Emergency Light Wiring Diagram

Emergency Light Wiring Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable emergency light diagram: download the diagram as SVG or open it and print to paper or PDF.

Understand how a maintained or non-maintained emergency light integrates a trickle charger, sealed battery, and inverter or LED driver into a circuit that fails safely when mains power is lost.

Emergency lighting is a life-safety system. Its purpose is to provide illumination for safe egress when normal lighting fails — during a power cut, a tripped circuit breaker, or a fire event that disrupts the mains supply. Understanding the circuit architecture is essential before servicing, because a wiring error that disables the charge circuit or prevents the battery from supplying the lamp can result in complete darkness during an emergency, with potentially fatal consequences.

Most self-contained emergency lights share a common internal circuit: a mains input feeds a trickle charger that maintains a sealed lead-acid (SLA) or nickel-cadmium (NiCd) battery at full charge under normal operating conditions. A transfer circuit monitors the mains input. When mains voltage drops below a threshold (typically 160–180 V AC on 230 V systems), the transfer circuit switches the lamp load from mains to the battery within one second (the maximum transfer time specified in standards such as AS 2293, BS EN 60598-2-22, and NFPA 101). The battery then powers the lamp either directly (for DC LED lamps) or via an inverter (for fluorescent tubes or high-voltage LED drivers), for the rated duration period.

Maintained emergency lights are energised at all times — both from mains under normal conditions and from battery during a power failure. They function as normal luminaires during regular operation. Non-maintained emergency lights are dark under normal conditions; they illuminate only during a mains failure. The wiring diagram differs: a maintained light connects the lamp load in parallel across both the mains and battery output circuits, while a non-maintained light connects the lamp load only to the battery/inverter output circuit.

The trickle charger is a current-limited power supply, typically delivering a charge current of C/10 to C/30 (a tenth to a thirtieth of the battery's ampere-hour capacity), maintaining the battery at float voltage without overcharging. Battery float voltage is approximately 2.25–2.30 V per cell for SLA (13.5–13.8 V for a six-cell 12 V battery). Exceeding float voltage causes water loss from SLA cells and shortens battery life significantly.

The test and inhibit function (marked on most units as a 'test' terminal or remote test input) allows testing the emergency function from a remote switch without disconnecting mains — this is the correct method for the routine function tests required by most standards.

How to wire emergency light diagram

Isolate mains power and verify dead At the distribution board, isolate the circuit feeding the emergency light. Lock off or tag the circuit breaker. Use an approved non-contact voltage tester and a calibrated multimeter to verify the circuit is dead at the emergency light terminals. Emergency lights may remain illuminated from battery after mains isolation — this is normal and does not indicate the circuit is live.
Disconnect the battery before working on internal wiring After verifying mains is isolated, open the fitting and disconnect the battery connector. The internal circuits of an emergency light remain live from the battery even with mains isolated. Disconnecting the battery eliminates this hazard. Note battery polarity before disconnecting.
Identify and connect mains input terminals Emergency lights have clearly marked terminal blocks: Live (L), Neutral (N), and Earth (PE) for mains input; and in many cases a switched Live input for maintained operation (allowing the fitting to follow a switched circuit for non-emergency normal use). Connect the permanent live to the permanent live terminal; connect the switched live (if required for maintained operation) per the manufacturer's diagram.
Connect the remote test and inhibit wiring (if applicable) Many installations include a central test switch or emergency lighting test controller. Connect the remote test cable from the controller to the fitting's test terminal per the manufacturer's wiring diagram. Verify polarity and voltage requirements for the test input — some accept a dry contact closure, others require a voltage signal.
Reconnect battery and verify charge voltage Reconnect the battery with correct polarity. Restore mains power. Using a multimeter, measure voltage across the battery terminals — it should read the battery float voltage (approximately 13.5–13.8 V for 12 V SLA) within a few minutes. A reading equal to open-circuit battery voltage (approximately 12.6–12.8 V) without any rise indicates the charger is not functioning.
Perform function test Activate the test function (test button or remote test switch). The lamp should illuminate within one second at rated brightness. Allow a partial duration test (typically three minutes, or as required by the maintenance schedule). Restore mains after the test and verify the lamp extinguishes (non-maintained) or remains on from mains (maintained), and that the charger resumes charging the battery.
Record test results Log the test date, outcome, battery voltage before and after test, and duration achieved. Regulations in most jurisdictions require a maintained log of emergency lighting test results. A minimum monthly function test and an annual full-duration test are required under BS EN 50172, AS 2293, and similar standards.

Specifications

Mains input (typical)	230 V AC 50 Hz (UK/AU/EU) or 120/240 V AC 60 Hz (North America)
Battery type (most common)	Sealed lead-acid (VRLA/SLA) or Nickel-Cadmium (NiCd)
SLA float voltage (12 V battery)	13.5–13.8 V (2.25–2.30 V per cell)
Minimum emergency duration (most standards)	90 minutes (BS EN 50172, AS 2293, NFPA 101)
Mains failure transfer time (maximum)	1 second (BS EN 60598-2-22, AS 2293)
Minimum maintained illuminance (egress)	1 lux on escape route floor level (BS EN 50172); 1 foot-candle (NFPA 101)
Battery replacement interval (SLA, indicative)	Every 3–5 years (environment- and depth-of-discharge dependent)
Required test frequency (typical)	Monthly function test; annual full-duration test

