A GFCI will open the circuit when the current to ground exceeds some predetermined value (such as 5 milliamperes). They can be found as part of a circuit breaker in a distribution box or in a load center. They can be found as part of a duplex receptacle. Or, they can be found as part of an assembly at the end of a line cord. TEST RESET TEST
LOADLOAD Hot Neutral Start with a toroid core. Add hot and neutral windings. Adding a load will cause current to flow and magnetic flux in the core. Note that the flux created by the neutral wire opposes the flux from the hot wire.
LOADLOAD Hot Neutral The flux cancels. No flux means no output from the sense winding.
LOADLOAD Hot Neutral Let’s add a ground fault. The fault current flows in the hot wire but not in the neutral.
LOADLOAD Hot Neutral The hot wire current is larger and its flux is greater. The net flux produces an output at the sense winding. (to service panel ground)
Hot Neutral LineLoad LM1851 MOV SCR Bridge rectifier Circuit breaker GFCI schematic A ground fault induces an output from the sense transformer which is amplified and causes the SCR to trigger. The SCR then activates the circuit breaker to disconnect the load circuit. Sense Drive
Hot Neutral Line LM1851 Sense 60 Hz 1. Ground fault 2. More current in hot wire 3. 60 Hz signal at sense winding 4. SCR fires 5. Circuit breaker trips
LOADLOAD Hot Neutral An unwanted low resistance path from ground to neutral at the load end can prevent the GFCI from working since the currents are no longer unbalanced. There is no net flux and no output at the sense winding. Unwanted ground to neutral path As we will see, a second drive coil is used to detect this fault.
Hot Neutral Panel side Load side LM1851 The bridge injects a 120 Hz signal into the drive coil. When there is an unwanted low-resistance path from the neutral wire to the ground wire at the load side, current flows in the neutral via the normal connection at the service panel. This current does not flow in the hot wire and the imbalance trips the breaker. Bridge rectifier Sense Drive Unwanted 120 Hz Normal 120 Hz
Hot Neutral Amplifier Class A GFCIs trip at 5 mA. This sensitivity is achieved by amplifying the output of the sense coil. The sensitivity is such that nuisance tripping can be experienced with exterior outlets during a rain storm. Check the gaskets when this happens. Moisture
Hot Neutral Amplifier The common mode rejection of the amplifier helps prevent false tripping. However, large signals can exceed the amplifier’s common-mode limits. Lightning strikes often trip GFCIs. A direct hit is not required to trip a GFCI since the wires act as antennas. Also, strong radio signals can cause tripping. The common-mode rejection usually decreases as frequency goes higher.