Electrical safety largely depends on persons who are responsible for designing, testing, installing, and inspecting electrical equipment. However, everyone should know how to use electrical equipment safely and how to avoid potentially hazardous situations.
Electrical appliances, power tools, and other home equipment operating on 120-volt or 240-volt circuits do not present shock hazards unless they are defective. Shock hazards are minimized by rigorous inspection, compliance with safety standards, and the use only of devices approved by Underwriters' Laboratories Inc. or other organizations of similar standing. Nevertheless, wear and tear or abuse may cause any apparatus to develop defects that may lead to electrical fire or shock hazards. Such defects include shiny copper spots in old wiring, wires protuding from power cords or plugs, broken appliances or plugs, and wiring that a novice has attempted to repair. Even when home appliances become defective, the hazard can be reduced by keeping one's body from becoming a part of the circuit.
A serious shock hazard develops only when current flows through the
body. Most electrical accidents involve a current pathway from one hand
to the feet or from one hand to the other hand. One should never hold a
metal handle of an energized appliance with a wet hand when wearing wet
shoes, standing barefoot on the ground, or standing on a wet floor.
Also, one should never use an electrical apparatus when barefoot in the
garden or when dangling feet in water, as from a boat or wharf. Water
provides a highly dangerous situation for receiving electric shocks. In
contrast, dry gloves, dry shoes, rubber overshoes, and dry floors
provide a high degree of protection against dangerous shocks from
120-volt or 240-volt circuits.
Dry shoes and a dry place to stand are inadequate insulations at high voltages. When a high-voltage wire falls on an automobile, the occupants should remain inside the vehicle because they can be killed by making any contact with the pavement. If one sees a high-voltage wire on an automobile, he should: (1) tell the occupants to stay inside the automobile; (2) warn would-be rescuers not to come close to the automobile or any fallen wires; and (3) summon a policeman and a utility company service man.
Equipment designers provide electrical safety by means of isolation, guarding, insulation, grounding, and shock limitation.
Isolation is achieved by placing high-voltage wires high overhead and far out of reach. When overhead wires are nearby, one should never fly kites or model airplanes with control wires. Also one should never climb a pole or tree or erect a TV antenna when overhead wires are nearby.
Guarding provides a barrier between a person and electric circuits. For instance, the chief function of the cabinet of a TV set is to guard against hands touching wires charged with voltages as great as 30,000 volts.
Insulation ensures that an electric current will be confined to its proper path. When the insulation is in good condition, electric apparatus will operate safely and reliably. However, some insulations become brittle and develop small cracks when they become old. Continual flexing also may cause cracks. Also, abuse or high temperatures can ruin any insulation. Water may bridge small defects in the insulation of an appliance and cause the metal frame of the appliance to become electrified.
In some power tools, the motor is insulated from the frame, and the frame is made of a plastic insulating material. These power tools, which are marked "doubly insulated," have provided a significant increase in electrical safety. However, any motor-driven device, such as a drill, electric razor, or vacuum cleaner, can be lethal if submerged in water.
Grounding for the electric wiring system in the home is accomplished by connecting the neutral wire at the utility's electric meter to an effective ground, such as a cold-water pipe or a grounding electrode buried in the earth. A wiring system with one line grounded: (1) provides protection against lightning or a fault in the utility's transformer; (2) limits the maximum voltage to ground under normal operating conditions; and (3) provides a low-resistance path so that a fault current will operate a fuse or circuit breaker in a live line, thereby shutting off the power to the fault location on the live line.
In addition, all noncurrent-carrying metal parts of equipment enclosures - including conduits, boxes, apparatus, and motor frames - are connected to the grounding wire of the wiring system. This is done: (1) to prevent any voltage above ground on an enclosure and (2) to provide a current path to operate a fuse or circuit breaker in a live line that has a fault.
The ground wire connected to the frame of a 120- or 240-volt power tool of a large appliance such as an air conditioner may be bare or insulated. If the wire is insulated, it must be colored green. The ground wire is connected to the marked prong of a 3-prong plug, and this prong connects to ground at a 3-hole receptacle. However, practically all 120-volt portable appliances - toasters, hot plates, irons, frying pans, radios, TV sets, public address systems, floor lamps, and desk lamps - are equipped with 2-prong plugs that fit into 2-hole receptacles.
In 1970 the National Commission on Product Safety reported that only about 15% of the homes in the United States have receptacles that accept a 3-prong plug. Consumers were expected to install an inexpensive adapter plug. It fits into a 2-hole receptacle, accepts a 3-prong plug, and has a green ground wire for connection to a screw at the receptacle plate. However, the connection to ground may be faulty or nonexistent even when an adapter plug is used. For example, the receptacle may not be grounded, especially in older homes.
The National Commission on Product Safety also reported in 1970 that a ground fault interrupter ensures nearly complete protection from shocks and electrical fires in and around the home. The instant a current of only a few milliamperes leaves the normal circuit and flows to ground, the ground fault interrupter senses the shift and opens a circuit breaker, thereby shutting off the power. A ground fault interrupter limits the shock intensity from line-to-ground contacts, and it operates very rapidly. Thus it provides an extremely high degree of safety against electrocution, even for small children. Ground fault interrupters are commercially available in the United States, Canada, and some European countries.
More by this Author
A muffler is a device for reducing noise propagated by a moving stream of gas. Although mufflers are used to reduce noise from such sources as air compressors, aircraft engines, ventilating systems, and industrial...
A tachometer an instrument that indicates the revolutions per minute (rpm) of a rotating shaft, such as an automobile or airplane engine crankshaft, or of other machinery. Most tachometers have a pointer that moves over...
The White House as it appears from the north. Photograph by David Lat. The White House is the official residence of the President of the United States, located in Washington, D.C. It is on Pennsylvania Avenue facing...
No comments yet.