Emergency Power & Lighting Systems for Airports
How UPS, diesel generators, independent feeders and changeover systems keep runway, taxiway, PAPI and stop-bar lighting available, and how switch-over performance is specified, tested and maintained.
Emergency Power & Lighting Systems for Airports
Airport lighting cannot be treated like ordinary commercial lighting. Runway edge, threshold and end lights, centreline, touchdown-zone and approach lighting, PAPI, stop bars and essential taxiway lighting may perform safety-critical functions. A power loss has to be managed so the required visual aids are restored within the applicable switch-over time.
In aerodrome engineering the more precise ICAO term is generally secondary power supply. ICAO allows it to come from an independent public power source or standby units such as engine generators and batteries. The goal is not merely lights during a blackout. It is the required continuity, availability, changeover performance and operational integrity of essential visual aids.
A modern airport power architecture may combine
Emergency Power vs Secondary Power Supply
Emergency power
A broad engineering term for backup electrical systems used when normal power fails.
Secondary power supply
The aerodrome-specific ICAO concept for supplying facilities that need continued service after failure of the primary supply.
Standby power
Power from a secondary source such as a generator or battery system.
UPS
An uninterruptible power system that holds selected loads without the interruption of a conventional generator start and changeover.
Why Airport Lighting Needs Backup Power
Generator only
Utility failure → generator starts → transfer → load restored. This can take several seconds, which can be operationally unacceptable for some visual aids.
UPS bridging to a generator
Utility → online UPS → AGL load, with UPS → DG as the longer-duration path. On utility failure the UPS carries the load at once, the DG starts, becomes available and then supplies the UPS. This gives fast continuity and extended backup.
Current AAI aeronautical information shows this kind of architecture in India. Hyderabad, for example, states that critical AGL circuits are supported by UPS and that a standby DG takes over automatically after the specified UPS backup period if commercial power is not restored.
Power Failure Sequence Explorer
Step through a UPS-plus-DG sequence to see how the status changes and what to confirm at each stage.
Status board
What engineering should confirm
Primary and Secondary Power Architecture
Representative arrangement. The actual design depends on the airport. The dashed line shows monitoring.
Sources
- Utility supply
- Independent feeder, separate to avoid common-cause failure
- Diesel generator
- UPS and battery bank
Switching and distribution
- Main switchgear
- Automatic transfer / changeover
- AGL power distribution
- Constant current regulators for series circuits
Supervision
- Source, UPS and generator status
- Alarms
- AGL availability
- Common status picture in ALCMS
UPS, Diesel Generator and the Combined Architecture
Online UPS
Normal: utility → rectifier → DC link / battery → inverter → AGL load. On utility failure: battery → inverter → AGL load. The protected output continues without waiting for the generator, which suits very short power-restoration times.
DG generator
Supplies longer outages: essential airport loads, AGL, and support to UPS systems. A set includes engine, alternator, fuel system, starting batteries, control panel, protection, exhaust, cooling, fuel tanks and paralleling equipment where needed. Large airports may use several sets for redundancy and load distribution.
Combined, the UPS is the bridge and the DG is the long-duration source. Current AAI information for Delhi describes multiple DG sets for airfield lighting with UPS-backed AGL, showing how large airports distribute and duplicate their power.
Utility and generator feed a changeover switch; the UPS carries control and ALCMS.
AGL Visual Encyclopedia: Standby power and UPS CCR power input Power quality monitoring
ICAO Maximum Switch-Over Times
Switch-over time is not one value for every light. It depends on runway type and operational category. Annex 14 Table 8-1 sets maximum values by visual aid. The extract below is for precision approach Category II/III.
| Visual aid (Cat II/III) | Maximum switch-over time |
|---|---|
| Inner 300 m of approach lighting | 1 second |
| Other approach lighting | 15 seconds |
| Runway threshold | 1 second |
| Runway end | 1 second |
| Runway centreline | 1 second |
| Runway touchdown zone | 1 second |
| Stop bars | 1 second |
| Essential taxiway | 15 seconds |
Do not apply one value to every airport
Requirements depend on runway classification, approach category, take-off RVR requirements, procedures, terrain, lighting configuration and authority requirements. The right question is “what switch-over performance does each protected visual aid need under the applicable category?”
How switch-over is measured
Switch-over time is defined by the behaviour of the actual light intensity: the time for measured intensity to fall to 50% and recover to 50% in the relevant direction. Generator start time is not AGL switch-over time, and UPS transfer time is not automatically the same as measured light recovery.
Where the interruption time goes during a generator changeover.
