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Si8 SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII
jaw end of the switch as in fig. 277, then when the switch is open the hinges and blades are ''dead" and can be put in order with
safety.
§ 714. Snap Switches. — These are sometimes used for turning on and off the current at the operating table. They are mounted on insulating material like porcelain, and are enclosed by a metal covering which is lined with insulating material. The key or button for turning on and off the current is also of insulating material. This form of a switch around the work table is convenient, and avoids any danger of accidentally short-circuiting the line. It should turn on the current and turn it off with a snap. It is also desirable that there should be a sign indicating when the current is on or off, as one cannot see directly as with the knife switch. If such a switch is used, make sure that it is of the right capacity for the maximum current and that it conforms in every way with the standard requirements. It will be plainly marked so that after it is installed one can see at any time the current and voltage for which it is designed. Snap switches are better adapted for small currents, than for large ones. Knife switches are to be used on lines with large currents.
FUSES AND CIRCUIT BREAKERS
§ 715. Fuses and circuit breakers are devices for opening or breaking the circuit whenever the current in any particular situation becomes too great. For example, if a part of the line should be short-circuited.
The devices used are of two kinds ; . fuses, and magnetic cut-outs or circuit breakers.
§ 716. Circuit breakers. — The circuit breaker is a device by which a magnetic trip releases a catch which allows a large switch to open, thus breaking the circuit.
The great advantage of a circuit breaker is that nothing is burned out or melted. It is only necessary to close the switch again and the current will be on. It acts instantly whenever the current rises above the amperage for which it is adjusted.