Nevelin Keepalite mercury-arc emergency lighting system
Background
Many buildings are required to have emergency lighting which remains alight for a minimum time in the event of the main electrical supply failing. Before small maintenance-free rechargeable batteries were available economically, emergency lighting installations were often supplied through a separate system of wiring from a central battery large enough to serve an entire building or wing. As nickel-cadmium and sealed-lead-acid batteries became more reliable and cost-effective, a trend started towards self-contained emergency light fixtures, but existing installations and high-specification designs continued to favour the more durable central-battery system. Although extra wiring is required to distribute the emergency supply, the system has a number of advantages including ease of maintenance and testing, increased backup time and simplicity of integrating backup lighting into any normal light fitting.
Many smaller central-battery systems used a battery voltage lower than that of the mains, typically 50-100 volts DC, to minimise the cost and size of the battery. For large buildings a mains-voltage battery (240V in the UK) could be justified by the lower cost of wiring (which could be of lighter gauge owing to the lower lamp current) and standardisation of lamp voltage. The batteries were commonly wet lead-acid accumulators built of individual 2V cells, which on account of their large size and ventilation requirements were often housed in dedicated rooms. Thus a large building might have one or more emergency lighting battery rooms adjunct to electrical switchrooms which would also house the chargers. The function of the charger or rectifier units would typically be threefold: To recharge the battery after discharge during a supply failure, to provide a top-up or 'trickle' charge to keep it in good condition, and to power the lighting installation without discharging the battery. Motor-generator sets, dry plate rectifiers, thermionic valves and mercury-arc rectifiers were all used for this purpose, with the mercury arc rectifiers being best suited to the largest installations.
'Nevelin / Chloride Keepalite' plant with mercury-arc rectifier