Download AG-AG-AMII-01422-02.1 GFCI-W

Electrical Wiring
How Ground Fault Circuit
Interrupters Work
GFCI-W
JMA
GFCI’s
A device that monitors the current flow in the
two wires of a circuit. If there is a difference in
the currents of as much as 5 millamperes the
circuit opens.
How is this done?
The following principles are used.
When a wire is moved through a magnetic
field a voltage is induced in the wire.

V
When current passes through a wire a
magnetic field is established.
i

If the current changes with time, as with AC,
the magnetic field changes with time.
If a second conductor is placed in the
changing magnetic field, a voltage is
induced in the second conductor.

i
V
If the conductor is looped so that the
conductor passed through the magnetic field
twice the voltage induced is doubled.
i
2V

The more turns, the greater the voltage
induced. For “n” turns the voltage is
increased by n times what it would be with
only a single conductor.
i
nV

The direction of the magnetic field is
dependent on the direction the current is
flowing. If one grasp the conductor in the
right hand, with the thumb in the direction
of the current flow, the magnetic field is in
the direction of the fingers.
i

Current is from L to R
Magnetic field is counterclockwise

i
i

Current is from R to L
Magnetic field is clockwise
If i1 is equal to i2 but in the opposite
direction, then 1 is equal to 2 and also in
the opposite direction.
i1
i2
2
1
If we bring the two wires, with equal and
opposite currents, very close together, then the
magnetic fields cancel. A third wire placed near
these two wires would not have a voltage
induced.
i1
i2
0V
If I1 is much greater than I2, then the voltage
induced is dependent on I1.
1
i1
i2
2
?V
If the two wires carrying the current for a circuit
pass through a coil, consisting of many turns, no
voltage will be induced in the coil if the current in
each wire is the same.
i1
i2
0V
If one wire carries more current than the other
then a voltage is induced on the coil, which is
proportional to the difference in the currents
I1>I2
i2
?V
GFCI’s
These principles are used in the GFCI
The two current carrying wires of a circuit pass
through a coil, often called a current
transformer. When the current in both wires are
the same the output of the transformer is zero.
If the difference in current is as much as 5 mA,
the output of the transformer is enough to trip a
relay opening the circuit.
GFCI’s
Line
Current
Transformer
Latched
Relay
Load
Electronics
Have a good Day!