What a ground fault circuit interrupter (GFCI) does?
A ground fault circuit interrupter (GFCI) monitors the current balance between the ungrounded “hot” conductor and the grounded conductor. As soon as the current flowing through the “hot” conductor is in the range of 4 to 6 milli-amperes more than the current flowing in the “return” grounded conductor, the GFCI senses this unbalance and trips (opens) the circuit off.
If the “other” path is through a human body, as illustrated in Figure 1a and 1b, the outcome could be fatal. Figures 1a and 1b show the basic principle of how a ground fault circuit interrupter operates.
When no current is induced in the coil
No current is induced in the coil because both wires are carrying the same current. The ground fault circuit interrupter does not trip the circuit off.
When small current is induced in the coil
A small amount of current is induced in the coil because of the unbalance of current in the conductors. This current difference is amplified sufficiently by the ground fault interrupter to cause it to trip the circuit off before the person touching the faulty appliance is injured or killed.
Note!! Current values above 6 milliamperes are considered dangerous! Ground fault circuit interrupters must sense and operate when the ground current exceeds 6 milliamperes.
UL Standard No. 943 covers ground fault circuit interrupters.
Class “A” GFCI devices are the most common.
They are designed to:
- Trip when current to ground is 6 milliamperes (6/1000 of an ampere) or greater.
- Not trip when the current to ground is less than 4 milliamperes (4/1000 of an ampere).
- May or may not trip when the current to ground is between 4 and 6 milliamperes.
Class “B” GFCI devices are pretty much obsolete.
They were designed to trip on ground faults of 20 milliamperes (20⁄1000 of an ampere) or more. They were used only for underwater swimming pool lighting installed before the adoption of the 1965 NEC.® For this application, Class “A” devices were too sensitive and would nuisance trip!
GFCI internal components and connections
Receptacle-type GFCIs switch both the phase (hot) and grounded conductors. Note that when the test button is pushed, the test current passes through the test button, the sensor, then back around (bypasses, outside of) the sensor, then back to the opposite circuit conductor.
Note that because both normal “load” currents pass through the sensor, no unbalance is present.
What a ground fault circuit interrupter does not?
- It does not protect against electrical shock when a person touches both circuit conductors at the same time (two “hot” wires, or one “hot” wire and one grounded neutral conductor) because the current flowing in both conductors is the same. Thus, there is no unbalance of current for the GFCI to sense and trip.
- It does not limit the magnitude of ground fault current. It does limit the length of time that a ground fault will flow. In other words, you will still receive a severe shock during the time it takes the GFCI device to trip “off.” See Figure 3.
- It does not sense solid short circuits between the “hot” conductor and the grounded “neutral” conductor. The branch circuit fuse or circuit breaker provides this protection.
- It does not sense solid short circuits between two “hot” conductors. The branch circuit fuse or circuit breaker provides this protection.
- It does not sense and protect against the damaging effects of arcing faults, such as would occur with frayed extension cords. This protection is provided by an arc-fault circuit interrupter (AFCI) discussed later in this chapter.
- It does not provide overload protection for the branch circuit wiring. It provides ground fault protection only.
How to wire a 20 amp GFCI receptacle (VIDEO)
Reference // Electrical Wiring—Residential by Ray C. Mullin (Purchase hardcopy from Amazon)