American Pneumatically and Electrically Controlled Switchgear.The electrical controlling arrangements of some of the large American

power plants represent a distinct departure from anything to be found in this country. It almost appears at first sight that the precautions that

1

1 Since the above was written three or four boards designed on these lines have been erected in this country.

have been taken are unnecessarily elaborate and costly. When it is remembered, however, that the power to be controlled in one of these generating stations exceeds 40,000 horse-power, and when the very serious consequences of even a momentary interruption to the supply are considered, one realises that the heavy expenditure incurred on switchgear is wholly justifiable.

Fig. 145 is a sectional elevation of the electrical controlling arrangements at the Metropolitan Street Railway Co.'s station in 96th Street, New York. This work has been carried out by the General Electric Co. of Schenectady. All the switching operations are normally conducted at the desk X. A number of miniature switches and 'bus bars on the face of this desk constitute a complete model of the electrical connections and switches in the entire station. These miniature switches are each electrically connected to one of the large circuit-breakers on the floors above. Small red and green lamps inserted at intervals in the miniature 'bus bars, etc., indicate what sections of the connections are dead or alive. The attendant thus has constantly before him a complete diagrammatic indication of the condition of the whole system. The final adjustment of the engines is also controlled from this operating desk by means of a relay acting on the steam governor. When an incoming generator has to be paralleled, the engine-driver starts the plant and runs it up to approximately the speed of the other plants, but the final adjustment is effected by the switchboard attendant. All the measuring instruments required for the system are mounted on panels M behind the controlling desk.

A diagram of the electrical connections of the plant in this station is shown in fig. 120, Chapter VI. The lettering of the various apparatus is the same in figs. 120 and 145. The three-core cable A is connected directly between one of the three-phase generators and the pneumatically controlled circuit-breaker B. The circuit is completed through the ammeter transformer C, a second generator switch D, to the group ammeter transformers and group switches E1, E2, and E3. From this point the current may be directed by selector switches F1, F2, F3 (indicated by dotted lines in fig. 145) to either of the 'bus bars G1 or G2 of each phase. The selector switches are merely heavy multiple-blade knife switches, and are not intended to break the circuit with current on. The construction is, in fact, such that it is impossible to accidentally draw out a heavy arc with these switches. This is effected by a catch which only permits the switch to be opened in the first instance a very short distance. If, therefore, a section carrying current is accidentally opened, a very short arc will result, and the attendant on seeing this can immediately reclose the switch and ascertain the source of the flow of current. From the respective 'bus bars current is conducted to group circuit-breakers H, and through these to the respective feeder circuit-breakers J and feeder ammeter transformers K.

The main 'bus bars of the respective phases are efficiently isolated from each other by double soap-stone slabs L.

The construction of the pneumatically controlled oil break circuitbreakers is illustrated in fig. 146. The terminals of each of the circuits to

FIG. 146.-Pneumatically operated three-phase circuit breaker.

be completed are connected respectively to the mushroom-shaped contacts A1 and A2. Metal oil pots B1 and B2 are supported by insulators in such a manner that they rest on, and make connection with, the contacts A1 and A2. Metal rods C1 and C2 connected at their upper extremities are guided through porcelain insulators D1 and D2 into contacts at the bottoms of the oil pots B1 and B2. The circuit is completed through these rods when they are lowered. The movable contacts referred to are carried at the end of

wooden rods E, supported from the cross-beam F. When the circuit-breaker is open, this cross-beam is held up by the catch G. The circuit-breakers inserted in series with the respective phases, though mechanically connected for simultaneous control, are efficiently isolated from each other by brickwork partitions covered at the top by a soap-stone slab H, and in front by an iron door J. It will thus be seen that, in the event of a heavy arc being started across the circuit-breaker controlling one of the phases,

FIG. 147.

Electrically operated three-phase circuit-breaker, open.

it is practically impossible for this arc to make contact with one of the other circuits, and thereby cause a short circuit.

To operate these circuit-breakers a current from a local secondary battery is caused to flow through the solenoid K by closing one of the controlling switches on the operating desk. This draws down the plunger and valve L, and admits air from an air compressor into the cylinder above the piston M. At the same time air is admitted above the small piston M1, and this is forced down against the spring N, thereby releasing the catch G, and allowing the air on the top of the piston M to close the circuit-breaker.