1. Principles & Applications
Electronics
Principles & Applications
Seventh Edition
Charles A. Schuler
Ground Fault
Circuit Interrupters
©2008 The McGraw-Hill Companies, Inc. All rights reserved.

2. A GFCI will open the circuit when the current to ground
exceeds some predetermined value (such as 5 milliamperes).
RESET
TEST
TEST
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.

3. 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.

4. No flux means no output from the sense winding.
The flux cancels. L O A D
Hot
Neutral
No flux means no output
from the sense winding.

5. Yow! Let’s add a ground fault. Hot Neutral
The fault current flows in the
hot wire but not in the neutral.

6. The hot wire current is larger and its flux is greater.
Neutral
The net flux produces an
output at the sense winding.
(to service panel ground)

7. 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.
MOV
Drive
Sense
Hot
Load
Line
Neutral
Circuit breaker
Bridge rectifier
LM1851
SCR

8. 2. More current in hot wire 3. 60 Hz signal at sense winding
1. Ground fault
2. More current in hot wire
Hz signal at sense winding
4. SCR fires
5. Circuit breaker trips
Sense
Hot
Line
Neutral
60 Hz
LM1851

9. 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
Unwanted ground to neutral path
As we will see, a second drive coil is used to detect this fault.

10. 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.
Drive
Sense
Hot
Load
side
Panel
side
Neutral
Unwanted
120 Hz
120 Hz
Normal
Bridge rectifier
LM1851

11. 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

12. 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.