Different cables
Primary, secondary, control and earthing: four cables that run side by side in the same ground but differ in what they carry and what a fault does.
An airfield lighting system uses more than one kind of cable, and they are not interchangeable. The primary series cable is the long loop that feeds every light, carrying the same constant current all the way round. The secondary cable is a short lead from one transformer to one fitting. The control cable carries signals, and the earthing conductor carries fault and surge current to earth.
Because the lights are in series, the primary cable is the one to protect most: a break puts every light on the circuit out, and one dig-in can do it. A damaged secondary cable usually darkens one fitting, and a control cable fault usually loses control or status rather than the lights. Cables of different types often lie in the same trench, so identification matters.
Primary and secondary cables carry lamp power. The primary loop serves the whole circuit; the secondary serves one fitting.
Control cables carry signals and earthing conductors carry fault current. Neither feeds the lamps, but both affect how the system is controlled and protected.
| Aspect | Primary series cable | Secondary cable | Control / signal cable | Earthing conductor |
|---|---|---|---|---|
| Does | Forms the series loop: leaves the CCR, passes through every transformer primary and returns | Connects one transformer’s secondary to one lamp or fixture | Carries control and monitoring signals between the CCR or control system and field equipment | Bonds metalwork to earth and gives fault and surge current a path |
| Where | Buried in ducts or direct along the route, with a joint or connector at each transformer | In the base can or pit, a short lead with slack for lifting the fixture or transformer | In ducts between the control room or CCR and the field equipment it serves | Laid with the cables, bonded to cans, fixtures, structures and earth electrodes |
| Carries | The constant circuit current, the same all the way round, at up to the highest circuit voltage | Lamp current for that one fixture, at a much lower voltage | Low-power signals, not lamp current | Fault and surge current, not normal lamp current |
| Built as | A stranded copper conductor with insulation and an outer jacket, to a specification such as FAA L-824 | A short multi-conductor cable rated for the secondary voltage, ending in secondary connectors | Multicore copper or fibre as the design requires, screened and labelled as needed | Copper conductor, bare or insulated, as the design and site conditions require; a bare conductor laid above the cables for lightning protection is often called a counterpoise |
| How many | One loop per circuit, as long as the circuit | One per light or sign | As the control design needs | Along the route, as the design needs |
| If it fails | A break opens the loop, puts every light out and trips the regulator’s open-circuit protection; an insulation fault gives an earth fault | Usually puts that one lamp out and leaves the loop closed | Control or status is lost; the lighting circuit may keep running, depending on design | Protection is reduced, so a fault or surge can leave metalwork at a dangerous voltage |
| Watch points | Depth and protection, digging damage, joints, insulation-resistance trend | Slack, connector sealing, damage when the fixture is lifted | Separation from power cables, screening, labelling, terminations | Continuity, joints and bonds, earth resistance, corrosion |
What to check on cables
Match the cable to its role
Primary, secondary, control and earthing cables are not interchangeable. Check type and rating against the design and the maker’s data.
Voltage and size
Check the insulation is rated for the highest voltage the circuit can reach and the conductor suits the current and run length.
Route, depth and protection
Check depth, bedding, warning tape and cover against the design, and keep the route on the as-builts.
Identification
Check tags at ends, joints and entries match the drawings. A tag is not proof of isolation.
Insulation trend
Test insulation to the limits in the procedure and record the values, so a falling trend shows up over the years.
Isolate before work
Treat every cable as live until the circuit is isolated, locked out and proved dead, including secondary leads.
Frequently Asked Questions
Q1What are the main cables on an AGL airfield?
Four common ones are the primary series cable, the secondary cable, the control and signal cable and the earthing conductor. The primary cable forms the series loop. The secondary cable is the short lead from a transformer to one fitting. The control cable carries signals, and the earthing conductor carries fault and surge current to earth.
Q2What is the difference between a primary and a secondary cable?
The primary cable is one continuous loop that leaves the CCR, passes through every transformer primary and returns, carrying the same current all the way round. The secondary cable is a short lead on the lamp side of one transformer and carries the current of that one fitting only, at a much lower voltage.
Q3Why does a primary cable fault matter more?
Because the loop is in series, a break in the primary cable opens the circuit and puts every light on it out, and an insulation fault can cause an earth fault on the whole loop. A damaged secondary cable normally darkens one fitting and leaves the loop closed.
Q4Can a control cable and a power cable be treated the same way?
No. A control or signal cable carries low-power signals, not lamp current, and its failure usually means lost control or monitoring rather than a dark circuit. It is routed, screened and labelled differently, and kept apart from power cables as the design requires.
Q5What limits and requirements apply?
Cable type, voltage rating, conductor size, construction and test limits come from the standard, such as the FAA L-824 specification or the relevant IEC requirements, the airport's design and the manufacturer's data. Use the requirements for your site, and treat every cable as live until the circuit is isolated, locked out and proved dead.
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