Ground FaultCircuit

Interrupters

A GFCI will open the circuit when the current to groundexceeds some predetermined value (such as 5 milliamperes).

They can be found as part of a circuit breaker in adistribution 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 theend of a line cord.

TEST

RESET

TEST

LOAD

Hot

Neutral

Start with a toroid core.

Add hot and neutral windings.

Adding a load will cause currentto flow and magnetic flux in the core.

Note that the flux created by the neutralwire opposes the flux from the hot wire.

LOAD

Hot

Neutral

The flux cancels.

No flux means no outputfrom the sense winding.

LOAD

Hot

Neutral

Let’s add a ground fault.

The fault current flows in thehot wire but not in the neutral.

LOAD

Hot

Neutral

The hot wire current is largerand its flux is greater.

The net flux produces anoutput at the sense winding.

(to service panel ground)

Hot

NeutralLineLoad

LM1851

MOV

SCR

Bridge rectifier

Circuit breaker

GFCI schematicA ground fault induces an output from the sense transformerwhich is amplified and causes the SCR to trigger. The SCRthen activates the circuit breaker to disconnect the load circuit.

SenseDrive

Hot

NeutralLine

LM1851

Sense

60 Hz

1. Ground fault2. More current in hot wire3. 60 Hz signal at sense winding4. SCR fires

5. Circuit breaker trips

LOAD

Hot

Neutral

An unwanted low resistance path from ground to neutralat the load end can prevent the GFCI from working since

the currents are no longer unbalanced.

There is no net flux and nooutput 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

Panelside

Loadside

LM1851

The bridge injects a 120 Hz signal into the drive coil. When there isan unwanted low-resistance path from the neutral wire to the groundwire at the load side, current flows in the neutral via the normalconnection at the service panel. This current does not flow in the hotwire and the imbalance trips the breaker.

Bridge rectifier

SenseDrive

Unwanted120 Hz

Normal120 Hz

Hot

NeutralAmplifier

Class A GFCIs trip at 5 mA. This sensitivity is achieved byamplifying the output of the sense coil. The sensitivity issuch that nuisance tripping can be experienced with exterioroutlets during a rain storm. Check the gaskets when thishappens.

Moisture

Hot

NeutralAmplifier

The common mode rejection of the amplifier helps preventfalse tripping. However, large signals can exceed theamplifier’s common-mode limits. Lightning strikes often trip GFCIs.

A direct hit is not required to trip a GFCI since the wires actas antennas. Also, strong radio signals can cause tripping.The common-mode rejection usually decreases as frequencygoes higher.