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A — Storage Battery
B — Safety or Secondary Lights
CI, C2— Relays
D — Delay SW. To allow filament to heat up
E — Full Wave Hot Cathode Valve
F — Regulating Switch
H — Adjustable Resistance Control
J — Transformer
Kl— Lights On
K2 — Charging
Rl, Gl — Red and Green Signal Lamps on Panel
R2, G2 — Corresponding Remote Lamps
Fig. I. Floating Battery Emergency System.
ready for emergency use. Furthermore, maintenance would be easier, and also the number of cells could be increased to give a voltage more nearly equal to the normal figure. With this arrangement, the battery on discharge is normally trickle-charged, i.e., it receives a very small charging current, which is sufficient to compensate for the open circuit losses of the cells and to maintain them in a fully charged condition. The current required to do this is approximately one tenth of an ampere for a 100 ampere-hour battery.
In the event of a mains failure, a gravity operated change-over contactor connects the battery to the emergency lighting circuits, which are normally supplied from a step-down transformer. When normal supply is resumed, the contactor is pulled back to its normal position and the battery can then be given charge at a higher rate, to bring it back to the fully charged condition. Fig. 2 shows the circuit employed for this scheme.
Evidently the Home Office was not satisfied that the operation of automatic switches was sufficiently reliable for them to be employed for emergency lighting of kinemas, though equipment of this type is largely employed for emergency lighting in hospitals, large stores, and other places where emergency supply is considered to be necessary. In Scotland the use of an automatic switch for this purpose has been permissible.
At this point the fact may be mentioned that improvements in the manufacture and a reduction in the price of contact rectifiers of the selenium type has led to their more general use, and the hot cathode gas-filled rectifier has tended to go out of use for these equipments.
Construction of Cells
Before dealing with the question of the care and maintenance of these emergency batteries, it would be as well to describe briefly the construction of cells of the type generally employed for this purpose. The positive plates are generally of the Plante type. The separator usually employed consists of a veneer of treated wood which is held between slit dowels which may be of ebonite or of treated wood.