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# What a ground fault circuit interrupter does and what it does not do

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## 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.

### 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.

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.

## 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.

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)

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### Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

1. kumaresan
Aug 26, 2022

Hi,
Í’m having a doubt on selection of CT for GFI. consider My Load is 230V,40A.How to choose the CT. need to select higher current ratio(50A/5A) than my load? Anyhow its going to measure the difference of Phase and Neutral if its Class A,The maximum measure current is 6mA then may i go with Low current ratio CT 1A/100mA(for example)??

2. Bill Cowhig
Apr 13, 2022

Now I have come across and read your very thorough article two times, and I want to thank you for putting so much energy into creating it. I do have a question, though.

When doing an Available Fault Current (AFC) study of my residence, the POCO stated that my AFC at my meter is 7,900 A. From there the service goes directly into the 200 A main breaker in a breaker panel located adjacent to the meter, outside.

I intend to add a new HVAC disconnect adjacent to the breaker panel, and it comes with a service GFCI receptacle (per NEC) located in the same 3R rated enclosure. It was my decision to look for and buy such a disconnect.

User information inside the disconnect says “A GFCI receptacle may present a risk of fire and injury to persons if installed on circuits capable of delivering an RMS symmetrical short circuit current (Available Fault Current) greater than 2,000 A.”

There is a lot of difference between the 2,000 A SCCR of the GFCI receptacle and the 7,900 A AFC that exists a couple of feet from it, so I need to somehow limit the AFC.

But, my question is would the GFCI sensing element be able to trip the GFCI should a SOLID ground fault occur, say if the hot leg gets BOLTED to the equipment grounding conductor somehow, triggering a huge flow of short circuit current, instantaneously.

My thinking comes from being told that a GFCI normally handles “leakage” currents, even though they are called “Ground Fault Circuit Interrupters.”

You posted this 6 years ago, so I know the chance exists that you are not even around any longer, but I cannot find this question being discussed anywhere.

3. 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…

• T Bennett
Feb 26, 2022

I believe the “let-go” current is also affected by the frequency. Therefore, 50 cycles is not the same as 60 cycles. My guess is 30mA is going to be fine in European circuits.