AGL Visual Encyclopedia: Generator changeover and switch-over time Standby power and UPS Mode changeover
Indian Practice and Real Configurations
| Airport / source | Configuration described | Note |
|---|---|---|
| Hyderabad | UPS-supported critical AGL circuits with seamless transfer, 30-minute UPS backup, then automatic DG takeover if commercial power is not restored. | The 30-minute figure is airport-specific. |
| Delhi | Multiple DG sets for airfield lighting with UPS-backed AGL arrangements and generator redundancy. | Shows a distributed, redundant design. |
| Cochin | Low-visibility procedures require standby power and describe critical lighting on fast-changeover arrangements. | Tied to LVP status. |
| Other AAI airports | UPS-supported critical AGL with transfer under one second at some CAT I/II installations. DG arrangements with about 15 seconds at others. | AAI publications regularly cite DGCA CAR Section 4 Series B Part I. |
There is no single emergency-power architecture for every Indian airport. Switch-over time and battery autonomy are separate parameters. Thirty minutes of UPS backup appears in some Indian configurations but is not a universal ICAO requirement.
Backup Power for Each Lighting System
Runway lighting
Often split into critical and less-critical loads. Critical: edge, threshold, end, centreline, touchdown zone and selected approach lighting. Loss of one source must not cause unacceptable loss of visual guidance.
Taxiway lighting
Not all taxiway lighting has the same requirement. ICAO separates essential taxiway functions. Classify each circuit by operational importance rather than putting the whole network on one backup arrangement.
PAPI
Visual approach slope indicators fall within the secondary-power framework. Include PAPI in the visual-aid power analysis and do not treat it automatically as critical or non-critical.
Stop bars
Safety-critical control lights at runway holding positions. For Category II/III the cited maximum is one second, so they need a very reliable and fast power and control architecture.
ALCMS
If the lights stay on but control loses power, remote switching, brightness control, monitoring, alarms, stop-bar control, route guidance and Follow the Greens are lost. Study power to the light and power to the control system.
A-SMGCS and Follow the Greens
Depend on surveillance, processing, networks, ALCMS and field controllers. Decide which equipment is UPS-protected, how stop bars and green routes behave in a power failure, what ATC sees and what the pilot sees. Validate in integrated commissioning.
Related Guides
ALCMS Follow the Greens concept A-SMGCS India PAPI engineering guide Runway guard lights and stop bars Taxiway lighting systemsBackup power depends on the lighting system, not the whole network.
AGL Visual Encyclopedia: PAPI power supply Stop bar monitoring ALCMS failure flow
Emergency Lighting vs AGL Secondary Power
AGL emergency / secondary power
Keeps aerodrome visual aids such as runway and taxiway lighting.
Terminal emergency lighting
Escape routes, stairs, corridors, critical spaces, fire-safety systems and exits. Governed mainly by building, fire, electrical and life-safety requirements, not AGL provisions.
Evacuation, apron and ATC lighting
Evacuation routes, operational or emergency-response apron areas and essential controller illumination where required.
Fire and rescue
ICAO lists essential equipment and facilities for aerodrome emergency-response agencies among loads that can need secondary power: fire station systems, emergency communications, access lighting, selected alarms. The list is airport-specific.
Load Classification and Load Schedule
Critical
Needs very fast continuity. Typically UPS-backed, such as critical AGL.
Essential
Tolerates a longer changeover within the applicable requirement. Typically DG-backed.
Non-essential
Can be shed during an emergency.
Example load schedule structure
| Load | Normal source | Backup source | Objective |
|---|---|---|---|
| Runway critical lighting | Utility | UPS + DG | Fast continuity |
| Approach lighting | Utility | UPS / DG as required | Applicable changeover |
| PAPI | AGL / utility | Secondary supply as required | Maintain visual guidance |
| Stop bars | AGL / utility | Fast backup | Protect runway holding |
| Essential taxiway | Utility | DG / UPS as required | Required continuity |
| ALCMS | Utility | UPS | Maintain control and monitoring |
| A-SMGCS | Utility | UPS | Maintain surveillance and control |
| ATC essential equipment | Utility | Dedicated backup | Operational continuity |
| Fire / emergency systems | Utility | Emergency source | Emergency availability |
| Apron lighting | Utility | DG as required | Operational support |
Generator, UPS and Battery Sizing
Generator sizing considerations
Do not size by adding nameplate wattages.
- Actual connected load and demand factor
- Motor starting current
- UPS input characteristics and harmonics
- CCR characteristics and non-linear loads
- Simultaneous operation and future expansion
- Redundancy, derating, altitude, temperature
- Fuel strategy
UPS sizing considerations
Size for the actual protected AGL load and required autonomy, not an arbitrary percentage.
