There are various reasons for emergency and exit lights to fail, the main reasons can be attributed to either globe failure or battery failure.
All emergency and exit lights must undergo a 90 minute discharge test of the battery every 6 months as stated under the Australian Standard: AS2293.1. Situations may arise where the backup battery fails due to to batteries becoming discharged – they do not hold a charge or do not receive a charge from the power converter.
With any light failure, it is important to realise that simply replacing the backup battery may not resolve the fault. The light may in fact function again due to any charge/backup power in the newly installed battery but it may fail to recharge beyond that due to a faulty power converter, therefore the most economical recommendation is to replace the complete unit.
During any test, tubes in exit light fittings are replaced and tubes or globes in emergency lights are replaced if the unit fails to illuminate. Any damage to the unit or diffuser constitutes an immediate fail and the unit must be repaired or replaced.