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Ground Fault Circuit Interrupters. RESET. TEST. TEST. 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.

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Ground Fault Circuit Interrupters

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Ground fault circuit interrupters

Ground Fault

Circuit Interrupters


Ground fault circuit interrupters

RESET

TEST

TEST

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.


Ground fault circuit interrupters

Start with a toroid core.

Add hot and neutral windings.

Adding a load will cause current

to flow and magnetic flux in the core.

L

O

A

D

Hot

Neutral

Note that the flux created by the neutral

wire opposes the flux from the hot wire.


Ground fault circuit interrupters

The flux cancels.

L

O

A

D

Hot

Neutral

No flux means no output

from the sense winding.


Ground fault circuit interrupters

Let’s add a ground fault.

Yow!

L

O

A

D

Hot

Neutral

The fault current flows in the

hot wire but not in the neutral.


Ground fault circuit interrupters

The hot wire current is larger

and its flux is greater.

L

O

A

D

Hot

Neutral

The net flux produces an

output at the sense winding.

(to service panel ground)


Ground fault circuit interrupters

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

MOV

Hot

Load

Line

Neutral

Circuit breaker

Bridge rectifier

LM1851

SCR


Ground fault circuit interrupters

1. Ground fault

60 Hz

2. More current in hot wire

3. 60 Hz signal at sense winding

4. SCR fires

5. Circuit breaker trips

Sense

Hot

Line

Neutral

LM1851


Ground fault circuit interrupters

Unwanted ground to neutral path

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.

L

O

A

D

Hot

Neutral

As we will see, a second drive coil is used to detect this fault.


Ground fault circuit interrupters

120 Hz

120 Hz

Bridge rectifier

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.

Sense

Drive

Hot

Load

side

Panel

side

Neutral

Unwanted

Normal

LM1851


Ground fault circuit interrupters

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.

Hot

Amplifier

Neutral

Moisture


Ground fault circuit interrupters

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.

Hot

Amplifier

Neutral

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.


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