How is a series circuit routed?
A series circuit is one closed loop. The primary cable leaves its CCR, passes through the isolation transformer at every light and comes back to the same CCR, carrying the same current all the way round.
In a series circuit the lights are not wired in parallel across a voltage. Instead one primary cable forms a loop that starts and ends at a constant current regulator, and the CCR holds the loop current at the selected brightness step. Every light connects to the loop through an isolation transformer, so the same current flows through every transformer primary whatever the length of the loop.
On the airfield the loop is routed along the edge of the pavement, usually in duct banks or direct-buried cable. It goes out past the lights, then returns to the vault, either beside the outgoing run or around the opposite edge. Where the design needs resilience, lights are interleaved between two circuits, so a cable fault on one circuit leaves every second light lit. A cut primary cable opens the loop and puts the whole circuit out, which is why the loop must be treated as live until its CCR has been isolated and made safe.
Frequently Asked Questions
Q1What is a series circuit in airfield lighting?
A series circuit is a single closed loop of primary cable that starts and ends at a constant current regulator (CCR). The same current flows through every part of the loop, and each light takes its power through an isolation transformer connected in that loop.
Q2How is a series circuit routed on the airfield?
The primary cable leaves the CCR in the airfield lighting vault, runs along the pavement edge through each light’s transformer connection, and returns to the same CCR. Depending on the layout it goes out and back along the same corridor or around both sides of the pavement, in duct banks or direct-buried cable with the design’s protection.
Q3Why does each light use an isolation transformer?
The transformer couples the light to the primary loop without a direct electrical connection. If a lamp fails, the transformer lets the loop current keep flowing, so one failed lamp does not put out the whole circuit. It also keeps the primary circuit isolated from the lamp and its wiring.
Q4What is interleaving of series circuits?
Interleaving alternates the lights of a pattern between two circuits so that one circuit failure leaves every second light on instead of a dark gap. It is used where the design calls for it, for example on CAT II/III runway centreline, touchdown zone and approach lighting, following FAA AC 150/5340-30 and ICAO Annex 14.
Q5What happens if a series circuit cable is cut?
The loop opens, current stops and every light on that circuit goes out. Treat a series circuit as live until the CCR has been isolated and locked out under site procedures; never break an energised loop, because the CCR will try to hold its current.
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