
    Yeah no crap its a bag of peanut M&M's.. It better contain peanuts. But too many people are stupid.

    My opinion is to remove all warning labels and let the stupid die off but I don't think that's gonna happen.. They spread faster than rabbits.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Or better yet "Processed in a facility that may process tree nuts" on a pecan log. :)
     
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