ืnonconductive materials cover most electrical wires.There are a variety of protective materials and types of coverings.Protective materials have ratings for specific kinds them,or both.A common example is extension cords that have light-,medium-,or heavy- duty ratings. The rating does not indicate electrical capacity.The rating depends on the thickness and type of covering and indicates how well the cord can withstand physical abuse
sealed equipment
when switches are turned on o off ,an arc may be generated as the electrical contacts approach or separate.similarly,electric motors arc as the brushes contact the commutator or slip rings.If an incandescent light bulb breaks,the filament will glow momentarily until it burns and breaks. When events such as these occur in an atmosphere containing a flammable mixture of air and gas or dust, a fire or explosion may result. To prevent this,special electrical equipment is installed in hazardous environments.This electrical equipment(switches,motors,lighting fixtures,conduits,etc.)is sealed to separate heat and sparks from the hazardous environment and to reduce the chance of physical damage.There are three classes of hazardous environments and divisions within the classes. Special electrical equipment is rated on its ability to comply with standards foe each class and division. Table12-3 gives a summary of hazardous location classification