Base Can

A Base Can (also known as a transformer can, fixture base, or ground can) is a heavy‑duty cylindrical steel housing that is embedded in the pavement (concrete or asphalt) to provide a robust foundation for elevated AGL light fixtures. It serves as the primary structural and protective enclosure for the isolating transformer, cable terminations, and connector interfaces that power the fixture. Base cans are supplied in two distinct load classes that are not interchangeable: FAA Type L-867 is rated for occasional light vehicular traffic only and must never be installed where it could see aircraft wheel loads, while Type L-868 is load-bearing and rated for direct aircraft traffic. Selecting the wrong class for a given location — for example, an L-867 can where an aircraft excursion or routine taxi could load it — is a genuine safety defect, not just a spec mismatch.

Key characteristics of base cans include:

• Robust Construction: Made from high‑strength steel (typically hot‑dip galvanised or stainless) to withstand heavy aircraft loads, impact, and corrosion.
• Watertight Design: Sealed to prevent moisture ingress, with gasketed lids and cable entry points.
• Integrated Mounting: Includes a top flange or threaded holes for attaching the fixture's base plate, and internal brackets for securing the isolating transformer.
• Cable Management: Provides internal space for primary and secondary cable connections, with cable glands or conduits for entry and exit.

Base cans are designated L-867 (light-duty, for locations subject only to occasional light vehicular loads — not aircraft or heavy vehicle traffic) or L-868 (heavy-duty, rated for aircraft and heavy vehicular loads). Both types support elevated fixtures; the choice depends on the load the installation location will see, not on whether the fixture is elevated or inset — flush-mounted inset fixtures use their own in-pavement fixture housings (e.g. the L-850 series) rather than an L-867/L-868 base can. Base cans are typically installed during pavement construction or retrofitted into existing pavement, and are available in various diameters (typically 150mm to 300mm) and depths (200mm to 500mm) to accommodate different transformer sizes and cable configurations.

The parts of an elevated fixture most likely to actually fail — the isolating transformer and the cable terminations — are also the parts least able to tolerate water, dirt, or physical disturbance. The base can's purpose is to give those components a dedicated, protected space below the pavement surface, separated from both the aircraft loading experienced above and the surrounding soil and groundwater. It also fixes the fixture's exact position and height before the fixture is even installed, so the frangible pole or base plate that sits on top of it starts from a known, level, and correctly located reference point rather than relying on field adjustment. In effect, the base can turns "install a light fixture in a runway" into two separable jobs — a below-grade electrical and structural foundation, and an above-grade optical fixture — that can be built, sealed, and maintained independently of each other.

Base cans are used wherever elevated AGL fixtures are installed on paved surfaces, including:

• Runway Edge Lighting: For elevated runway edge light fixtures, especially on curves, intersections, or where flush mounting is not feasible.
• Taxiway Edge Lighting: For elevated taxiway edge lights, particularly on curves and at intersections.
• Approach Lighting: For elevated approach light fixtures, barrettes, and flashing light units.
• Obstruction Lighting: For elevated obstruction lights on poles or structures.
• Airfield Signage: For elevated mandatory instruction signs and direction signs that require a stable foundation.
• Sensor Mounting: For anemometers, wind cones, and other instrumentation that requires a raised mounting.

The base can is designed to be installed flush with or slightly above the pavement surface, with a removable top cover that allows access to the transformer and connections for maintenance. The can's internal volume is sized to accommodate the isolating transformer, cable splices, and any necessary connectors, while maintaining adequate clearance for heat dissipation.

The base can is set into a formed or cored foundation pit at the fixture's surveyed position, with its top rim set flush with or at the specified height above finished grade depending on whether the installation is elevated or inset. Backfill around the can is compacted in layers to prevent later settlement, which would otherwise leave the can proud of or sunk below grade; cable entry to the can is made through a sealed gland or knockout kept below any water table in the surrounding backfill where possible. Can position and level are checked against the pavement or shoulder survey before backfilling, since a can set at the wrong height is far more disruptive to correct once the surrounding pavement or turf is finished.

The base can functions as a combined structural foundation, protective enclosure, and cable management hub for the elevated fixture. Its design ensures that the fixture is securely mounted, electrically connected, and protected from the environment:

• Structural Support: The can is embedded in the pavement with a concrete or asphalt surround, providing a rigid, stable base that can withstand the dynamic loads from aircraft, snowploughs, and maintenance vehicles.
• Transformer Housing: The isolating transformer is mounted inside the can on a bracket or base plate, keeping it dry and protected from mechanical damage. The secondary cable runs from the transformer to the fixture, while the primary cable enters the can through a cable gland or conduit.
• Weatherproofing: A gasketed lid or cover plate seals the top of the can, preventing water, de‑icing fluids, and debris from entering. Cable entry points are also sealed with glands or compound to maintain a watertight seal.
• Accessibility: The top cover can be removed for maintenance, allowing technicians to access the transformer, cables, and connections without disturbing the pavement.
• Frangible Feature (if applicable): In some designs, the base can includes a frangible coupling or break‑away flange that allows the fixture to shear off on impact, minimising aircraft damage.

The installation process typically involves:

• Excavation: A hole is dug in the pavement to the required depth and diameter.
• Placement: The base can is positioned in the hole, levelled, and secured with a concrete or asphalt surround.
• Wiring: The primary cable is routed into the can, and the isolating transformer is connected. The secondary cable is routed from the transformer to the fixture location.
• Fixture Mounting: The fixture's base plate is bolted to the top flange of the can, and the fixture is installed.
• Sealing: The lid is installed and sealed to prevent water ingress.

The base can is designed to provide a service life of 20+ years, with regular inspection of the seal, cables, and transformer condition.

The base can is the one point in the installation where the electrical and mechanical sides of the architecture physically meet. The series circuit cable arrives from the trench and terminates inside it, generally at the isolating transformer housed in the same can; at the same time, the can itself bolts down onto the anchor bolts and carries the structural load of whatever sits above it — a frangible coupling and elevated fixture, or an inset fixture seated directly in it. Because both the power connection and the mechanical foundation converge at this one component, a base can problem can present as either an electrical fault (a wet or failed transformer) or a mechanical one (a loose or corroded mounting), and diagnosing which requires opening the can rather than reading a single status from either the ALCMS or a structural inspection alone.

A base can is fabricated from structural steel plate, rolled and welded into a cylindrical body, then hot-dip galvanised or specified in stainless steel for installations demanding higher corrosion resistance. The top of the can terminates in a welded flange or ring, machined or drilled with a bolt pattern matched to the fixture's base plate, so the fixture bolts directly onto the can once it is set into the pavement. A removable steel lid, gasketed with a neoprene or rubber seal and secured with bolts or quick-release clamps, closes the top opening around the fixture base, keeping the transformer and connector compartment inside protected from surface water while still allowing access for maintenance. Cable entries, typically two to four gland or conduit fittings, are welded or threaded into the can's side wall below grade, sized to accept the incoming and outgoing series circuit cables with a sealed, strain-relieved connection. The can's wall thickness and overall structural design are engineered to support the vertical load of the fixture above and the surrounding pavement load without deforming, while its buried depth and diameter are matched to the isolating transformer and connector hardware it houses.

An airport light base can is the cylindrical housing set into the pavement or ground that supports an elevated or inset light and holds the transformer and connections beneath it. The right load class (L-867 or L-868) must be used for the location. After installation, check that the base can is level, correctly set to height, sealed, and that its cable entries are watertight, because a poorly set base can causes tilted lights and water ingress.