- Load kW and kVA, power factor, efficiency
- Autonomy and battery ageing
- Temperature and future load growth
- Redundancy, recharge time, fault current
- Bypass arrangements
Online vs offline UPS
An online UPS supplies the load continuously through the inverter: no conventional transfer delay, power conditioning and a stable output. An offline or standby UPS switches to the inverter after detecting failure, which introduces a transfer event. Choose from the required switch-over performance and the project specification.
Battery technology
VRLA, flooded lead-acid, lithium-ion or other approved types. Consider autonomy, temperature, fire protection, ventilation, maintenance, life cycle, replacement, monitoring and environment. Base selection on the manufacturer’s certified system design and airport electrical requirements.
Independent Supply, Feeders and Generator Redundancy
Independent public power
ICAO allows an independent public power source as the secondary supply, provided it is sufficiently independent and simultaneous failure is extremely remote. Feasibility depends on the airport’s utility infrastructure.
Genuine feeder independence
Two cables leaving one vulnerable point are not independent. Consider separate routes, switchgear, protection and substations, common-cause failure, fire separation and physical separation.
Generator redundancy
N, N+1, dual-source, geographically separated or dedicated runway-area generators, so one failure does not take out the protected lighting. Set the level from the airport’s reliability and operational design.
Common-cause check
Supposedly redundant systems may share switchgear, a cable route, control power, a fuel system or cooling. True redundancy covers the whole chain.
Two feeders and a normally open bus tie.
AGL Visual Encyclopedia: Dual feeders Standby CCR changeover Single point of failure analysis
Power Monitoring and ALCMS Status
Backup power only helps if the airport knows it is available. Monitor utility (voltage, frequency, breakers), UPS (mode, battery state, inverter, bypass, alarms, battery health), DG (running status, fuel, oil pressure, coolant, battery, breaker, load, faults) and AGL (CCR and circuit status, alarms, serviceability). An integrated ALCMS can show one common status picture and flag a problem before a lighting outage.
| State | Normal power | UPS | DG-1 | AGL critical |
|---|---|---|---|---|
| Normal | Available | Healthy | Standby | Available |
| Utility failed | Failed | On battery | Starting | Available |
| DG stabilized | Failed | Supplied by DG | Running | Available |
Related Guides
ALCMSTesting, Switch-Over Measurement and Commissioning
Regular tests
- Utility failure simulation
- UPS transfer
- DG auto-start and load test
- Automatic changeover
- Return to normal
- Battery autonomy
- AGL functional test
- ALCMS alarms and source status
- Integrated: utility failure → UPS → DG → AGL → control → monitoring
Verify, do not assume
Do not take switch-over time from a generator specification. Verify actual AGL performance with the applicable test method, because final behaviour depends on the complete electrical and lighting system.
Commissioning sequence
Document review
Single-line diagrams, load schedules and backup-power philosophy.
Equipment inspection
UPS, batteries, DG, switchgear, ATS and distribution.
Electrical testing
Insulation, protection and connections.
UPS testing
Normal, battery and bypass modes.
DG testing
Auto-start, synchronization where applicable, loading and shutdown.
Transfer testing
Simulate loss of the normal source.
AGL testing
Verify critical lighting stays operational.
Switch-over measurement
Measure actual performance.
ALCMS integration
Verify source and equipment alarms.
Failure scenarios
Test defined abnormal conditions.
Restore normal supply
Verify safe return to utility.
Documentation
Record results and set the commissioning baseline.
Electrical continuity alone does not prove the lighting is still compliant. Follow the power test with a functional test and a photometric test: runway edge, centreline, threshold and end lights, stop bars and PAPI output.
Test the chain end to end, then measure at the light.
AGL Visual Encyclopedia: Generator changeover and switch-over time Testing requirement flow Test documentation
Backup Power and Low-Visibility Procedures
At airports operating LVP, backup power becomes an operational readiness item, not only an electrical-maintenance matter. AAI information describes engineering staff verifying secondary-power availability before or during LVP and monitoring AGL status, with inspections shortly before LVP and at set intervals while it remains in force.
Possible sequence during a power failure
Common Emergency Power Failures
Click a failure to see what to investigate.
