What You Need To Know About GFCIs

A Ground Fault Circuit Interrupter or GFCI is an electrical safety device. It is designed to trip or cut power to electrical circuits by monitoring the amount of current flowing from hot to neutral wiring. GFCIs are not to be confused with fuses or circuit breakers. Here is what you need to know about Ground Fault Circuit Interrupters.

What is a GFCI used for?

If a GFCI detects a difference between the current flowing from hot to neutral wiring, the GFCI will trip and cut the supply of electricity. A ground fault circuit interrupter (GFCI) can help prevent electrocution.

GFCIs are available as breakers or receptacles. Homeowners can tell which receptacles are GFCI protected because they will have a test button and a reset button in the middle of the outlet.

How do GFCIs work?

The purpose of a GFCI outlet is to prevent electrical injury. This is not something regular outlets will do. A regular outlet in your home features two, three-prong plug-in slots. A GFCI outlet has the same plug-in configuration plus two buttons on its face: a “TEST” button and a “RESET” button.

In some cases, one GFCI receptacle can protect all of the outlets downstream. You’ll find this in bathrooms where one bathroom would have a GFCI receptacle and the other bathrooms would have regular receptacles. When wired correctly all the bathroom receptacles would be GFCI protected.

The GFCI picks up on the difference in the amount of electricity flowing into the circuit to that flowing out, to put it simply. The GFCI reacts quickly (less than one-tenth of a second) to trip or shut off the circuit.

Normally, electrical current flows at a uniform rate through wiring. When a ground fault occurs, the flow of electricity surges as it jumps to the unintended conductor. A GFCI outlet contains a sensor that monitors the flow of the electrical current through the wires, and when it senses a ground fault (in electrical terms, “fault” means any variation from the normal current), the GFCI, which also contains an internal switch, shuts off the flow of electricity in the outlet.

Where should GFCIs be installed?

The National Electric Code (NEC) requires the installation of GFCI outlets in new construction in areas where electrical outlets are close to water. Older homes are not required to have GFCI outlets unless the wiring is being updated. I recommend installing them anyway.

The NEC requires GFCIs on:

all exterior and bathroom receptacles

all receptacles serving kitchen countertops.

In laundry rooms and utility rooms. GFCIs should be installed on outlets within six feet of sinks, washing machines, and water heaters.

Within six feet of a wet bar

In garages and unfinished basements

GFCIs were introduced as codes in the early 1980s to prevent fatal shocks. A GFCI protected outlet must be installed on outdoor outlets, bathroom outlets, kitchen counters, and hot tubs. Simply, anywhere there is the potential for water and electricity to come into contact.

Products and reference designs of Ground Fault Circuit Interrupter (GFCI)

As shown in the figure below, AFCI consists of seven parts, each part is Non-isolated AC/DC power supply、Current sensing analog front end、Voltage sensing analog front end、Ground fault current sensing analog front end、MCU、Self-diagnostics/monitoring、Output user interface.

Non-isolated AC/DC power supply

DC voltage is derived from the main lines to power the breaker. Multiple architectures with different efficiencies are used to do the AC/DC rectification and regulation. Non-isolated AC/DC power supply includes Buck converters (integrated switch)、Flyback controllers、Linear & low-dropout (LDO) regulators, the corresponding products are LM5168、UCC28881、TLV709

Current sensing analog front end

Detection of an arc in the line current is done by looking at high frequency components. Depending on the range of frequencies, filters, log amplifiers and comparators can be used. Also, tripping an AFCI is a function of the magnitude of the current flowing through the line which is measured using the current sensing analog front end. Current sensing analog front end includes Comparators、Precision op amps (Vos<1mV)、High-speed op amps (GBW ≥ 50 MHz)、General-purpose op amps、Analog switches & muxes, the corresponding products are TLV7011、OPA2188、OPA835、TLV9052、TMUX4051

Voltage sensing analog front end

For measuring voltages and detecting zero cross over events voltage sensing analog front end can be used. Also, a comparator can be used to identify miswiring and alert the system. Voltage sensing analog front end includes Precision op amps (Vos<1mV)、Comparators、General-purpose op amps, the corresponding products are SM73307、TLV1851、TLV9002

Ground fault current sensing analog front end

Ground fault current sensing analog front end is used for monitoring leakages between line and neutral. If the leakage is above the thresholds defined in UL943 then tripping action is initiated. Ground fault current sensing analog front end includes Digital power monitors, the corresponding products are INA232

MCU

Advanced algorithms for detecting arcs faults are implemented using a microcontroller. Multiple peripheral such as ADCs, Timers, GPIOs dictate the MCU requirements. MCU includes MSP430 microcontrollers, the corresponding products are MSP430FR5994、MSP430FR5969、MSP430FR5992

Self-diagnostics/monitoring

A supervisor IC is used to monitor the power supply levels. It provides a fixed delay during a power glitch or when the reset function is forced upon the microcontroller. Self-diagnostics/monitoring includes Supervisor & reset ICs, the corresponding products are TPS3840、LM8364

Output user interface

LEDs are used for status indication. Output user interface includes RGB LED drivers, the corresponding products are LP5812、LP5810、LP5811

What Is The Difference between AFCI and GFCI?

To put it simply, GFCIs prevent shocks, and AFCIs prevent fires. With Arc fault circuit interrupters (AFCIs) when an arc is detected, power to the circuit is interrupted. AFCI receptacles will recognize an arc fault and quickly trip to stop the flow of electricity. This prevents the electrical system from being an ignition source of a fire.

AFCI circuit breakers and GFCIs complement and work together to provide electrical safety and protect against electrical fires in your home.