Fig. 995.—Double throw switch for use in isolated plants when auxiliary power is used from the central station in case of breakdown.
Ques. When power from an outside source only is to be used what must be determined before wiring?
Ans. The system of wiring of the supply. If a three wire system be used, the general arrangement will be as shown in [fig. 997].
Fig. 996.—Double throw three pole switch for use in isolated plants where auxiliary power is brought in through three wire system. The side of the switch controlling the current is bridged as shown.
Ques. Would it be expensive to change a regular three wire system to a two wire system?
Ans. It would require the reinforcement of all mains and feeders by an additional wire. This wire would be connected with the neutral wire so as to make the capacity of the neutral equal to the sum of the other two. If a three wire two wire system had been originally installed, no change in the wiring system would be necessary. The only change would be at the service end of the switchboard, and the doubling of the size of the center fuses.
