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Building Design and Construction Handbook - Merritt - Ventech!

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15.24 SECTION FIFTEEN<br />

FIGURE 15.9 Single- <strong>and</strong> double-throw switches: (a)<br />

Double-pole, single-throw switch; (b) triple-pole, singlethrow<br />

switch; (c) single-pole, double-throw switches used<br />

for remote control of lights from two locations.<br />

An isolating switch may not be used to interrupt current. It should be opened<br />

only after the circuit has been interrupted by another general-use switch. Since<br />

isolating switches are very light, an arc will create high temperatures <strong>and</strong> can severely<br />

burn the operator.<br />

For control of large, separate loads, the live copper blades of the various switches<br />

are concealed in steel enclosures, <strong>and</strong> the movable blades are operated by insulated<br />

levers on the front of the board. The equipment is called a dead-front switchboard.<br />

15.7.4 Protective Devices for Circuits<br />

In an electrical distribution system for a building, each electric service must have<br />

a means of disconnection, but it may not consist of more than six service switches.<br />

Each service switch may disconnect service to a panelboard from which lighter<br />

feeders extend to other distribution points, up to the final branch circuits with the<br />

minimum size wire, No. 12. This panelboard contains switches with lower disconnecting<br />

ratings than that of the service switch <strong>and</strong> that serve as disconnecting means<br />

for the light feeders. The rating <strong>and</strong> type of each switch must correspond to the<br />

size <strong>and</strong> kind of load <strong>and</strong> the wire size.<br />

There must also be in every circuit some protective device to open the circuit<br />

if there is an unexpected overload, such as a short circuit or a jammed motor that<br />

prolongs a high inrush current. These protective devices may be fuses combined<br />

with knife switches, or circuit breakers, which provide both functions in one device.<br />

In addition, electrical systems should be protected against power surges caused by<br />

lightning strokes. See Art. 15.19.1.<br />

Fuses in lighting <strong>and</strong> applicance circuits with loads up to 40 A may be the<br />

screwed plug type, with a metallic melting element behind a transparent top. For<br />

each rating, plug fuses are given different colors <strong>and</strong> the screw size is made intentionally<br />

different, to prevent errors.<br />

Cartridge fuses are another type of fuse. They are cylindrical in form <strong>and</strong> are<br />

available in any size. They are classified for special purposes, <strong>and</strong> the rating is

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