To open the circuit-breaker the movement of the slide-valve L is reversed, thus admitting air below the piston M.

None of the circuit-breakers will be accidentally operated by a failure of the air pressure. The circuit-breakers already closed can only be opened by admitting air pressure below the piston M, and those that are opened can only be closed by admitting air pressure above the piston M1.

A recent modification of the circuit-breakers referred to above is illus

FIG. 148. Electrically operated three-phase circuit-breaker, closed. trated in figs. 147 and 148. Fig. 147 is a front and sectional view of one of these circuit-breakers opened, and fig. 148 shows the same circuitbreaker closed. The construction of the oil pots is somewhat similar to that shown in fig. 146, the main difference being that, in addition to the circuit being completed through the rods and contacts in these oil pots, external contacts A1 and A2 are provided to make connection with the oil-containing pots. When the switch is closed the main current is carried by these external contacts, but the circuit is finally broken by the contacts under oil as before.

This circuit-breaker is operated electrically only, without the aid of pneumatics. This is effected by the small series wound electric motor B. A diagram of the connections to this motor is shown in fig. 149, the lettering of this diagram corresponding to that in figs. 147 and 148. When the circuit-breaker is open, a powerful spring C is compressed; this spring tends to force down the cross-arm D. This movement is prevented, however, by the toggle-jointed lever E, the three fulcrums of which are in line with each other. When the circuit is completed through the motor B, this commences to rotate, and turns the wheel F in the direction indicated by the arrow. As, however,

K1

K2

L

B

FIG. 149.-Diagram of connections for controlling electrically operated circuit-breakers.

one end of the toggle-jointed lever E is connected to the shaft driven by the wheel F, the centre joint of this lever is moved out of the straight line between the fulcrums at each end of the double lever, and consequently this joint is unable to resist the tension of the spring C. This, therefore, causes the cross-arm D to descend, and the lower end of the lever E rotates with the ratchet wheel in a clockwise direction. This rotation will, to start with, be considerably faster than the movement of the driving wheel to which the pawl is

attached. As, however, the speed of the motor accelerates, which it will do rapidly, having no work to do, the driving wheel will gain on the ratchet wheel, and will finally drive this through the ratchet and pawl, thus completely closing the circuit-breaker, and compressing the lower spring C1 in doing so. The motor is thrown out of gear when the centre joint of the toggle-jointed lever E has been rotated through an angle of 180 degrees round the shaft. This is effected by means of the commutator G in fig. 149. This commutator is carried and rotated by the same shaft as the ratchet wheel F, the movement being also in a clockwise direction.

H is a two-way switch on the operating desk, and J1 and J2 are the respective red and green indicating lamps which show whether the circuit

breaker is opened or closed. The diagram indicates the position in which the switch and commutator are left after the operation of closing the circuit-breaker. The lower brush K2 has just broken circuit with the segment of the commutator connected to the motor and electro-magnet L controlling the clutch M on the motor shaft, and the upper brush K1 has just made contact with this segment. With the switch H in the position shown, no current will pass through the motor except that through the indicating lamp J2, which is incandesced, and indicates that the switch. is closed. No current will pass through the lamp J1, as this is shortcircuited by the switch H and open-circuited at the commutator brush K2. The small current through the lamp J2 will be insufficient to start the motor B. To open the circuit-breaker the switch H is thrown over to short-circuit the lamp J2. This allows a sufficiently heavy current to pass through the commutator brush K1, the electro-magnet L, and motor B, to throw the clutch in circuit and start the motor. The first movement of the motor carries the centre of the toggle-jointed lever into the position shown in fig. 148, and thus allows the powerful spring C1 to lift the cross-head and rapidly open the circuit; the action of the compression spring C1 is assisted by the extended spring C2. The continued rotation of the motor again compresses the upper spring C until it is thrown out of gear by the circuit being broken at the contact K1. The circuit will be completed through K2, and the green lamp J1 will be incandesced, showing that the circuit-breaker has been properly opened. The positive and negative 'bus bars shown at the bottom of the diagram, fig. 149, are excited by secondary batteries. The chances of this supply failing are, therefore, very remote. It will be seen, however, that, should it fail, none of the circuit-breakers will be affected, and the failure will be immediately shown by the extinction of the indicating lamps.

The circuit-breakers may be opened manually by means of a handle fitted to the projecting end P of the shaft carrying the ratchet wheel F.

The use of two circuit-breakers B and D, fig. 145, is to enable either of these switches to be tested. If one circuit-breaker only was used, it is evident that this could not be closed without connecting the generator on to the 'bus bars, but by closing one at a time the operation of the relay control may be frequently tested.

A modification of the switching arrangements at the New York stations has recently been installed in the Niagara Falls Power Co.'s new powerhouse. In this case the oil break circuit-breakers C1 C2 are placed over a subway which runs parallel with the generators. This subway carries the main 'bus bars B1 B2. Fig. 150 shows a sectional elevation across this subway.

The relay controlling switches and indicating instruments are arranged on panels A mounted on a raised gallery in the centre of the engine-room.

These panels are equipped with dummy 'bus bars similar to those used in the New York stations. In this case, however, the instruments are arranged in their proper positions in the dummy 'bus bar circuits. An objection raised against the New York system is that the switchboard attendant, in carrying out any switching operations, must first go to the operating desk or bench-board, as it is termed, and pick out the relay

FIG. 150.-Section showing general arrangement of Niagara switchgear. switch required, then taking his eye off this switch, he must select from a number of instruments in front of him on entirely separate panels the instruments involved in the operation he is about to effect. In the Niagara modification of this arrangement each generator or feeder panel constitutes a complete unit, and has on it all the instruments, relay switches, and dummy 'bus bars appertaining to that particular generator or feeder. The generator field rheostats and field switches are located under the switchboard gallery.

CHAPTER VIII.

GENERAL ARRANGEMENT OF CONTROLLING APPARATUS FOR LOW-TENSION SYSTEMS.

B.O.T. traction panel-Newington switchboard-M'Donald Road, Edinburgh, switchboard-'Glasgow': generator panels opposite each machine, feeder panels arranged on gallery above in groups of eight, with alternate groups of positive and negative feeders' Hackney': generator and feeder panels arranged back to back—' Willesden': modification of 'Ferranti' high-tension board, with special selector switches for connecting generators to 'bus bars--' Kelvin and White' switchboard at Glasgow Exhibition positive and negative panels placed one over the other-Boston' switchgear, equipped with motor-operated switches.

:

ALTHOUGH the design of low-tension boards does not perhaps vary to quite the same extent as that of high-tension boards, there are, nevertheless, very marked differences in the general arrangement of the switching apparatus for controlling low-tension systems. The usual switchboard for low-tension. three-wire systems, at any rate for small installations, consists of a number of slate or marble panels supported on an iron framework, with the instruments and switches, etc., on the face of the board, and with the 'bus bars and connections at the back of the board. The battery controlling switches. and middle wire switches and instruments are as a rule mounted on a panel in the centre of the board, and the positive and negative generator and feeder panels are fixed respectively to the right and left of this centre panel.

Board of Trade Traction Panel.

In the case of traction switchboards, the Board of Trade panel is usually located in the centre of the switchboard. This panel is equipped with the instruments necessary for making and recording the various tests specified in the Board of Trade regulations relating to this class of work. Fig. 151 shows Messrs Nalder Bros. & Thompson's standard B.O.T. panel A is an ammeter for indicating the line leakage of any feeder. It is calibrated with two scales, one reading from 001 to 05 ampere, and the second from 01 to 5 ampere. Bis a two-way switch enabling either of these scales