Approach Light Junction Box

An Approach Light Junction Box is a fundamental yet critical component of the airfield ground lighting (AGL) infrastructure. It serves as a central connection point for primary and secondary cables, housing splices, terminations, and providing an accessible interface to isolating transformers. Built from materials such as glass-fiber reinforced polyester (GRP), polycarbonate, or cast aluminium/steel, these enclosures are designed to provide a watertight and corrosion-resistant environment for electrical connections.

The junction box acts as the backbone of the series lighting circuit, ensuring electrical continuity while providing mechanical protection and preventing moisture ingress. It facilitates maintenance and troubleshooting by providing organised access to circuit connections, making it an indispensable asset for the reliable operation of approach lighting systems (ALSF, MALSF, Calvert, etc.).

The approach light junction box's purpose is to give the extensive network of cable splices and terminations that an approach lighting system depends on a protected, organised, and accessible home, rather than leaving them buried and unreachable or exposed to the elements. Because an approach lighting array like ALSF-2 involves hundreds of individual connections between primary cable, isolating transformers, and fixtures spread across a long approach path, the junction box's watertight, corrosion-resistant enclosure is what keeps each of those connections electrically sound over years of ground moisture and temperature cycling, while its accessible lid means a technician can open, test, and re-seal a specific connection during troubleshooting without having to dig up or replace the whole run of cable.

Approach light junction boxes are deployed across all types of approach lighting systems that require secure and protected electrical connections:

• ALSF‑1 and ALSF‑2 systems – protecting connections for centreline, crossbar, barrette, and SFL fixtures in CAT I, II, and III precision approaches.
• MALSF and MALSR systems – providing weatherproof enclosures for medium‑intensity approach lighting connections.
• Calvert I and II systems – housing connections for barrette and crossbar configurations.
• Runway and Taxiway Lighting – used extensively in edge and centreline lighting circuits.
• Equipment Shelters – mounted on concrete pads inside shelters to protect transformer and splice connections.
• Direct Burial Applications – buried underground to maintain airfield surface clearance while providing access points for maintenance.

Approach light junction boxes are sited at intervals along the approach lighting cable run, at the base of each structure or grouped where several fixtures share a nearby splice point, breaking the primary and secondary cabling into manageable sections between the CCR and the fixtures. Buried units sit in a handhole or pit set to grade in the often unpaved or rough ground beneath the approach lighting array, sealed against water ingress and set out so they remain locatable and accessible despite being off any paved surface. Where a box is mounted inside an equipment shelter, it sits on a concrete pad close to the transformer and CCR feed it interfaces with, and box positions throughout are tied into the same survey used to set out the structures and cabling above them.

The approach light junction box operates by providing a secure, organised, and protected environment for electrical connections within the series circuit:

• Cable Termination: Primary and secondary cables enter the box through cable glands or knockouts. Inside, they are terminated to terminal blocks, isolating transformers, or directly spliced using insulated connectors.
• Transformer Housing: In many configurations, the junction box houses the isolating transformer itself, providing a compact, protected assembly that simplifies installation and maintenance.
• Environmental Protection: The enclosure is sealed to IP67 or IP68 standards, preventing ingress of water, dust, and de‑icing chemicals. This ensures long-term reliability of the electrical connections.
• Mechanical Protection: The robust construction protects internal components from physical damage due to excavation, ground movement, or vehicle loading.
• Accessibility: The box features a removable or hinged lid with secure fastenings (stainless steel captive screws), allowing authorised personnel to access the connections for testing, maintenance, and fault finding.
• Earthing: Provides a designated earthing point to ensure the safety of the circuit and protection against lightning strikes.

Junction boxes sit at the physical joints of the approach lighting series loop and are, functionally, the sectionalising points a technician uses when the ALCMS reports a loop-level fault. Because the CCR's fault detection only identifies that loop impedance has risen, not where, fault-finding in practice works backward through the string of junction boxes: opening each box in turn and taking a loop-resistance or insulation reading isolates which segment of cable, which transformer, or which fixture is responsible. Where the box houses the isolating transformer itself, it also becomes the natural point to swap a failed transformer without disturbing the fixture or the buried primary cable. The box's own contribution to reliability is therefore indirect but structural — every termination inside it is a potential source of the very high-resistance faults the CCR is designed to detect, which is why sealing to IP67/IP68 and using corrosion-resistant, captive-screw connections matters as much to the control architecture's fault-reporting accuracy as it does to weatherproofing.

The enclosure body is moulded from glass-fibre reinforced polyester (GRP), polycarbonate, or cast aluminium/steel, chosen for a combination of mechanical strength and resistance to de-icing chemicals, salt spray, and jet fuel. Cable enters through multiple pre-installed glands or knockouts (typically M20, M25, or M32) sized to the primary and secondary conductors passing through, and gasketed lids sealed with stainless-steel captive screws or a padlock hasp give the box its IP67 (temporary immersion) or IP68 (continuous immersion) rating. Inside, terminal blocks or sealed splice connectors organise the primary-to-transformer and transformer-to-fixture connections in a layout that a technician can trace without disturbing adjacent circuits, and an integrated earthing terminal ties the enclosure into the site's grounding system. Depending on the installation, the box is set for direct burial, mounted on a concrete pad, or fixed to a pole, with the same enclosure design serving all three.

An airport lighting junction box is a sealed enclosure that holds the cable joints, splices and terminations of an airfield lighting circuit. On approach lighting lines it is the central connection point between the primary and secondary cables and the isolating transformers, and it gives a place to open the circuit for testing or fault finding.

It must stay watertight and accessible. Check the seals, cable entries and lid on every inspection, and record any signs of water ingress, because moisture in a junction box is a common cause of insulation faults. For the general-purpose version, see the junction box page.