Office Emergency Lighting on Switched Local Circuits
A 9-storey office building has a new emergency lighting system using self-contained LED luminaires with 1-hour integral batteries. The contractor has taken the unswitched supply for each corridor luminaire from the load side of the corridor lighting switch, so the batteries charge only when the corridor lights are on. When the corridor is switched off at night, the luminaires illuminate from battery until exhausted. The building has a 500 kVA generator with 12 s start and 30 s transfer; the fire alarm is addressable with a two-stage alarm. The corridor is 32 m long, 2.4 m wide, with one exit at each end. The owner wants to omit central battery monitoring because 'each luminaire has a battery and a green LED.' During a test, several luminaires in a 20 m section failed to switch to emergency when the upstream submain was opened. You must review the scheme for compliance and reliability.
1. Diagnose the wiring error and state the correct supply arrangement for the self-contained emergency luminaires.
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A self-contained emergency luminaire needs a permanent unswitched live supply for battery charging and for detecting loss of normal supply. Taking the supply after the corridor switch means the luminaire sees a 'power failure' every time the corridor is switched off, so the battery discharges nightly and is unavailable for a real emergency. The correct arrangement is to feed the emergency luminaire from the permanent live side of the local lighting circuit, ahead of the switch, or from a dedicated emergency final circuit. If the luminaire has separate switched and unswitched terminals, the switched line may control normal illumination while the permanent live maintains charging. Test by opening the local switch: the luminaire should remain in charge mode. Then open the upstream submain: it should transfer to battery.
2. Evaluate the generator backup and battery duration, and the significance of the 20 m section failure.
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A 12 s generator start and 30 s transfer is within typical emergency supply criteria, but the 1-hour battery duration must be verified for the actual luminaire load and ambient temperature. The generator does not remove the need for a duration test, because the battery must cover the transfer period and any generator failure. The two-stage fire alarm should initiate emergency lighting in the affected zone and along escape routes; verify the cause-and-effect matrix. The failure of several luminaires in a 20 m section when the upstream submain was opened indicates a wiring or device fault, not merely a battery issue. Check whether those luminaires are connected to the correct permanent live, whether their batteries are charged, and whether the test switch or addressable monitoring is wired correctly.
3. What commissioning evidence should be required before accepting the system, and what is wrong with relying on the green LED?
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Require a commissioning certificate with a 1-hour duration test on every luminaire, illuminance measurements along the 32 m corridor and at both exits, a functional test on local and generator supply, and a test of the fire-alarm interface. The measured illuminance must meet the design values for the escape route and exit areas, not just show that the lamp is lit. A green LED typically indicates only that the battery is connected or charging; it does not prove battery capacity, changeover, or emergency duration. If central monitoring is omitted, the owner must accept a documented monthly inspection and annual duration-test regime. Do not accept the 20 m failed section until it is rewired and retested.