I AM BUMPING THIS Q BECAUSE I WANT SOMEBODY TO EXPLAIN HOW IN THE HL THIS GFI BREAKER IS GONNA WORK W/O A LOAD NEUT.
my text book says that a gfi "monitors the current of the hot and the neutral wires (talking about a 120V gfi) comparing them, and if it senses a difference of 3-5 mA, trips)
It gets more complicated in my eyes, when you have a 240v gfci, say for a hot tub, that enlists a neutral. How does the 50 DP gfci know whether the load is 240 or 120. Thus you may have a 240v motor and a 120v light. Obviously the current will be different one one phase. It seems that the device can monitor this somehow.
Anybody know????
The GFCI doesn't "need to know" whether there's a grounded circuit conductor, whether there's one or two hots, or anything else. The only 'requirement' is that the system is grounded, as compared to a floating (non-bonded) source.
With apologies to the previous posts, I'm goint to start from scratch:
A GFCI device works by comparing the total current, algebraically speaking, through all of a circuit's conductors. If all of the current being supplied through any conductor is being matched, or returned, through the other conductor(s), the net total current is zero.
This is accomplished by passing all of the circuit conductors through a ferrous ring which has a small coil wrapped aorung one side. As long as all of the currents match, no magnetism is developed in the ring, and no voltage is developed in the small coil.
Added: This is basically the same theory as why we want conductors of every phase in a metallic conduit: so the net current will be zero and not heat the conduit. The currents of each wire need not match for this to work, only that the total algebraically adds to zero.
If someone is getting a shock, that means that the current flowing through their body is flowing from the source, through one hot wire, but is returning to the source through a pathway other than another wire, meaning through the earth or another grounded, conductive surface.
The difference in conductor currents means there is now a sum of other than zero current, which creates a magnetism in the ring and a voltage in the small coil, which is amplified, and trips the GFCI mechanism when the detected current exceeds about 5 milliamps (5/1000 of an amp).
So, the very practice which makes the GFCI necessary, grounding one conductor of our supply, is also necessary for it to function. Whether there is one or more hot wires, or whether the grounded conductor is required by the load, don't matter. It only matters that some current is getting around the ring.
Note that someone getting shocked between two circuit conductors, again regardless of whether it's line-to-line or line-to-neutral, will not be protected, because all of the shocking current is passing through the ring, and appears to be normal current.
So, the GFCI doesn't compare hot-to-neutral, per se, but wire to wire (to wire). They could make a 3-phase GFCI, but I don't know if there is one. There are also GFCI devices that have the ring exposed, and you pass your own wires through the ring when you install it.
Hope that was helpful.