AGL Primary Cable

The AGL Primary Cable (also known as the series cable or loop cable) is the main power distribution cable used in airfield ground lighting systems. It carries the constant current (typically 6.6A or 20A) from the Constant Current Regulator (CCR) to all isolating transformers and light fixtures in a series circuit. The cable forms a continuous loop that connects all fixtures on a given runway, taxiway, or approach lighting system, with each fixture connected to the loop via an isolating transformer.

AGL primary cable is typically:

• Single‑core or multi‑core: Often single‑core (one conductor) with a heavy insulation and sheath, but multi‑core versions (2‑core, 3‑core) are used for twin‑circuit or earth‑included systems.
• Heavy‑duty construction: Designed to withstand mechanical stress, moisture, chemicals, and temperature extremes. Common insulation materials include XLPE (cross‑linked polyethylene), EPR (ethylene propylene rubber), or PVC.
• Rated for continuous operation: At the full primary current (6.6A or 20A) with voltage drops within acceptable limits over long cable runs (up to several kilometres).
• Colour‑coded: Typically black or red for primary circuits, but colour coding varies by standard and region.

The cable is installed in underground conduits, direct‑buried in trenches, or laid in cable ducts within concrete. It is terminated at each isolating transformer using L‑823 connectors (or similar) and at the CCR output. Proper cable selection, installation, and maintenance are critical for the reliable operation of the AGL system.

The AGL primary cable's purpose extends beyond simply carrying current: because every fixture on a series circuit shares the same loop, the cable's integrity along its entire length determines whether the whole circuit stays lit, not just the section nearest the fault. A single break, chafe, or moisture ingress point anywhere in the loop can take down every downstream fixture until the fault is located and repaired, which is why the cable's insulation and sheath are built to withstand mechanical stress, chemical exposure, and temperature extremes well beyond what an individual fixture connection would need. This shared-fate characteristic of the series loop is what makes primary cable condition, rather than any single fixture, the dominant factor in circuit-wide reliability.

The AGL primary cable is used throughout the airfield for all primary series circuits, including:

• Runway Lighting Circuits: Powering runway edge lights, centreline lights, threshold lights, end lights, touchdown zone lights, and other runway fixtures.
• Taxiway Lighting Circuits: Powering taxiway edge lights, centreline lights, stop bars, clearance bars, and guidance signs.
• Approach Lighting Systems: Powering ALSF‑1, ALSF‑2, MALSR, and other approach lighting configurations.
• Obstruction Lighting: Powering low‑, medium‑, and high‑intensity obstruction lights on structures.
• Airport Beacons: Powering rotating beacons and heliport beacons.
• CCR to Field Interface: Connecting the CCR output to the field distribution panel or directly to the series loop.

The cable is available in various conductor sizes (typically 6mm², 10mm², 16mm², or 25mm²) to suit the circuit length and load requirements. It is designed for use in temperatures ranging from -40°C to +85°C and is resistant to oils, fuels, and de‑icing chemicals commonly found on airfields.

The cable is laid in a trench or drawn through duct at a depth typically specified in local design standards (commonly 450–600 mm below the pavement, or deeper under paved surfaces), bedded in a sand or fine-fill layer that protects the sheath from sharp stones and allows even heat dissipation along the run. Where the loop crosses beneath a runway, taxiway, or apron, the cable is drawn through a rigid duct sized with spare capacity for future pulls, with marker tape laid above the trench to warn against later excavation damage. Route is planned to follow the series circuit's fixture spacing as directly as practicable, minimising unnecessary length (and the voltage drop that comes with it) while keeping clear of other buried services such as drainage, fuel lines, and communications ducts. At each isolating transformer, junction box, or CCR interface, the cable is terminated with an L-823 (or equivalent) connector, with sufficient slack looped into the pit or handhole to allow the termination to be remade without disturbing the buried run.

