Ground fault circuit interrupters.
A ground fault exists when electric current leaves its intended path and flows to ground. If a person is a part of the path to ground, an electric shock or more serious accident may occur.
A ground fault circuit interrupter (GFCI) is a safety device that is designed to prevent accidental electrocution (see drawing at left). It senses an imbalance of current flow in a 120-volt circuit to which an appliance, lighting, or other device is connected. When even a minute imbalance above a predetermined threshold is detected, the current in the circuit is interrupted (turned off) immediately. Therefore, a person involved in causing the imbalance is protected from electrocution.
The Consumer Product Safety Commission (CPSC) has estimated that in recent years about 150 lives are lost annually by electrocution involving power tools in and around the home. Of these deaths, CPSC estimates that 125 could have been prevented if a GFCI had been in use in the circuit. Electrocution can occur from accidents other than those involving power tools, and many of them can also be prevented through the use of a GFCI.
Even through the risk of death by electrocution is greatly reduced by a GFCI, there may be other types of accidents resulting from an electrical shock. A GFCI may not trip fast enough to completely avoid the shock sensation and the person may be startled. If an electric tool is being used, the minor shock may cause the operator to drop the tool; if the tool falls against some part of the body, an injury could result. Similarly, a shock to a person handling a pot of coffee or other container of scalding food or beverage might spill the contents causing serious burns.
HOW A GFCI WORKS
How does a GFCI work? When everything is working properly, current to the appliance, or load, through the line ("hot") conductor equals current returning through the neutral conductor. If current leaks from an appliance to ground, the difference between the two currents is not zero. In that situation, a ground fault exists. A GFCI will detect the ground fault and act by interrupting the circuit.
The GFCI can sense differences as low as 4-6 millilamps (0.005 amps) and immediately turn off the current. "Immediately' in this case usually means a second or less, perhaps a time span of the order of 0.025 seconds. The exact length of time depends on the amount of imbalance; greater leakage current interrupts the circuit in less time than a smaller imbalance. A person can feel a shock from a current below the threshold required to activate the GFCI, but although such a shock may cause discomfort, it will not cause death.
For new homes, the National Electrical Code (NEC) requires GFCIs on 120-volt, single-phase, 15- or 20-ampere receptacles in bathrooms, garages, and outdoor circuits. Local codes may require installation of GFCIs when remodeling/rewiring of older homes is done. Other rooms which might also benefit from GFCI protection are the workshop, laundry and kitchen.
GFCIs come in four types:
.a portable GFCI (below, left) that can be plugged in and provide protection wherever needed.
.a combined circuit breaker and ground fault circuit interrupter (below, right) that is installed in the distribution panel and protects all outlets on the circuit.
.a ground fault receptacle that provides protection for itself and for all receptacles/wall outlets further along ("downstream") on that circuit (feed-through type), and .a ground fault receptacle that provides protection at that receptacle/wall outlet only (termination type).
One might ask whether a GFHI is needed if appliances are grounded. A GFCI will provide even greater protection than grounding. Of the approximately 150 people who are accidentally electrocuted by electric tools per year, a study sponsored by CPSC estimated that 45 would not be killed if properly-installed, three-wire grounding were used on the tools. That number is in contrast to 125 who would be expected not to be killed if a CFCI had been connected in the circuit. A GFCI will provide greater protection because it will protect "grounded" appliances and tools even when the grounding is improperly installed or is inoperative.
Although GFCIs were developed for underwater lights for swimming pools, they can increase electrical safety in many home situations. Their use is expected to increase as new homes are built and as the wiring of older homes is improved.
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|Publication:||Pamphlet by: Cornell University Cooperative Extension|
|Date:||Jul 1, 1993|
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