What is a GFI outlet or circuit?

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GFCI Outlet

GFCI Outlet

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Why do I use GFIs on knob and tube lines?
(From OSHA)
Ground-Fault Circuit Interrupters (GFCI)  Come also in circuit breaker form!  Inside every GFI device, there is a current transformer device, that senses the leakage (shock) current to ground not returning back through the current transformer inside the GFI over 1/40th of a second, and 5/1000th of an amp, and the GFI relay shuts down the flow of electricity so it cannot harm you.  1/3 of an amp at 120 volts can stop your heart.

A ground-fault occurs when there is a break in the low-resistance grounding path from a tool or electrical system. The electrical current may then take an alternative path to the ground through the user, resulting in serious injuries or death. The ground-fault circuit interrupter, or GFCI, is a fast-acting circuit breaker designed to shut off electric power in the event of a ground-fault within as little as 1/40 of a second. It works by comparing the amount of current going to and returning from equipment along the circuit conductors. When the amount going differs from the amount returning by approximately 5 milliamperes, the GFCI interrupts the current and saves you, your family, your children, your pets, and workers on your property!

The GFCI is rated to trip quickly enough to prevent an electrical incident. If it is properly installed and maintained, this will happen as soon as the faulty tool is plugged in. If the grounding conductor is not intact or of low-impedance, the GFCI may not trip until a person provides a path. In this case, the person will receive a shock, but the GFCI should trip so quickly that the shock will not be harmful.

The GFCI will not protect you from line contact hazards (i.e. a person holding two “hot” wires, a hot and a neutral wire in each hand, or contacting an overhead power line). However, it protects against the most common form of electrical shock hazard, the ground-fault. It also protects against fires, overheating, and destruction of wire insulation.

For construction applications, there are several types of GFCIs available, with some variations:

Receptacle Type

  • The Receptacle Type incorporates a GFCI device within one or more receptacle outlets. Such devices are becoming popular because of their low cost.
Two receptacle outlet with red and black test and reset buttons
Cord-Connected Type

  • The Cord-Connected Type of GFCI is an attachment plug incorporating the GFCI module. It protects the cord and any equipment attached to the cord. The attachment plug has a non-standard appearance with test and reset buttons. Like the portable type, it incorporates a no-voltage release device that will disconnect power to the load if any supply conductor is open.
Attachment plug with test and reset buttons incorporating the GFCI module

Because GFCIs are so complex, they require testing on a regular basis. Test permanently wired devices monthly, and portable-type GFCIs before each use. All GFCIs have a built-in test circuit, with test and reset buttons, that triggers an artificial ground-fault to verify protection. (These will not work if there is no ground present, or if the GFI is attached to knob and tube lines, as those lines have no ground.  This is legal under NEC code section 210-7D, as long as the GFI is labelled: no ground present.)  I have also seen GFIs pop off when a drunk hits the power pole, and sends high voltage, 3300 volts, through house lines, frying everything and everyone in its pathway.  Those people who had GFIs suffered no damage or shocks or burns, while those who did not, lost computers, tvs, irrigation clocks, cable modems, online modems, transformers in the attic to door bells, etc.  It’s cheaper to replace a GFI, than it is to replace your hard drive, or your child for that matter!  The construction worker in this picture was burned by electrical current, which would have been prevented by GFI.  GFI stands for: Ground Fault (shock to ground or neurtal) Interruption.
   Where are they required:  In water areas such as kitchens, bathrooms, garages (Below 6’6″), swimming pools, outside, basements, concrete floor areas, dirt floor areas, water covered areas, anywhere you can be electrocuted by coming into contact with the ground on your feet.

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