The AGL primary cable functions as the series conductor that carries the constant current through the entire lighting loop. Its design ensures efficient and reliable power transmission:

• Conductor: Typically made of high‑conductivity annealed copper (or aluminium in some cases) to minimise I²R losses. The conductor is stranded to provide flexibility and reduce the effects of skin effect at power frequencies.
• Insulation: A thick layer of cross‑linked polyethylene (XLPE) or ethylene propylene rubber (EPR) provides high dielectric strength, resistance to moisture, and excellent thermal properties. XLPE is preferred for its higher temperature rating (up to 90°C continuous).
• Sheath: An outer PVC or thermoplastic polyurethane (TPU) sheath provides mechanical protection against abrasion, impacts, and chemical attack. For direct‑burial applications, the sheath may be reinforced with armour (steel wire or tape) to protect against rodent damage and mechanical stress.
• Voltage Drop: The cable is sized to ensure that the voltage drop along the loop does not exceed acceptable limits (typically 5‑10%) at the maximum current, ensuring that each isolating transformer receives sufficient voltage to drive its connected fixture.
• Fault Current Capacity: The cable is rated for the prospective fault current of the system, ensuring it can withstand short‑circuit conditions without damage.

In operation, the CCR feeds a constant current (e.g., 6.6A) into the primary cable loop. The loop passes through each isolating transformer's primary winding, and the current returns to the CCR. The isolating transformers step down the voltage to the appropriate level for each fixture. The primary cable is designed to have a low resistance per metre, minimising voltage drop and ensuring uniform current distribution across all fixtures.

The primary cable carries no intelligence of its own and is not individually addressable — it is the physical medium through which the series circuit's commanded current flows. An Airfield Lighting Control and Monitoring System (ALCMS), operated by the tower or an automated visibility-linked routine, selects the required brightness step, and the CCR converts that command into a fixed current on the loop (typically one of 5 steps between 2.8A and 6.6A). That current travels the full length of the primary cable — out to the last isolating transformer and back to the CCR — with every fixture on the loop drawing from the identical current simultaneously. The cable's resistance per metre is built directly into the loop's control logic: the CCR continuously compares the commanded current against loop voltage, and because the cable's own resistance forms part of that voltage budget, an oversized run, a poor joint, or a corroded termination reduces the headroom available to fixtures further down the loop. A complete break in the cable presents at the CCR as a simple open-circuit loop fault — the regulator sees only that voltage has risen without limit at the commanded current, with no indication of where along the run the break has occurred until the loop is sectionalised and tested in the field.

Build‑up is concentric, from the conductor outward. The core is a stranded, high‑conductivity annealed copper (or aluminium) conductor, stranding rather than a single solid wire being used to keep the cable flexible enough to draw through duct and route around obstacles in the trench. Over this sits an extruded layer of cross‑linked polyethylene (XLPE) or ethylene propylene rubber (EPR), chosen for its high dielectric strength and continuous operating temperature rating; lower‑duty runs may instead use a PVC insulation. An outer sheath of PVC, thermoplastic polyurethane (TPU), or HDPE provides mechanical protection against abrasion, chemical attack from fuel and de‑icing fluid, and moisture ingress, and for direct‑burial sections this sheath is often reinforced with steel wire or steel tape armour to resist rodent damage and backfill impact. Cable ends are terminated in an L‑823 (or equivalent) connector, with the conductor exposed, ferruled, and seated in the connector's pin or socket contact before the assembly is sealed against the elements.

Runway Lighting Cable, Runway Light Cable & Taxiway Lighting Cable

The same primary cable family serves every system. Runway lighting cable (also called runway light cable) and taxiway lighting cable differ mainly in loop length, load and route, not in construction. The cable type and rating still need to match the CCR and the project specification.

Airport lighting cable sizing. Conductor size is set by the loop current, loop length, CCR capacity, voltage drop and installation method. A longer loop or higher load needs a larger conductor or a different circuit split. Confirm the sizing against the circuit design, not from a rule of thumb.

Airport lighting cable insulation. The insulation and sheath keep the high-voltage conductor separate from earth. Moisture, damage during installation and ageing at joints are the usual causes of breakdown, so insulation condition is the main thing tested over the life of the cable.

Airport Cable Testing

Airport cable testing normally combines two checks, made with the circuit isolated and earthed: insulation resistance, and continuity, which is covered next.

• Insulation resistance: measure between the conductor and earth with an insulation tester, and compare with the commissioning value and the project limit.

Airport Cable Continuity Test

Check the conductor with a low-resistance meter to confirm there is no open circuit along the loop.

Airport lighting cable fault types are earth faults, open circuits, shorts and insulation breakdown. They are often found at joints, terminations, or places where the cable was damaged by digging. Splitting the loop at junction points narrows the search. An airport lighting cable connector that has let in water is a common cause as well, so check the L-823 connector before suspecting the cable itself. A cable tester can help locate the fault.