| Description |
 |
VHS English Version Product Number: 1018HEVHS
DVD English Version Product Number: 1018HEDVD
Excellent program that discusses what GFCIs are all about. Explains how they work, and the general safety procedures for protecting against electrical shock.
Excerpt: Insulation and grounding are two recognized means of preventing
injury during electrical equipment operation. Conductor insulation may be
provided by placing non-conductive materials such as plastic around the
conductor. Grounding may be achieved through the use of direct connection
through a known ground such as metal cold water pipe. The metal housing or
enclosure around a motor or the metal box in which the electrical switches,
circuit breakers, and controls are placed. These enclosures protect the equipment
from dirt or moisture, and prevent accidental contact with exposed wiring.
There is, however, a hazard associated with housings and enclosures. A
malfunction within the equipment such as deteriorated insulation may create an
electrical shock hazard. Many metal enclosures are connected to a ground to
eliminate the hazard, but if a hot wire contacts a grounded enclosure a ground
fault results which normally will trip a circuit breaker or blow a fuse. Metal
enclosures and containers are usually grounded by connecting them with a wire
going to ground. This wire is called an equipment grounding conductor. Most
portable electric tools and appliances are grounded by this means. There is one
disadvantage to grounding, a break in the grounding system may occur without
the users knowledge. Insulation may be damaged by hard usage on the job, or
simply by aging. If this damage causes the conductors to become exposed the
hazards of shocks, burns, and fire will exist. Double insulation may be used as
additional protection on the live parts of the tool, but double insulation does
not provide protection against defective cords and plugs, or against heavy
moisture conditions.
The use of ground fault circuit interrupter or GFCI is one
method used to overcome grounding and insulation deficiencies. The GFCI is a
fast acting circuit breaker which senses small imbalances in the circuit caused
by current leakage to the ground, and in a fraction of a second shuts off the
electricity. The GFCI continually matches the amount of current going to an
electrical device against the amount of current returning from the device along
the electrical path. Whenever the amount going differs from the amount
returning by approximately 5 milliamps the GFCI interrupts the electric power
within as a little as 1/40th of a second. The GFCI will not protect
a person from line to line contact hazards such as a person holding two hot
wires, or a hot and a neutral wire in each hand. It does provide protection
against the most common form of electrical shock hazard, the ground fault. It
also provides against fires, overheating, and destruction of insulation on
wiring.
With the wide use of portable tools, and the use of flexible cords,
connectors, receptacles, and cord and plug connected equipment, hazards are
created by improper use and maintenance. Flexible cords are more vulnerable to
damage than is fixed wiring. Flexible cords must be connected to devices and to
fittings so as to prevent tension at joints and at terminal screws. Because a
cord is exposed these terminals become more vulnerable. Improperly connected
terminals is another common situation. When a cord connector is wet hazardous
leakage can occur to the equipment grounding conductor and to humans that pick
up that connector if they also provide a path to ground. Such leakage is not
limited to the face of the connector, but also develops at any wet portion of
it. When the leakage current of tools is below one ampere, and the grounding
conductor has a low resistance no shock should be perceived. However, should
the resistance of the equipment grounding conductor increase, the current
through the body also will increase. Therefore, if the resistance of the
equipment grounding conductor is significantly greater than 1 ohm, tools with
even small leakages become hazardous. GFCIs can be used successfully to reduce
electrical hazards on construction sites. Tripping of GFCIs or the interruption
of current flow is sometimes caused by wet connectors and tools. It is a good
practice to limit exposure of connectors and tools to excessive moisture by
using water-tight or sealable connectors.
|
|
