What is reliability in airfield lighting?
The chance that the lights stay lit and correct. A series circuit works like a chain, so reliability comes from fewer weak links and from having a second way to do the job.
Reliability is the probability that a system does its job, here keeping the right lights lit, for a stated time in stated conditions. In airfield lighting it is designed in through the layout, the equipment and the upkeep, because the lights are a safety aid and cannot be “mostly” on.
1A series circuit is a chain
The same current passes through every element in turn, so a break in any link can darken the whole circuit. Even if each link is very reliable, reliability multiplies down: in the example, links that are each 99.9% reliable give a chain of 99.0% at 10 links but only about 82% at 200. That is why every joint and connector counts, and why fewer of them is better.
| Failure | Effect | Defence |
|---|---|---|
| Lamp or LED fails | One light out; the loop keeps running because of the isolation transformer | Isolation transformer, replacement lamps |
| Cable break or bad joint | Series loop opens and the whole circuit can go dark | Fewer joints, good jointing, routing, spares |
| Insulation breakdown to earth | Often still lit, but a warning that a second fault will cause an outage | Earth-fault monitoring, prompt repair |
| CCR failure | The circuit it feeds goes dark | Standby regulator, interleaved circuits |
| Power loss | Every circuit on that supply goes dark | Secondary power source and changeover |
| Control or communication loss | Remote control is lost; the lights themselves keep working | Local control, resilient communications |
2Break the chain, then back it up
The isolation transformer is the key design move: a failed lamp does not open the loop, so one light goes out instead of the circuit. Redundancy then covers what is left, with standby regulators, alternate feeds and a secondary power source. Interleaving puts neighbouring lights on different circuits, so one lost circuit does not leave a dark stretch.
3Build it well and watch it
Most failures start at a joint, connector or seal, so quality cable, potted connectors and good drainage pay back directly. Monitoring through alarms and the control system, plus planned maintenance, finds faults while they are still warnings, and failure records show where the next improvement is worth making.
- Define the function. Which lights must stay lit, and for which operations.
- List the chain. Supply, regulator, cable, joints, transformers, fixtures, control.
- Find single points of failure. Anything whose loss darkens a circuit or an area.
- Add redundancy where it counts. Standby equipment, alternate feeds, interleaving.
- Monitor and maintain. Alarms, earth-fault checks, inspection and testing.
- Learn from failures. Record causes and feed them back into design and spares.
| Term | Meaning |
|---|---|
| Reliability | Probability of working as required for a stated time |
| Series system | Elements in a chain where the failure of one stops the whole |
| Single point of failure | One element whose loss takes out the function |
| Redundancy | A second means of doing the job, held ready |
| Interleaving | Neighbouring lights fed from different circuits |
| Availability | Share of time the system works, which also depends on repair time |
Frequently Asked Questions
Q1What is reliability in airfield lighting?
The probability that the lighting does its job, staying lit and correct, for a stated time under stated conditions. It is designed in, by the layout, the equipment and the maintenance, not added afterwards.
Q2Why does a series circuit need special care?
The same current flows through every element in turn, so a break anywhere in the loop can darken the whole circuit. Every cable, joint and connector is therefore a link in a chain.
Q3How do isolating transformers help?
Each fixture is fed through its own isolating transformer, so a failed lamp or open secondary does not break the series loop. One light goes out, not the whole circuit.
Q4What is redundancy?
Having a second way to do the job, such as a standby regulator, an alternate feed or a secondary power source, so that one failure does not take the lighting out.
Q5What does interleaving do?
Adjacent lights are put on different circuits, so that losing one circuit leaves every other light lit and the pattern still recognisable instead of leaving a long dark stretch.
Q6How is reliability different from availability?
Reliability is how rarely failures happen. Availability also counts how quickly they are put right, so it depends on repair time as well. Availability is covered in Entry 26.
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