Lights & switches

[Paul W. Abernathy]

During the normal operation of a typical 2-wire circuit, the current returning to the power supply will be equal to the current leaving the power supply (except for some very small leakage). If the difference between the current leaving and returning through the current transformer of the GFCI protection device exceeds 5 mA (± 1 mA), the solid-state circuitry opens the switching contacts and de-energizes the circuit. The mA used above stands for one thousands of an amp, so 5 mA is equal to 5/1000th of an ampere.

Anyway......here is a good link that explains it probably better than I can tonight as I am tired..been putting together a seminar material all day as I took off today.

http://ecmweb.com/mag/electric_gfcis_work/

[Jason A. Sieg, CMI]

Originally Posted by brepanshek

Hay Paul I was trying to explain the working of the GFCI when connected to a ground and when not. Why doesn't it trip when I use my tester, is it because the tester creates a short to ground? When I have to press the test button on the GFCI outlet to test when there is no ground wire going to the outlet what is accually happening?? Is the GFCI sensing something across hot and neutral?? I know what to do to test but want to explain what is happening within. thanks

I am not going to try to explain anything like Paul can, but...

A GFCI is continuously measuring the current difference between the hot & neutral. They should be the same at all times. When they are different then it trips. A GFCI tester shorts to the ground to create the difference. The Test Button uses circuitry in the GFCI it's self to create the difference without using the ground.

Very basic wording that most people would except. Not entirely accurately written but it gets the point across.

(I'm half a sleep right now. I will probably read my post in the morning and yell at myself!)

[Jae Williams]

A while back someone posted this...it may help.

[A GFCI receptacle, new or old, can't trip with a plug in tester unless there is an EGC connected to the GFCI. There is no path for the test current to flow on without an EGC. When testing GFCIs with the internal test button, you must test for voltage on the GFCI receptacle after you push the test button. A GFCI receptacle that has the power connected to the load terminals will still have power( on older models) on the receptacle even when the button shows that the device has tripped.

The GFCI works because it detects the difference between the grounded and ungrounded conductor.


GFCI is a Ground Fault Circuit Interrupter. The Ground Fault Interrupter is a receptacle that has the ability to open or disconnect the power from the output of the receptacle. The Ground Fault Circuit Interrupter is a breaker that has the ability to disconnect the power from a circuit. The ground fault senses a difference in the flow of current from the hot wire through the neutral, if that difference is about 5 milliamps or more the ground fault will trip the circuit out. It actually assumes that if the current is not flowing in the neutral it is flowing through something else. Some motor windings have sufficient losses to cause one to trip out so don't use a GFCI circuit for a refrigerator or washer outlet. You should use (and the NEC requires) the use of GFCI protected outlets within 6 feet of a sink, anywhere in a bathroom, in a garage or outside; anywhere an outlet can be reached from a water source, a wet area, or earth ground, you should use GFCI protection. A GFCI receptacle has a line side (incoming power) and a load side (outgoing power). The receptacle will not work if the incoming power is connected to the load side of the receptacle. Connect the incoming power to the line marked terminals and the continuation of the circuit (the next outlet) to the load terminals. The one GFCI will protect all the following plugs or receptacles connected in this way. Even if you don't have a continuation of the circuit, connect the power to the line side of the receptacle. GFCI receptacles and GFCI breakers have a test button that should cause the circuit to trip, operate the test button after installing and regularly there after to be sure it works properly.]

[Paul W. Abernathy]

actually I think you did a WONDERFUL job Jason at explaining it yet keeping it VERY basic....BRAVO !

[Jason A. Sieg, CMI]

Originally Posted by pabernathy

actually I think you did a WONDERFUL job Jason at explaining it yet keeping it VERY basic....BRAVO !

Thank You Paul! You brightened my morning!

It tends to be differcult to get compliments here when you try to keep things simple. That was the explanation I used to use when I worked at Builders Square in my late teens/early twenties, when I was the electrical Guru.

[brepanshek]

perfect guys

[Paul W. Abernathy]

lol...ok now fella.......ain't no GURU's working at Builders Square...thehehhe