Central Battery System

This will depend how it has been wired in - if for example the batteries are only connected to the EM fittings via a common bus that is dead most of the time then the switching to energize that is central switch that would be harder to change as it is designed for one-out all-out operation.

If however battery power is available at all the fittings all the time, and there  is some sort of alarm wire that puts the lights on, then you already have half of what you need.
It may be easier to add an additional stand-alone EM fitting or two that are supplied by the local circuits you wish to monitor if the re-wiring becomes too difficult and its only a couple of exits that need illumination.

Mike.

Central battery powered Emergency lighting systems have power supply monitoring devices to install in the local DB. Which is connected to the central battery system Via data cables (Beldon cable). If your Central battery supplier has such devices, I reckon that is the safest and technically sound method. 

Hope this helps. 

PAB

You will need to monitor each and every lighting circuit, or a % of lighting circuits in an open space to ensure adequate emergency lighting. Generally this is done using phase sensors at each lighting DB connected back to the CBS with a data cable. The tripping of an MCB would then force a programmed zone into an ON state. 

Is this an AC/DC or an AC/AC system? I suspect the latter given the description. Generally with AC/AC systems, there are dual supplied luminaires with change over relays, with the backup supply live 24/7. This protects against local circuit failure as well as a general power failure. AC/DC systems have a single feed to the luminaire that provides AC during normal operation, and then DC during backup operation. 

Do you have stand-alone dedicated emergencies or conversions? 

Hold Off Relays:

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