Ground fault circuit interruptors

Ground fault circuit interruptors

GFCIs do not measure the difference between voltage in (through the hot wire) and voltage out (through the neutral return). Instead, they measure the current differential. They do this by passing both the hot and neutral lines through the center of a toroidial current transformer (essentially a voltage transformer with a fixed, known resistor across the secondary output). If the current through the hot line exactly equals the current through the neutral return line, the voltage level, and therefore the current measured, are exactly 180 degrees out of phase and, as such, “buck out” or cancel each other out, and the induced voltage on the secondary is equal to zero. If they are not the same, a small voltage is induced across the secondary which is proportional to the difference (exactly how much is determined by the number of windings on the secondary, typically around 1000 turns or so) between them. Here’s where the resistor across the secondary comes into play: per Ohm’s law, the current through a resistor is equal to the voltage across it divided by the resistance, and there is circuitry in the GFCI which monitors the voltage across the resistor and “computes” the current passing through it. If the measured current is greater than 5 milliamperes (the lowest value determined to cause heart defibrillation…not 100mA as the article states) the device trips and shuts off the flow of current to the connected device. What the Test button does is actually insert an active network (composed of transistors, resistors and other components) into the hot lead to “burn off” 5mA of current, so that the imbalance will trip the device if it is working properly.

Yes, I do know it’s “fibrillation” not “defibrillation”. It was late…sue me.

Your complaint boils down to:

In the sentence “If volts in exceeds volts out by more than a certain percentage, that means some of the current is taking a detour, possibly by way of your trembling body.”, the word “volts” should be “amps”.

So sue Cecil.

Well, yes. but wasn’t my version so much more interesting and informative? :smiley:

Informative, without a doubt, interesting only for us who work with them I imagine. :slight_smile:

By the way I 100ma is the lowest value determined to cause heart fibrillation but the trip setting of the GFCI remains at 5ma.
From the link I give below

“In fact, as little as 100ma (milliamps) or
1/10th of one amp can cause a fatal heart fibrillation.”

You would not want it to trip at the threshold level, its purpose is to safeguard so why push your luck.

I used to work for a transformer manufacturer, which, among other things, made current transformers–some of which got used for differential current protection on commercial airliners. So, I guess to me it’s interesting. :cool:


You are on the money concerning GFI being current driven, since its the anomalous current that transforms the ticker into a “bag o’ twitching worms”, not the voltage.

Given the fact that I never did master the basics of applying ac/dc circuit analysis (Kirkhov’s [sic] Current Law, etc.), I am a little confused by the term of “180° out of phase”. Are we talking about vector aspect of this (VI over time, or is it just a matter of two wires side by side where the current is literally physically moving in opposite directions (at least for part of the 1/120th of a second)? :confused:

I’ll put it in different words for you, user_hostile.

If the currents in the hot line and the neutral return line are equal and opposite, their magnetic fields cancel each other out, and so the induced voltage on the secondary is equal to zero. Equal and opposite means equal in magnitude but opposite in direction.

That help?

**Of course, magnetic fields cancel. **

** Desmostylus **

Yeah . . . the simpler explaination was what I thought it was. But I got carried away with the problem (perhap’s I still suffer from Post Tramatic Circuit Analysis).

I need to sharpen my Occam’s razor-- it’s bit little rusty. :dubious: