Approach Light Conductor/Cable

An Approach Light Conductor/Cable is a specialised electrical cable used to carry power from the Constant Current Regulator (CCR) to the approach lighting fixtures in a series circuit. These cables are designed to operate in a constant‑current environment (typically 6.6A or 20A) and are engineered to withstand the harsh outdoor conditions of an airfield, including exposure to jet fuel, de‑icing chemicals, UV radiation, moisture, and extreme temperature variations.

The approach light conductor system typically consists of a primary cable (running from the CCR to the isolating transformers) and secondary cables (running from the isolating transformers to the individual light fixtures). The primary cable is the main backbone of the series circuit, while secondary cables provide the final connection to each fixture. Both are available in various conductor sizes (typically 6 mm², 10 mm², or 16 mm² for primary; 1.5 mm² or 2.5 mm² for secondary) and insulation types, depending on the system voltage, current rating, and environmental conditions.

The approach light conductor cable's purpose is to carry the CCR's regulated current the full length of the approach lighting array — often hundreds of metres out from the runway threshold — while keeping every fixture along that run receiving the same current and therefore the same intensity, which is what the series-circuit design depends on to work at all. Its insulation and sheathing exist specifically to survive an environment that would degrade an ordinary cable within a few seasons: constant exposure to jet fuel, de-icing chemicals, UV, and temperature extremes, any of which could otherwise cause an insulation failure that either drops a fixture out of the loop or creates a dangerous fault. The distinction between primary and secondary cable sizing reflects this same purpose at two different scales — the primary cable minimising voltage drop and I²R losses over the full loop length, the secondary cable making the short, lower-current connection from transformer to fixture.

Approach light conductors are deployed in all approach lighting systems that require electrical power transmission. They are used in:

• ALSF‑1 and ALSF‑2 systems – providing power to centreline, crossbar, barrette, and SFL fixtures for CAT I, II, and III precision approaches.
• MALSF and MALSR systems – supplying power for medium‑intensity approach lighting.
• Calvert I and II systems – powering barrette and crossbar configurations.
• Runway edge lighting systems – where similar cable types are used for elevated and inset edge lights.
• Taxiway lighting systems – using similar cable types for taxiway edge and centreline lights.
• All airfield lighting circuits – where series circuit constant‑current technology is employed.

Conductor cable for the approach lighting system is routed from the CCR or field distribution panel out to the approach light structures, typically in underground ducts or direct-buried trenches beneath the extended runway centreline, then up the frangible structure to each barrette, crossbar, or sequenced flashing light. Underground runs are laid at a depth consistent with the rest of the airfield's series circuit cabling, with junction boxes at intervals to break the run into manageable sections and provide splice or transformer connection points. Where the cable climbs a structure, it is secured and protected along the mast so wind and vibration don't work the conductor loose, and slack is left at the top to allow the frangible coupling to break away without immediately snapping the cable.

Approach light conductors operate as an integral part of the series‑connected AGL circuit:

• Series Circuit Configuration: The primary cable connects all light fixtures in a series loop, with the CCR maintaining a constant current (typically 6.6A) regardless of the number of fixtures or cable length. This ensures all lights have the same brightness.
• Primary Cable: The main cable runs from the CCR through the airfield, with isolating transformers connected in series along its length. The primary cable carries the full loop current and must have a sufficiently large conductor (typically 6–16 mm²) to minimise voltage drop and I²R losses.
• Secondary Cable: Short cables (typically 1.5–2.5 mm²) connect the secondary side of each isolating transformer to the individual light fixture. These cables carry the transformed current (also constant) to the lamp or LED module.
• Insulation and Sheathing: The cables are insulated with robust materials such as cross‑linked polyethylene (XLPE), EPR, or PVC, and are often sheathed with a tough outer layer (e.g., neoprene or PE) for mechanical protection and moisture resistance.
• Colour Coding: Primary and secondary cables are typically colour‑coded to distinguish them and to identify phase and neutral conductors (e.g., black, red, blue) for ease of installation and maintenance.
• Connectors: The cables terminate in specialised connectors (e.g., L‑823 connectors) that provide reliable, watertight connections to the isolating transformers and fixtures.
• Earth/Shield: Many cables include a copper screen or earth conductor to provide protection against electromagnetic interference and ensure electrical safety.

The conductor cable is not a controlled element in its own right — it is the physical layer that every ALCMS/CCR control decision described elsewhere in this system actually travels through. Because the CCR holds the loop at constant current rather than constant voltage, the cable never experiences a variable electrical load; what changes is the voltage the CCR must produce to push that current around the loop, and that voltage is exactly what the control system watches. A healthy cable is electrically invisible to the ALCMS — it simply reports a stable loop voltage. Degradation shows up as a control-system event only once it changes loop impedance: insulation breakdown, a damaged screen, or moisture ingress at a connector raises resistance and therefore loop voltage, which the CCR reports as an open-circuit or high-resistance fault indistinguishable, from the control room, from a failed transformer or lamp. Locating the actual point of cable degradation is done outside the control loop entirely, with insulation-resistance (megger) testing and loop-resistance measurement carried out section by section during scheduled maintenance or after a reported fault.

The cable is built from annealed, Class 2 or 5 stranded copper conductors — bare or tinned, depending on soil chemistry at the site — surrounded by a cross-linked polyethylene (XLPE) or EPR insulation layer chosen for its wide temperature range, then an outer sheath of neoprene, polyethylene, or PVC formulated for UV stability and resistance to jet fuel, de-icing fluid, and hydraulic fluid. Primary cable is sized at 6, 10, or 16 mm² to carry the full loop current from the CCR to each isolating transformer with minimal voltage drop, while lighter 1.5 or 2.5 mm² secondary cable carries the transformer's stepped-down output the short distance to the fixture. Conductor insulation is colour-coded per FAA/ICAO convention so primary and secondary runs, and different circuits sharing a trench, stay distinguishable during installation and fault-finding years later. Terminations at each end — whether a sealed L-823 splice connector or a transformer lead — are what actually deliver the cable's rated IP67/IP68 water resistance; the cable jacket alone is not a sealed system until it is properly terminated.