What are AGL circuit zones?
The airfield is lit as separate zones, each on its own series circuit and CCR, so every lighting function can be dimmed, switched and isolated without touching the rest.
An AGL circuit zone is a group of lights with the same job that share one series circuit: a closed loop from a constant current regulator (CCR) in the vault, through the isolation transformer at each light, and back. Approach, threshold, runway edge, runway centreline, touchdown zone, runway end, taxiway edge, taxiway centreline and stop bars are each normally given their own circuit, or a few circuits where the load is too large for one CCR. Where combined threshold and runway-end fixtures are used, those two functions share one circuit.
Splitting the airfield this way lets each function run at its own brightness step, be switched by ATC on its own, and fail on its own. Lights inside a zone are often interleaved across two circuits, so losing one circuit leaves every second light lit instead of a gap in the pattern.
One circuit per function. Independent brightness, switching and fault isolation.
Every light shares one fault and one brightness. Load can exceed one CCR.
Frequently Asked Questions
Q1What is an AGL circuit zone?
A group of airfield lights with the same function that share one series circuit, or a few interleaved circuits, fed from a constant current regulator in the airfield lighting vault.
Q2Why not put all the lights on one circuit?
One fault or one CCR failure would darken the whole airfield, brightness could not be set per function, and the combined load would be more than a single CCR is normally rated to carry.
Q3What does interleaving mean?
Alternate fixtures in a pattern are fed from two different circuits. If one circuit fails, every second light stays lit, so pilots still see the shape of the pattern.
Q4Which lights normally have their own circuits?
Approach, threshold, runway edge, runway end, runway centreline, touchdown zone, taxiway edge, taxiway centreline and stop bars are typically separated. Where combined threshold and runway-end fixtures are used, those two functions share a circuit. The exact grouping is fixed by the design.
Q5Are stop bars circuited differently?
They are commonly fed and switched separately from other taxiway lighting so ATC can control each bar and interlock it with the lead-on lighting beyond it.
Q6Who decides the circuit layout?
The airfield lighting designer, working to the applicable standard, the CCR ratings and the operating needs of the airport. The circuit schedule is the working reference.
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