Quite often the local circuit will power the coil in an emergency lighting relay unit, this opens the NC contacts and the emergency lights in that area are off. If the local circuit loses power the relay lets go and the emergency lights illuminate fed from the supply from the central battery system. the only ones I've done anything with all output 240v all the time from the mains to all the relay contacts, if the central battery unit loses power the inverter then supplies the 240V.
The other type of system I've worked with has the same arrangement but the local circuit feeds the relay coil, the rooms switched supply feeds the relays NO contacts, the battery supply feeds the NC contacts and the common connection is to the rooms light fittings, so the main lighting becomes emergency lighting if the power fails. I've only seen this setup in listed buildings where emergency bulkheads would not look good in the property.
