Ground fault circuit breakers...

Thread Starter


Joined Nov 29, 2005
How do they work ? The extra wire, triggers it to disconnect ? What signal takes the extra wire ? Is it like a remote controlled 'tripper' ?
Trying to find other purposes for the gadget but first must learn
how it works...:rolleyes:

Analog Ground

Joined Apr 24, 2019
There is no "extra wire" on a GFCI. The breaker compares the current in the hot wire with the current in the return wire. Everything is OK if the currents in the two wires are equal. If they are different, then there is a problem such as some of the current going to earth ground which is a "fault". If there is an additional wire, like a white "pig tail" wire, the breaker is probably an AFCI which also detects "arc faults". This is different than a GFCI.
Here is an early datasheet: Sheets/Texas Instruments PDFs/LM1851.pdf

and a later one:

neutral ans line are wound on a current transformer such that the currents cancel. A third winding picks up the difference.
So, it's pretty much a direct compare.

They also generally do polarity detection. The LM1851 does grounded neutral protection, so grounding the neutral should trip it too.

In an installation, you don;t have to use ground if you label the outlet "No protective ground.

In a grounded system, all you have to do to trip it, is shunt 10mA from hot to ground with a resistor.