Fig. 2,206.—Westinghouse fused starting switch for squirrel cage motors. It is arranged for National Electric Code fuses on one end only and has springs on the other end to open the switch automatically if left closed at this end. The corresponding terminals at both ends of the switch are connected in grooves in the back of the slate base so that the wiring need be connected to one set of these terminals only, thus decreasing the number of connections necessary, as shown in fig. 2,207. In starting an induction motor, the switch is thrown to the end that is not fused and held there until the motor is up to running speed; then it is quickly thrown to the fused position, thus protecting the circuit under running conditions.
Fig. 2,207.—Diagram of connections of Westinghouse fused starting switch for squirrel cage motors. The starting current of induction motors is several times the normal running current and, when the controlling switch is fused to carry the running load only, the fuses are apt to blow when the motor is started. The fuses must be of a capacity to prevent overloads under running conditions. These switches are designed to meet this difficulty and are used without auto-starters to control motors up to 5 horse power rating.
Ques. What is the objection to open air break?
Ans. The relatively long gap required to extinguish the arc, limiting this form of switch to low or moderate pressure circuits.
The open air arc may cause very high voltage oscillations when the circuit contains inductance and capacity unless the break occur at zero value.
Fig. 2,208.—Westinghouse single pole disconnecting switch. Disconnecting switches are used primarily for isolating apparatus from the circuit for purposes of inspection and repair; also for sectionalizing feeders. They are not designed for opening under load, and therefore no attempt should be made to open them with current in the circuit. In connection with lightning arrester installations, disconnecting switches are particularly useful, providing a simple and effective means for isolating the arresters while cleaning and inspecting. The switch is opened and closed with a hook on the end of a wooden pole, which hook engages in a hole provided in the switch blade. This type of disconnecting switch is intended for wall mounting. The live parts are mounted on porcelain insulators carried on a cast iron yoke or base, forming a simple and substantial construction.
Ques. What are disconnecting switches?