Buried Isolating Transformer

The Buried Isolating Transformer is a heavy‑duty, fully encapsulated single‑phase transformer designed for direct burial in the ground. Unlike standard isolating transformers that are typically installed in concrete pits or handholes, this variant is engineered to withstand the mechanical loads, moisture, and chemical aggression of direct soil contact. Its robust construction, enhanced corrosion protection, and specialised cable sealing make it ideal for installations where a pit is impractical or where maximum protection against environmental factors is required.

The transformer provides the same essential galvanic isolation between the 6.6A (or 20A) primary series circuit and the secondary fixture circuit, ensuring safety and fault containment. It is available in a range of power ratings (15W to 100W) and secondary voltages, catering to most AGL fixture requirements. The direct‑burial design eliminates the need for additional protective enclosures, reducing installation time and material costs.

The buried isolating transformer exists to let a transformer live directly in the ground — without the added cost, footprint, and installation time of a concrete pit — while still surviving prolonged exposure to soil moisture, chemical content, and mechanical loading from vehicles and aircraft overhead. Its enhanced sealing and corrosion protection are what make this direct-burial approach viable rather than a shortcut that compromises reliability.

This matters because a transformer that fails from water ingress or corrosion doesn't just lose its own function; it opens the series loop and can affect every fixture downstream of it, depending on circuit design. Building the unit to withstand direct soil contact from the outset removes a failure mode that a standard pit-mounted transformer would otherwise depend on the pit itself to prevent.

The buried isolating transformer is specifically suited for installations where:

• Space is Limited: In congested airfield areas where there is insufficient room for a concrete pit or handhole, direct burial is the only viable option.
• High Water Table: In regions with a high water table or where flooding is common, the buried transformer's superior sealing prevents water ingress.
• Chemically Aggressive Soils: In areas with corrosive soils (e.g., coastal regions with salt content, industrial areas with chemical spills), the specialised encapsulation and corrosion‑resistant materials ensure long‑term reliability.
• High Mechanical Loads: In areas subject to heavy vehicle traffic (e.g., runway turn‑offs, heavy apron areas), the buried transformer can withstand the compressive loads without damage.
• Expeditionary or Temporary Airfields: Quick installation without the need for concrete works, allowing for rapid deployment and removal.
• Retrofit Projects: Replacing existing buried transformers or upgrading hard‑wired systems with direct‑burial units.

The transformer is typically buried at a depth of 300‑600mm (12‑24 inches) below the surface, with the primary and secondary cables routed through specially designed sealing glands to prevent moisture ingress. It is suitable for both 6.6A and 20A series circuits.

The buried transformer is installed directly in the ground rather than in a handhole or concrete pit, typically at a depth of 300-600mm below the surface, with its encapsulated body and sealing glands providing the moisture and mechanical protection that would otherwise come from a separate enclosure. Primary and secondary cables are routed through the glands and spliced or terminated using waterproof connectors before backfilling, with the unit positioned close enough to the fixture to keep the secondary run short. Because it has no accessible chamber, its exact location is recorded against the runway or taxiway survey so it can be located for testing or replacement without random excavation, and it is set below plough or frost depth in areas subject to those conditions.

Electrically, the buried isolating transformer operates identically to standard AGL transformers: the primary winding is connected in series with the constant‑current loop, and the secondary winding delivers the appropriate voltage to the fixture. The key differentiator lies in its mechanical and environmental design:

• Reinforced Encapsulation: The transformer is encapsulated in a high‑strength, chemical‑resistant epoxy or polyurethane compound that provides mechanical protection and a barrier against moisture, soil acids, and salts.
• Heavy‑Duty Cable Entries: The primary and secondary cables enter the transformer through specialised compression glands or moulded seals that maintain IP68 integrity even under high external pressure (e.g., water table).
• Corrosion‑Resistant Terminals: Internal connections are made with tinned copper, stainless steel, or other corrosion‑resistant materials to prevent degradation from any moisture that may penetrate the seals.
• Robust Core and Windings: The magnetic core and windings are designed to withstand the higher thermal and mechanical stresses associated with direct burial, including potential ground movement and thermal cycling.
• Grounding Provisions: A dedicated earthing terminal (often an external lug) ensures proper grounding of the transformer casing to protect against lightning and fault currents.

The transformer is compatible with standard CCR dimming profiles and meets all relevant electrical and safety standards. Its direct‑burial design does not compromise electrical performance; efficiency and regulation remain within the same tolerances as conventional transformers.

Burial changes the transformer's mechanical environment, not its place in the control chain. An Airfield Lighting Control and Monitoring System (ALCMS) selects the required brightness step based on prevailing RVR, ATC instruction, or pilot-controlled lighting input, and the CCR regulates the whole loop to the corresponding fixed current; the buried transformer's primary winding carries that current exactly as an elevated or handhole-mounted unit would, with no addressable identity of its own and no way to be commanded independently of the loop. This has a practical consequence specific to the buried variant: because the unit has no accessible chamber, a loop fault traced to its general vicinity by CCR voltage monitoring cannot be visually confirmed the way an open handhole lid would allow — locating the specific failed unit relies on the as-built survey record and, where available, sectionalised loop testing from adjacent accessible points, since excavation on suspicion alone is impractical.

A buried unit has no accessible chamber or removable cover by design: the windings and core are fully encapsulated in a chemical-resistant epoxy or polyurethane compound, UV-stabilised for the brief period before backfill covers it, and this potting is the only barrier between the winding and the surrounding soil moisture — specified more robustly than the resin used on a handhole-mounted unit, since it is never opened again once installed. Primary and secondary cable entries use compression glands rather than a gasketed lid, maintaining IP68 integrity even under sustained water-table pressure at the specified 300–600mm burial depth. Terminals are tinned copper or stainless steel to resist any moisture that eventually works past the seal over a multi-decade service life, and because there is no cover to open for a visual check, every electrical connection is made and tested before the unit goes into the ground. Weight is kept comparatively low, around 2.0–4.0 kg, since the unit carries no enclosure or mounting hardware — only the potted winding assembly itself.