Replies

1. 
   FastEddie | Sep 08, 2005 06:26am | #1

   Not very well.

    

    

   "When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it."  T. Roosevelt

1. 
   DanH | Sep 08, 2005 01:51pm | #5

   Fast Eddie is wrong -- it works quite well in a 2 wire circuit.

1. 
   FastEddie | Sep 08, 2005 03:48pm | #6

   Well that's two.  I thought it had to be a grounded circuit.  Another reason I sub all my electrical work.

     

   "When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it."  T. Roosevelt

1. 
   DanH | Sep 08, 2005 05:07pm | #7

   Yep, what the others said about comparing current in and out (via a current transformer). The test button may not work without a ground (though I believe most do), and a feature that detects short between neutral and ground will (of course) be inoperative, but the GFCI will do it's job and protect people from shock.(Of course, a GFCI should never be relied on to protect folks from stupidity -- it's intended as a "fallback" safety measure, not a first line of defense.)

2. 
   4Lorn1 | Sep 08, 2005 06:59am | #2

   A GFCI, for short GFI, on a standard 120v circuit works by measuring the amount of current that travels down the wires to the load and comparing this with what flows back from the load. Any significant difference in the readings is taken to mean current is diverted from where it is supposed to be going and the GFI turns off the juice.

   As an example let's say your toasting some bread. The bread sticks and the toaster stays on burning the bread. Without thinking you grab a butter knife and stick it down into the slot without realizing your other hand is on the metal sink.

   When the knife contacts the coil some of the current travels up the knife, arm to arm and through to the sink and on to the grounded plumbing.

   What the GFI on the circuit feeding the toaster 'sees' is that the toaster is turned on when the bread is inserted and the lever pushed down. The amount of current going to the toaster is the same as the amount of current coming back.

   When you contact the knife to the coil in the toaster. Some of the current travels by way of you to the sink. The GFI 'sees' the current going to the toaster go up slightly while the amount of current coming back is divided between what is coming back through the taster's cord and what is going through you. The current going out and the current coming back are no longer balanced. Within a few milliseconds the GFI trips and the power to the toaster is cut off.

   You get a shock but it is a very short one. Sometimes it is stopped so early and quickly that you may not feel it. This sort of shock is unlikely to stop your heart or otherwise cause direct harm.

   A GFI designed and hooked up to a 240v circuit works much the same except instead of monitoring one hot and the neutral it compares two hots and a neutral.

   Be aware that a GFI has limitations. If you get between the hot and neutral the GFI will not normally trip. You can be cooked like a hotdog while the GFI sits fat,dumb and happy. You 'look' like a legitimate load unless some of the current is diverted to ground.

   Also GFIs are not foolproof. They are electro-mechanical devices that wear out and can be damaged both electrically or physically. They are safety devices. Great when they save a life by preventing a worse shock but don't take them for granted. Don't assume it will save you from yourself. Don't make the GFI the first or main line of defense. Use common sense and good electrical safety practice and then, when all precautions fail, be grateful when the GFI saves the day.

1. 
   BillHartmann | Sep 08, 2005 07:29am | #3

   "A GFI designed and hooked up to a 240v circuit works much the same except instead of monitoring one hot and the neutral it compares two hots and a neutral."While that is the concept in fact they work a little differently.All the wires (one or 2 hots and the neutral) go through center of the a torriod transformer. They are wired so that the magnetic field generated by them cancel if there equal currents.There is a secondary or sense winding on the transformer that generate a signal proportianal to the difference in currents and is used to trip the relay.

1. 
   4Lorn1 | Sep 08, 2005 08:49am | #4

   Figured I would keep it on a conceptual and functional level and avoid the science lesson.Yours is a fuller explanation. Essentially a torroidal coil around all the circuit conductors, excluding the ground of course, using the ever lovin left-hand rule. Balanced the forces cancel. Unbalanced the control IC, hooked to the coil, senses a current. The IC controlled circuit, when activated, drives a solenoid that mechanically trips a set of contacts that are being forced open by a spring. The effect being that of a NO relay being mechanically held closed and the mechanical stop being removed.Fun to disassemble GFIs. The parts can be used in interesting ways. And to think it all started with me shoving tweezers into wall outlets.

3. 
   bosn | Sep 09, 2005 03:28am | #8

   I'll  add this...

   Because most GFCI testers allow a bit of current to go to ground to cause a fault,  a GFCI tester and the test button on some GFCI's will not work if there is no equipment ground connected to the GFCI. 

   If you wish to test a GFCI connected to a two wire system, you can use one of those adapters that allow a three prong plug to be inserted into a two prong receptacle.   Use one that has a little green wire coming out of it.  Connect the green wire to ground.  Plug the adapter into the GFCI and now you can test using either your hand plug-in tester.

   If you haven't drawn blood today, you haven't done anything.