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What a ground fault circuit interrupter does and what it does not
What a ground fault circuit interrupter does and what it does not (photo credit: rvhomestead.wordpress.com)

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.

The unbalance indicates that part of the current flowing in the circuit is being diverted to some path other than the normal return path along the grounded return conductor.

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.

No current is induced in the coil because both wires are carrying the same current
Figure 1a – No current is induced in the coil because both wires are carrying the same current


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.

A small amount of current is induced in the coil because of the unbalance of current in the conductors
Figure 1b – A small amount of current is induced in the coil because of the unbalance of current in the conductors

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.

This is how the “unbalance” is created then monitored by the electronic circuitry to signal the GFCI’s contacts to open.

Note that because both normal “load” currents pass through the sensor, no unbalance is present.

Ground fault circuit interrupter internal components and connections.
Figure 2 – Ground fault circuit interrupter internal components and connections.

What a ground fault circuit interrupter does not?

  1. 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.
  2. 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.
    The time/current curve shows the tripping characteristics of a typical Class A GFCI
    Figure 3 – The time/current curve shows the tripping characteristics of a typical Class A GFCI. Note that if you follow the 6-mA line vertically to the crosshatched typical time/current curve, you will find that the GFCI will open in from approximately 0.035 second to just less than 0.1 second. One electrical cycle is 1⁄60 of a second (0.0167 second). An air bag in an automobile inflates in approximately 1⁄20 of a second (0.05 second).


  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.
  4. It does not sense solid short circuits between two “hot” conductors. The branch circuit fuse or circuit breaker provides this protection.
  5. 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.
  6. 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)

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

2 Comments


  1. David Renshaw
    Sep 12, 2016

    A very good description of N. American devices & installation practice. A 6mA-tripping device gives better shock protection than the European counterpart which would nearly always be a nominally 30mA trip, whether it was protecting a single outlet or an extensive final sub-circuit. In Europe & UK a widely-assumed threshold for ‘let-go’ current is 10mA, so someone unlucky enough be conducting a current of 20mA after grasping a live object would be unable to let go of it, but would be suffering it indefinitely because the 30mA RCD (or GFCI) would usually not trip. OK, ventricular fibrillation may not be likely at this level, but even so…


  2. Amol wadkar
    Sep 12, 2016

    How to calculate TVSS sizing??? Pls help

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