Junction Box

A Junction Box (also called a connection box or distribution box) is an enclosure used to protect and organise electrical connections, splices, and terminations in AGL power and control circuits. In airfield ground lighting systems, junction boxes are critical for:

• Housing cable joints and splices for primary and secondary AGL cables
• Providing termination points for isolating transformers and light fixtures
• Enabling branch circuits and tap connections in the series loop
• Offering a protected interface for testing, maintenance, and troubleshooting

Junction boxes are available in various sizes, materials, and ingress protection ratings to suit different installation environments — from above-ground equipment shelters to underground manholes and direct burial applications.

The junction box's purpose is to give the many cable joints, splices, and taps in an AGL circuit a protected, accessible home rather than leaving them exposed underground or inside conduit where a fault could go undetected and unreachable. Because AGL series circuits run for kilometres and connect to hundreds of individual isolating transformers, the connections between primary cable, transformer, and fixture are inevitable — the junction box's sealed, strain-relieved enclosure is what keeps those connections from becoming the weak point in an otherwise reliable circuit, protecting them from moisture ingress and mechanical damage while still allowing a technician to open the box, test the joint, and re-seal it during maintenance or troubleshooting.

Junction boxes are deployed throughout the AGL infrastructure, including:

• Runway and Taxiway Lighting Circuits: At the interface between primary cables and isolating transformers, and between transformers and fixtures.
• Power Distribution Systems: In CCR rooms and substations to distribute power to multiple circuits.
• Control and Monitoring Networks: In ALCMS cabinets and remote I/O panels for signal and communication wiring.
• Underground Networks: In manholes and handholes for cable splices and branch connections.
• Equipment Shelters: Inside electrical rooms to terminate incoming and outgoing cables.

Junction boxes are installed wherever cables need to be spliced, tapped, or transitioned along the AGL network — between primary cable and isolating transformer, between transformer and fixture, in manholes and handholes along underground runs, and inside CCR rooms and equipment shelters to terminate incoming and outgoing circuits. Field-buried boxes are set in a handhole or pit at a depth consistent with the surrounding cable trench, with gland entries sealed against water ingress and enough internal space to make and insulate all splices without crowding. Indoor units are wall- or frame-mounted at a height and position that keeps terminations accessible, with the box location chosen so cable runs on either side stay within a straight, well-supported path rather than adding unnecessary bends or slack.

A junction box provides a safe and accessible enclosure for electrical connections. Cables enter through cable glands or conduit entries and are terminated inside using terminal blocks, splice connectors, or direct soldering. The box is designed to:

• Protect connections from moisture, dust, and mechanical damage (IP rated).
• Organise wiring with sufficient space for cable bending and labelling.
• Allow access for routine inspection, testing, and maintenance via removable covers.
• Provide strain relief through secure cable entry points, preventing pull-out forces from damaging connections.

In AGL series circuits, junction boxes are often used to splice the loop conductor through in series to successive isolating transformers, or to tap off the loop for a local fixture connection.

A junction box has no control logic — it is a passive electrical node whose only architectural role is to preserve continuity of the circuit passing through it while providing a sealed, accessible tap-off point. In an AGL series circuit, the loop conductor typically runs straight through the box, entering and leaving through separate glanded ports, while a spliced tap inside feeds the isolating transformer or fixture local to that box; this means a fault at the junction box (a failed seal allowing moisture ingress, for example) shows up to the CCR as a loop-impedance anomaly rather than as a report from the box itself, since the box has no way to communicate its own condition. Detecting a faulty junction box is therefore a matter of loop-level fault indication from the CCR narrowing the search, followed by physical inspection of the boxes on that loop — the box's role in the architecture is purely to be found and opened, not to report.

The enclosure body is moulded or cast in one of several materials depending on where it sits in the field — UV-stabilised thermoplastic (polycarbonate or polyamide) for lightweight above-ground boxes, cast aluminium or stainless steel for heavier-duty or corrosive environments, and glass-reinforced polyester (GRP) where chemical resistance is a priority — each finished to achieve the box's rated ingress and impact protection. A gasketed lid, secured with captive stainless-steel screws or a quarter-turn latch, seals the enclosure against water and dust while remaining quick to open for cable work. Cable entries are formed as knockouts or pre-drilled and threaded holes sized to metric (M20–M63) or imperial (PG9–PG48) glands, with blanking plugs fitted to any entries left unused so the enclosure's IP rating isn't compromised. Internally, a mounting plate or DIN rail carries terminal blocks, tap-off connectors, or splice kits, and buried boxes sit on a bed of compacted stone or sand with a surrounding drainage layer so water doesn't pool against the enclosure or its cable glands.

An airport junction box is the sealed enclosure that holds cable joints, splices and terminations in an airfield lighting circuit. It gives a protected point for connecting primary and secondary cables, for branch connections and for testing and fault isolation.

Choose the box by its location, load class, sealing rating, size for the cables and joints it must hold, and the voltage of the circuit. For a box used on approach lighting lines, see the approach light junction box, and for PAPI circuits the PAPI junction box.