Understanding GFCIs

Advancements in electrical safety occur continuously. New products make the use of electricity safer every year. Some devices are ubiquitous but their function may not be fully appreciated. One of the most common devices in residential, commercial, and industrial safety is the ground fault circuit interrupter (GFCI).


Advancements in electrical safety occur continuously. New products make the use of electricity safer every year. Some devices are ubiquitous but their function may not be fully appreciated. One of the most common devices in residential, commercial, and industrial safety is the ground fault circuit interrupter (GFCI).

GFCIs are well known because the National Electrical Code, NFPA 70, has mandated them in residential and commercial applications for a relatively long time. Starting in 1973, GFCI protection became a requirement for many places in the home. However, GFCIs are required for industry too. NFPA 79, the Electrical Standard for Industrial Machinery , also dictates their use in industrial applications:

"16.1.2 Receptacles, which are part of the industrial machine, either internal or external to the control cabinet and intended for use by maintenance personnel, shall have ground fault circuit interrupter (GFCI) protection for personnel."

GFCI receptacles have push buttons on the front labeled "Test" and "Reset." They protect both the outlet and the downstream circuit (Fig. 1). These buttons exercise the mechanical components of the receptacle to verify its operation. It is recommended that operation be checked monthly. Other GFCIs are built into circuit breakers and are located in panel boxes, providing both overcurrent and ground fault protection to the entire circuit.

Purpose of GFCI

The purpose of GFCIs is to prevent bodily harm from an electrical fault that causes electricity to flow through you to ground. But wait, isn't that what fuses and circuit breakers are supposed to do? Not exactly. Fuses and breakers were designed to reduce death and property damage caused by electrical fires started by short circuits and other large magnitude faults. The trouble is that the energy needed to trip a standard breaker far exceeds the lethal amount. Ground faults are most likely to occur when you are standing on ground or near a grounded piece of equipment.

How GFCIs work

GFCIs work by quickly (1/40 sec) disconnecting a current flowing through an unintended ground path (you) even if the current is too small (5 mA) to trip a normal circuit breaker (20 A). Inside a GFCI is a device that monitors the amount of current flowing through the line and compares it to the amount that returns in the neutral. If less current is present in the neutral, then a ground fault must exist somewhere in the circuit. The receptacle in turn trips (See "GFCI concepts").

At the heart of a GFCI circuit is a device that measures current in both the line and neutral. This current transformer (CT) has both the line and neutral passing through the hole and is called a differential transformer. When CTs are used for measurement you place only the line through the hole. A magnetic field is created that is proportional to the magnitude of the current passing through it. This magnetic field then creates a secondary current that is a fraction of the original, depending on its turn ratio. This transformer-type function allows you to read very high currents (for example, 100 A or even 1000 A) without having to break the conductors to install an ammeter.

As long as the same amount of current comes back on the neutral, the fields cancel out. If less current comes back, then a field is created and can be detected. This is done with a circuit that monitors the field (Fig. 2). The circuit that makes the GFCI a GFCI is a comparator. Shown in Fig. 2 as a block comprised of (and labeled as) "amplifier, comparison, and 'decision making' circuit," the comparator is what makes the decision.

The signal from the CT is amplified to a sufficient level to operate the comparator. A comparator circuit operates by comparing two inputs and reacting according to the difference it senses between its inputs. Therein lies the decision-making aspect of the circuit. The circuit is designed so that when line current and neutral current are equal, the comparator is balanced and it does not react. When a ground fault occurs, it unbalances the respective currents, raising the difference in potential applied to the comparator's inputs.

The comparator reacts by changing states. Its output is connected to a silicon controlled rectifier (SCR) trigger that disconnects the circuit. Typically the GFCI is spring-loaded, which provides the mechanical energy necessary to open the internal contacts and cut power to the circuit.

Test early; test often

GFCIs should be tested after installation and periodically thereafter, according to the manufacturer's instructions. The two buttons on the front provide an easy way to test and reset the circuit, which makes sure the spring is operating properly. Also, there are devices available for about $15 retail that will test the GFCI with a very small amount of ground fault current, ensuring that it will operate under a "real world" event. Some models indicate if the ground is connected properly or if the line and neutral are reversed.

Don't confuse GFCI with a new type of breaker called the arc fault circuit interrupter (AFCI). The purpose of the AFCI is to protect against a fire hazard that results from a low-magnitude (30 mA) short circuit between the line and neutral. Typically such a fault is caused by wire insulation damage and neither a standard circuit breaker nor a GFCI will protect against it.

Author Information
Mike Nager is the Industry Marketing Manager for Phoenix Contact Inc., Harrisburg, PA. He holds a BSEE from the University of Scranton and has 15 years experience in industrial control. Mike is Director-Elect of ISA's Food and Pharmaceutical Division. He can be reached at 800-888-7388 or mnager@phoenixcon.com .

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