UPS battery failure
The UPS may work electrically but lack autonomy:
- Battery ageing
- Capacity not tested
- Temperature effects
DG starting failure
The generator does not start when needed:
- Weak starting battery
- Fuel problem
- Starter or control fault
- Insufficient maintenance
ATS failure
The generator runs but the critical load does not transfer:
- Transfer device fault
- Control logic or interlock
- Return-to-normal failure
Overload
The generator or UPS trips when all loads energize:
- Poor load segregation
- Inrush or starting current
- Undersized source
Poor power quality
Sensitive equipment is affected:
- Voltage or frequency deviation
- Harmonics
- Generator–UPS interaction
Incorrect load segregation
A backup-protected circuit holds loads that should be on another source:
- Wrong bus allocation
- Undocumented changes
- Load schedule not kept current
Control system failure
Power is available but ALCMS or field controls do not operate:
- Server or network not on UPS
- Field controller unprotected
- Alarms lost
Common-cause failure
Two redundant systems fail together:
- Shared switchgear or cable route
- Shared control power
- Shared fuel or cooling
Maintenance Programme
Daily / operational
- Source status
- UPS alarms
- Generator availability
- Fuel level
- Control-panel status
Weekly / periodic
- Generator auto-start
- Battery condition and charger
- ATS operation
- UPS status
Monthly
- Generator load test
- Battery inspection
- Transfer-system inspection
- Alarm verification
Detailed periodic
- UPS battery capacity
- Generator engine service
- Protection-system testing
- Cable inspection and switchgear
- Thermal inspection and earthing
- AGL circuit performance
Renewables, Microgrids and Control-Room Power
Renewables and storage
Solar PV, battery storage, microgrids and smart energy management can improve resilience. But renewable generation does not automatically make a supply suitable as a safety-critical secondary source. Critical AGL needs availability, stability, switch-over performance, fault behaviour and approved operating requirements, with a dedicated secondary-power strategy.
Control rooms and field controls
ATC, ALCMS, A-SMGCS, communications and surveillance should be considered together. Keeping the light but losing control or monitoring is a problem, and so is the reverse. Protect the ALCMS server, workstation, field controllers, network, CCR interfaces and PLC/RTU, and plan functional continuity, not only equipment continuity.
CCRs and isolating transformers
Test the whole path: backup source → CCR → primary circuit → transformer → light. Do not test each block alone and assume the result. Judge the light itself, not only the CCR output reading.
Full chain
Power source → switchgear → UPS / DG → CCR → primary cable → isolating transformer → connector → AGL light. A defect anywhere affects the final visual aid.
Working Safely on Emergency Power Systems
Hazards include high voltage, electrical isolation, generator backfeed, UPS stored energy, batteries, fuel, rotating equipment, airside vehicle movements, runway and taxiway access, live circuits, lockout/tagout and permit systems. Do generator and UPS maintenance only under appropriate electrical safety procedures.
Standards and Design Baseline for India
ICAO
Annex 14 Volume I is the core international reference. The ICAO Store lists the 9th Edition, July 2022, including Amendment 18. Its secondary-power provisions allow an independent public source or standby units and set category-dependent switch-over times. ICAO guidance also says power, lighting and control for very low RVR operations must be designed so that an equipment failure does not leave the pilot with inadequate or misleading visual guidance.
For an Indian project, base the design on
Check the DGCA and project requirements against the current regulatory framework, not an international table alone.
- DGCA CAR, including Section 4 Series B Part I
- AAI requirements
- Aerodrome operating procedures
- Project specifications
- Current ICAO provisions as applicable
New Airports, Upgrades and Gap Assessment
New airports
Set the power philosophy before finalizing AGL circuit design: operational category → visual-aid requirement → critical / essential load classification → power architecture → UPS / DG sizing → AGL circuit design → ALCMS integration → switch-over testing → photometry → commissioning. This avoids costly redesign after circuits are installed.
Existing airports
Start with an audit of utility feeds, DG and UPS capacity, battery age, CCR configuration, circuit segmentation, switchgear, ATS, cable routes, ALCMS, actual measured changeover time and current operating category. Find the gap between existing and required capability.
| Area | Existing condition | Required condition | Gap |
|---|---|---|---|
| Utility and DG | Existing capacity | Required capacity | — |
| UPS and battery | Existing capacity and autonomy | Required protected load and autonomy | — |
| ATS and CCR | Existing | Required transfer and configuration | — |
| AGL and ALCMS | Existing circuits and monitoring | Required critical circuits and monitoring | — |
| Testing | Existing records | Required evidence | — |
Service scope
New airports set the philosophy first; existing airports find the gap.
Emergency Power Acceptance Checklist
Click an item to mark it complete.
Sources and UPS
Generator and changeover
AGL, control and documents
Quick Knowledge Check
Q1. Which term does ICAO generally use for aerodrome backup power?
Q2. Does generator start time equal AGL switch-over time?
Q3. What is the role of the UPS in a UPS + DG architecture?
Q4. Is 30 minutes of UPS autonomy an ICAO requirement?
Q5. Why include ALCMS in the backup-power study?
