What is switchover time on a generator changeover?
How long the lights are interrupted when the load moves from the primary supply to the standby generator, and the limit that interruption must stay under.
Changeover is the act of moving the load from the primary supply to the standby generator, normally through a changeover or transfer switch. Switchover time is the length of the interruption the lights actually see. ICAO Annex 14 defines it for a light as the time for the intensity to fall to 50 percent and recover to 50 percent during a power supply changeover, with the light running at 25 percent intensity or above. It is a measure of what the pilot sees, not of how fast the switch operates.
Annex 14 Volume I gives each facility a maximum switchover time in its secondary power supply table. Most facilities are given 15 seconds, the most critical elements of Category II and III runways are given 1 second, and a 1 second limit also applies to some other facilities where the table notes require it, for example where approaches are over hazardous or precipitous terrain. A generator can serve the 15 second class, but it cannot start in 1 second, so the 1 second class needs a source that is already running. A slow start, a sticky switch or regulators that restart slowly can push the interruption past the limit, and this is often found only in a real outage.
- Know the class. Find which facilities are on the circuit and which limit applies to each in the current Annex 14 table and the authority’s requirement.
- Budget the chain. Add detection, generator start and run-up, transfer and regulator restart, and compare the total with the limit.
- Arrange the source. A generator for the 15 second class; a source that is already running for the 1 second class.
- Time it. Simulate loss of the primary supply on load and record the interruption, ideally with a light sensor and a recorder.
- Transfer back. Return to the primary supply in a controlled way once it is stable. This is a second interruption, so plan it with air traffic control.
- Record & repeat. Log the result against the limit and repeat the test on a regular schedule and after any change.
Frequently Asked Questions
Q1What is the difference between changeover and switchover time?
Changeover is the act of moving the load from the primary supply to the standby source, usually by a changeover or transfer switch. Switchover time is the length of the interruption the lights actually see. It includes detecting the failure, starting the generator, the transfer itself and the regulators restarting, so it is much longer than the switch operation alone.
Q2What switchover time does airfield lighting have to meet?
ICAO Annex 14 Volume I sets a maximum switchover time for each facility in its secondary power supply table. Most facilities are given 15 seconds, the most critical elements of Category II and III runways are given 1 second, and a 1 second limit also applies to some other facilities where the table notes require it, for example where approaches are over hazardous or precipitous terrain. Always check the table in the current edition and the authority's requirement for your runway.
Q3How is switchover time defined?
Annex 14 defines it for a light as the time for the intensity in a given direction to fall to 50 percent and recover to 50 percent during a power supply changeover, with the light operating at 25 percent intensity or above. It is a measure of what the pilot sees, not of how fast the switch moves. Check the exact wording in the current edition.
Q4Can a standby generator meet a 1 second requirement?
No. A generator needs several seconds to start and reach rated voltage and frequency, so a 1 second limit needs a source that is already running, such as a second independent utility feed with a fast transfer or a no-break power system. Which arrangement is used depends on the design and the authority.
Q5How is switchover time tested?
By simulating a loss of the primary supply with the system on load, then recording how long the lights are interrupted, ideally with a light sensor and a recorder rather than a stopwatch. Repeat the test at the intensity steps in use, test the transfer back to the primary supply as well, and log the results against the required limit.
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