What are interleaved series circuits?
Alternate lights of one pattern fed from two different circuits, so one failure thins the row instead of blacking it out.
A series circuit is one loop, so an open circuit or a regulator failure puts out every light on it. Interleaving avoids that for a pattern that must not disappear: neighbouring lights are fed from two different circuits, odd-numbered lights from circuit A and even-numbered lights from circuit B.
Lose either circuit and the other still lights every second light along the whole row. The pilot sees a gap twice as wide, not a dark stretch. The price is two regulators and more cable and transformer connections, and the two circuits must be switched together at the same brightness step.
Two circuits, alternate lights. One failure thins the row.
One loop, all lights. One failure blanks the row.
| Situation | Lights lit | Effect for the pilot |
|---|---|---|
| Both circuits healthy | All | Normal spacing |
| One circuit lost | Every second light | Spacing doubled; row still continuous |
| Both circuits lost | None | Row dark; interleaving gives no protection |
| Circuits on different brightness steps | All, unevenly | Alternating bright and dim lights |
| Same row on one circuit, circuit lost | None | Whole row dark at once |
Frequently Asked Questions
Q1What are interleaved series circuits?
Interleaving feeds neighbouring lights of one pattern from two different series circuits, usually odd-numbered lights from circuit A and even-numbered lights from circuit B. Each circuit has its own constant current regulator and its own loop of primary cable.
Q2Why interleave instead of using one circuit?
A series circuit is one loop, so an open circuit or a regulator failure puts out every light on it. If a whole row were on one loop, the row would go dark at once. With two interleaved circuits, losing one leaves every second light lit, so the pattern is thinner but still continuous.
Q3What does the pilot see when one circuit fails?
Every second light stays lit, so the spacing along the row doubles but there is no long dark stretch. The row is still recognisable as the same line of lights, which matters most in low visibility.
Q4What does interleaving cost?
Two circuits usually mean two regulators, each carrying about half the load, plus more cable runs and transformer connections to install and maintain. The two circuits must be switched together and set to the same brightness step, or the row looks bright and dim in turn.
Q5Does one circuit failure need to be reported?
Yes. A half-lit row can look acceptable at a glance, so monitoring should report the loss of one circuit as an alarm so it is repaired rather than overlooked. If both circuits are lost the row is dark, and interleaving gives no protection.
Q6Is interleaving required?
It is common practice where continued guidance matters, such as taxiway centreline lights for low-visibility operations, but whether it is required, and which lights it covers, depends on the standard and the operation. See ICAO Doc 9157 Part 5, Annex 14 Chapter 8 and FAA AC 150/5340-30, and confirm against your design basis.
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