Is a GFCI like an LVDT? (long, may be out there...)

Factual Questions
1

July 13th, Straight Doper Worldwide Home Wiring Day. :slight_smile:

Speaking of these widgets, how are GFCIs built? I assumed that they were similar to an LVDT, or linear voltage differential transducer.

A GFCI acts as a current differential solenoid (to make up a name); when the current in the hot wire of a circuit exceeds the current in the neutral wire by a small amount (10 milliamps sounds right), the GFCI trips, opening the circuit.

Now, an LVDT is used to record linear motion as a difference in voltage. I believe that they are designed as a transformer with two secondary coils wound separately, side by side, on a round hollow core; a prmary coil is shared by both secondaries. An inductor passes through the core. As the inductor moves back and forth inside the core, the difference in output of the two secondary coils will change.

I assume that the GFCI is designed like the LVDT in that the current on each leg, or a small part of it, goes through a coils which act as electromagnets on an actuator shaft that goes through the middle of both coils. If the currents for each leg are equal, then the magnetic fields in each coil are egual and the shaft stays in place. If there’s a ground fault, then the coil on the hot side will pull more than the neutral one, causing the actuator to move out of position, tripping the interrupter.

So, having said all this, is this how a GFCI is designed?

2

According to House Wiring Simplified:

“These devices sense ground (leakage) currents when the currents entering and leaving the circuits are not identical and automatically turn off the primary power within 25 to 20 milliseconds.”

As to how it monitors current, I’m pretty sure it’s all solid state. No coils or moving inductors, although I did see one do an incredible simulation of a jacob’s ladder once. It does have a mechanical disconnect circuit breaker that pops the button which I believe operates thermally (like a regular breaker) as opposed to a solenoid.

3

cornflakes is right, mostly. There’s a coil around both the hot wire and the neutral. When the current in both is equal, and in phase, no current is induced in the coil. If the hot wire goes to ground, the hot (phase) current exceeds the neutral (because some current goes to ground) and induces a current in the coil, causing the GFCI to trip mechanically.
I think that a LVDT is too complex to use in a GFCI, and that kind of precision isn’t needed.
Peace,
mangeorge