Explore More AGL Engineering
CCR Working Principle
Constant current regulators and series circuits.
Open CCR Working Principle →Airport Isolating Transformer
The transformer between circuit and light.
Open Airport Isolating Transformer →PAPI Engineering Guide
Precision approach path indicators.
Open PAPI Engineering Guide →Testing & Commissioning Guide
Test and commission AGL systems.
Open Testing & Commissioning Guide →AGL Preventive Maintenance Guide
Maintenance cycles and checks.
Open AGL Preventive Maintenance Guide →How Airfield Lighting Works
The full AGL system explained.
Open How Airfield Lighting Works →ICAO Aerodrome Lighting Requirements
Framework and requirements.
Open ICAO Aerodrome Lighting Requirements →AGL Photometry India
In-field photometric testing for Indian airports.
Open AGL Photometry India →Airport Lighting Companies India
Company types across the Indian AGL market.
Open Airport Lighting Companies India →Keep the Lights On. Prove It.
From UPS and DG integration to switch-over measurement and photometry, explore the complete AGL knowledge base.
AGL Circuit Design for Emergency Power
AGL circuit design
AGL circuit design for emergency power decides which series circuits and loads are fed from the backup source, how the CCRs are supplied and how switch-over is handled. It starts from the operational category and the required switch-over time, then sizes the UPS and generator and plans the ALCMS interface. See the load classification and sizing sections of this page.
Explore the AGL Engineering Knowledge Hub
Fundamentals
Standards & Compliance
Runway, Taxiway & Approach Lighting
Surface Movement & Safety
PAPI
Electrical Engineering & Power
Photometry & Testing
Maintenance, Inspection & Safety
Resources, Documentation & Careers
Emergency Power & Lighting FAQ
Q1What is emergency power in an airport?
Backup electrical infrastructure used to maintain critical airport facilities when the normal source fails. For aerodrome visual aids ICAO commonly calls it a secondary power supply.
Q2What is the difference between UPS and a diesel generator?
A UPS gives rapid or uninterrupted power to protected loads. A diesel generator gives longer-duration standby power. They are often used together.
Q3Why is UPS used for runway lighting?
A UPS can hold the protected AGL load through the transition caused by failure of the normal source, helping the lighting meet the applicable switch-over requirement.
Q4Does every airport need the same backup-power system?
No. The architecture depends on runway category, operational conditions, visual aids, airport configuration and applicable regulations and specifications.
Q5What is the required AGL switch-over time?
There is no single value. ICAO Table 8-1 gives category- and visual-aid-specific maximums. For example it gives one second for the inner 300 m of approach lighting and for all stop bars in precision approach Category II/III.
Q6Is 30 minutes of UPS backup required by ICAO?
No. Thirty minutes appears in some Indian airport configurations, such as Hyderabad, but it is an airport-specific implementation detail.
Q7Can an independent utility feeder be used instead of a generator?
Yes. ICAO allows an independent public power source as a secondary supply when it meets the required independence conditions.
Q8Does generator start time equal AGL switch-over time?
No. Switch-over is evaluated at lighting-system level, from the measured light intensity response through the power transition.
Q9Does PAPI need emergency power?
It may, depending on runway category and applicable requirements. Include it in the airport’s visual-aid power analysis.
Q10Does emergency power need to supply ALCMS?
Include the control and monitoring system in the continuity analysis wherever its loss would affect the required operation of critical AGL or safety-related guidance functions.
Q11What happens if UPS fails during low visibility?
The airport’s approved LVP procedure decides the response. AAI information shows examples where UPS or DG unserviceability can lead to restrictions or suspension of affected low-visibility operations.
Engineering & Expert Notes
Separate Emergency Lighting From Secondary Power
ConceptsThese serve different purposes. Be clear which requirement a specific design is meeting.
Verify Switch-Over Times by Test
Switch-OverMeasure the actual interruption on the equipment, not just the specification of the source.
Check Which Loads Are on Which Source
Load ScheduleKeep the schedule accurate. Loads added over time may end up on the wrong supply.
Test Generators Under Realistic Load
GeneratorsA no-load run does not show how the system behaves with the full airfield load.
Confirm Monitoring Shows Source Status
MonitoringOperators should see clearly which source is feeding the airfield and any alarms.
Record Tests and Findings
RecordsDated logs of tests and defects support both safety and compliance.
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Please read before relying on any content on this page or on any record downloaded from it.
- Purpose and intended users. This page is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome personnel. It is not professional engineering, legal or regulatory advice, and using it creates no professional or client relationship. Learning-path and Academy references describe stages of learning only and are not a certification, accreditation or authorisation.
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