Safety warnings

Emergency lighting wiring and installation must comply with applicable standards: NFPA 101 and NEC (USA), BS EN 60598-2-22 and BS EN 50172 (UK/Europe), AS 2293 (Australia), IEC 60598-2-22 (international). Installation must be performed or verified by a licensed electrician.
Always isolate the mains circuit AND disconnect the internal battery before working on emergency light wiring. The battery will maintain the internal circuit live even when mains is isolated — failure to disconnect the battery has caused burns and short-circuit damage.
Do not substitute a battery with a higher capacity than specified without verifying the charger circuit is rated for the new battery — an undersized charger on a large battery will never achieve full charge, and battery life and emergency duration will be compromised.
Emergency lighting is a life-safety system. Do not disable, bypass, or leave in a non-functional state. If a fitting cannot be repaired immediately, inform the building owner and fire safety officer and implement alternative measures.
Routine testing, maintenance, and log-keeping are legal requirements in most jurisdictions. Failure to maintain and test emergency lighting correctly creates regulatory liability and, more importantly, a genuine risk to occupant safety.

Tools needed

Approved non-contact voltage tester
Digital multimeter (AC and DC voltage, resistance)
Lockout/tagout equipment for circuit breaker
Screwdrivers (for terminal block and fitting cover)
Torque screwdriver (terminal block connections)
Emergency lighting test log book
Stopwatch (for duration test timing)

Common mistakes

Working on the fitting with the battery still connected after isolating mains — the battery keeps internal circuits live and can cause burns if a tool bridges battery terminals.
Connecting the permanent live to the switched live terminal (or vice versa) causing the fitting to operate incorrectly in maintained or non-maintained mode.
Replacing a failed battery with a physically similar but higher-capacity unit without checking the charger's maximum charge current output, resulting in chronic undercharging.
Failing to perform or log regular function tests, resulting in both regulatory non-compliance and an undetected battery or charger failure that would only become apparent during an actual emergency.
Using the main circuit breaker as the test method — disconnecting mains supply from the distribution board to test an emergency light disturbs all other loads on that circuit and risks mains wiring faults during reconnection.

Troubleshooting

Lamp illuminates continuously even with mains power on: Cause: Mains not reaching the fitting's detection circuit, or failed mains detection component Fix: Verify mains voltage at the fitting's L and N terminals with a multimeter. If mains voltage is absent at the terminal block, trace the supply fault. If mains is present but the fitting behaves as if on battery, the internal mains detection circuit has failed — replace the fitting or the PCB (if the manufacturer supports component-level repair).
Battery voltage drops rapidly during test — rated duration not achieved: Cause: Battery has lost capacity (typically after 3–5 years for SLA, 2–3 years for NiCd in warm environments) or battery was never fully charged due to charger failure Fix: Measure battery open-circuit voltage after a 24-hour charge period — a healthy 12 V SLA should read 12.7–12.8 V. If below 12.5 V after full charge, the battery has lost capacity and must be replaced. Verify the charger is delivering correct float voltage before fitting a new battery.
Charger not maintaining battery voltage (battery reads below 12.0 V after 24 h on mains): Cause: Failed charger circuit, loose mains connection, or blown internal fuse on the charger supply Fix: Verify mains voltage at the fitting input terminals. Check for an internal fuse on the charger PCB (some fittings include one). If mains and fuse are correct but float voltage is absent, the charger circuit has failed — replace the fitting or PCB.

Frequently asked questions

What is the difference between maintained and non-maintained emergency lighting?

A maintained emergency light is energised continuously — it operates as a normal light under mains power and automatically switches to battery when mains fails. A non-maintained unit is dark under normal conditions and illuminates only during a mains failure. Maintained fittings are used where continuous illumination is required (exit signs, high-risk areas); non-maintained in areas that have adequate normal lighting.

How long must emergency lighting operate after a mains failure?

Minimum duration depends on the applicable standard and occupancy type. BS EN 50172 and AS 2293 require a minimum of 90 minutes for most occupancies. NFPA 101 requires 90 minutes for means of egress lighting. Higher-risk occupancies, hospitals, and underground spaces may require extended durations of two or three hours. Always verify the requirement for the specific occupancy.

Why does my emergency light illuminate immediately when the mains is on?

This indicates the transfer circuit is not detecting mains voltage correctly — it believes mains has failed even though it has not. The most common causes are a failed mains detection circuit inside the fitting, a loose mains connection at the terminal block, or a wiring error leaving the mains input terminal disconnected while the battery is connected. Check mains voltage at the input terminal with a multimeter.

How do I test emergency lighting correctly without disconnecting the mains?

Use the test function built into the fitting (a push-button or remote test terminal). This simulates a mains failure by inhibiting the charger and transfer circuit, causing the fitting to switch to battery. The lamp should illuminate at full or rated output immediately and sustain for the test period. Do not disconnect mains as a test method — this risks mains faults during reconnection and can disturb other loads on the circuit.

What is the battery float voltage for a 12 V sealed lead-acid emergency light battery?

A 12 V six-cell SLA battery should be maintained at approximately 13.5–13.8 V (2.25–2.30 V per cell) in float mode. Charge voltage above 14.4 V (2.4 V per cell) causes electrolyte loss and premature battery failure. Most emergency light charger circuits are factory-set; do not adjust unless you have the manufacturer's service documentation.