8/12/2019 GFCI Presentation

1/12

2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-1

Ground Fault

CircuitInterrupters

8/12/2019 GFCI Presentation

2/12

2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-2

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

8/12/2019 GFCI Presentation

3/12

2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-3

LO

A

D

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.

8/12/2019 GFCI Presentation

4/12

2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-4

LO

A

D

Hot

Neutral

The flux cancels.

No flux means no output

from the sense winding.

8/12/2019 GFCI Presentation

5/12

2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-5

LO

A

D

Hot

Neutral

Lets add a ground fault.

The fault current flows in the

hot wire but not in the neutral.

8/12/2019 GFCI Presentation

6/12

2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-6

LO

A

D

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)

8/12/2019 GFCI Presentation

7/12 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-7

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.

SenseDrive

8/12/2019 GFCI Presentation

8/12 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-8

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

8/12/2019 GFCI Presentation

9/12 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-9

LO

A

D

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.

8/12/2019 GFCI Presentation

10/12 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-10

Hot

Neutral

Panel

side

Load

side

LM1851

The bridge injects a 120 Hz signal into the drive coil. When there is

an unwantedlow-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

SenseDrive

Unwanted 120 Hz

Normal120 Hz

8/12/2019 GFCI Presentation

11/12 2013 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill

1-11

Hot

NeutralAmplifier

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 thishappens.

Moisture

8/12/2019 GFCI Presentation

12/12 2013 The McGraw-Hill Companies, Inc All rights reservedMcGraw-Hill

1-12

Hot

NeutralAmplifier

The common mode rejection of the amplifier helps prevent

false tripping. However, large signals can exceed the

amplifiers common-modelimits. 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.