If operating conditions admit, these devices are made non-automatic and are left disconnected except in case of emergency; but if it be necessary for them to be continually in service, they may be made automatic by means of instantaneous overload relays connected to current transformers in the low voltage bus; the relays being adjusted to trip the circuit breaker under short circuit conditions, confining the trouble to one section and preventing the circuit breakers rupturing more than their rated capacity.
Installations with but one bank of power transformers, and without high voltage bus, are provided with automatic circuit breakers operated by an inverse time limit relay.
The relay is connected to the secondaries of current transformers, which in turn are connected in the low voltage side of the power transformer.
Stations with more than one bank of power transformers, a high voltage bus, and high and low voltage circuit breakers, may have both circuit breakers arranged to trip at the same time or one after the other. As in the former case, they are operated from the inverse time limit relay connected in the low voltage side.
Figs. 2,332 and 2,333.—Diagram showing two phase four wire no voltage connections for I-T-E circuit breaker. The two no voltage coils for two phase four wire circuits are connected respectively to binding posts B, C and A, D on the face of the base. B and D are connected to lower spring contacts 2 and 1 respectively, of the small disconnecting switch. (In instruments supplied on individual bases, these connections are made in the factory, let into channels in back of base and covered with wax.) Each of the upper contacts a and b of the disconnecting switch is connected respectively through resistance R2 and R1 to one main in each phase at aa and bb. C and A are respectively connected to the other main in each phase at 3 and 4. Thus each of the no voltage coils operates across one phase independent of the other. The terminals 3, aa, bb and 4, must, in all cases, be so connected that they will be subject to the full voltage of the circuit, irrespective of the position of the starting switch.
Figs. 2,334 and 2,335.—Diagram showing two phase three wire no voltage connections for I-T-E circuit breaker. The two no voltage coils for two phase, three wire circuits are connected respectively to binding posts B, C and A D on the face of the base, and from A and C connections are made to lower contacts 2 and 1 respectively of the disconnecting switch. Binding posts B and D are connected together on the back of the board. (In instruments supplied on individual bases, these connections are made in the factory, let into channels in back of base and covered with wax.) Each of the upper contacts a and b is connected respectively through resistance R2 and R1 to one of the mains at aa and bb as shown. D is connected through resistance R3 to the common wire of both phases at 3 B and D being connected as aforesaid, thus forming a common connection for both no voltage coils. Terminals aa and bb of the resistances must be connected to the outside main across the two phases, terminal 3 to the main common to both phases, the connections being so made that these terminals will be subject to the full voltage of the circuit irrespective of the position of the starting switch.