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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.