20A Isolating Transformer

The 20A Isolating Transformer is a heavy‑duty, single‑phase transformer engineered for use in 20A constant‑current AGL series circuits. Unlike the industry‑standard 6.6A transformers, the 20A variant is deployed in specialised installations where the lighting load exceeds the capacity of conventional 6.6A loops — such as high‑intensity approach lighting systems (e.g., ALSF‑2 with extended bars), large obstruction lighting arrays on very tall structures, and military airfields with exceptionally high lighting power requirements.

The transformer provides the same galvanic isolation, fault tolerance, and fixture‑level control as its 6.6A counterparts, but is built with substantially heavier gauge windings, a larger core, and enhanced thermal management to handle the elevated primary current and associated power transfer. It is typically deployed in specialised pits or enclosures with dedicated ventilation or cooling provisions.

The 20A isolating transformer exists because some lighting loads — extended approach lighting bars, large obstruction arrays on very tall structures, or heavy-duty military installations — simply draw more power than a standard 6.6A loop can practically deliver. Rather than forcing these high-demand systems onto multiple parallel 6.6A circuits with all the added complexity that implies, a 20A primary lets a single loop carry the higher current these fixtures need.

Handling that higher current safely is what drives the heavier windings, larger core, and enhanced cooling: a transformer under-built for its primary current risks overheating or premature insulation failure, which would undermine the very isolation and fault-tolerance the transformer exists to provide in the first place.

The 20A isolating transformer is reserved for the most demanding AGL applications, including:

• High‑Intensity Approach Lighting Systems: Extended ALSF‑2 and military approach configurations requiring very high luminous output from barrettes, crossbars, and sequenced flashing lights, where the total loop load exceeds the 6.6A capacity.
• High‑Power Obstruction Lighting: Extremely tall structures (e.g., >500m communication towers, chimneys) requiring high‑intensity white flashing lights with peak intensities exceeding 200,000 cd, often using 500W or 1,000W lamps.
• Large Airport Beacons: High‑power rotating beacons with multiple 500W lamps, requiring significant primary current to drive the beacon's illumination and rotation.
• Military Airfields & Specialised Operations: Military airfields often specify 20A circuits for enhanced reliability, redundancy, and the ability to power high‑intensity tactical lighting systems in combat or extreme weather conditions.
• Long Runways with High Fixture Density: Very long runways (e.g., >4,000m) with densely spaced lighting may exceed the power capacity of a single 6.6A loop, necessitating a 20A loop.
• Multi‑Fixture Circuits: Configurations where a single transformer powers multiple high‑intensity fixtures (e.g., five 100W fixtures, ten 50W fixtures, or combinations) requiring total secondary power up to 2,000W.

The transformer is typically installed in large, well‑ventilated handholes or dedicated transformer pits, using specialised primary connectors rated for 20A (often L‑823 or larger variants) and heavy‑gauge secondary cabling (≥ 16mm²) to handle the increased current.

Because it carries the higher-current 20A series loop, this transformer is installed in a larger, well-ventilated handhole or dedicated transformer pit rather than a standard small enclosure, to dissipate the greater heat load and accommodate the heavier-gauge cabling. Primary connections use L-823 (or larger) connectors rated for 20A service, and secondary cabling of at least 16mm² is run to the fixture to keep voltage drop and heating within limits. These pits are typically sited on high-fixture-density runways, extended approach lighting systems, or military circuits, and are spaced and sized so maintenance crews can access and test each unit without disturbing the loop cabling.

The 20A transformer operates on the same electromagnetic induction principle as other AGL isolating transformers, but with significant design enhancements for high‑current operation. The primary winding is connected in series with the 20A constant‑current loop. The primary current induces a secondary voltage — typically in the range of 6V to 120V depending on the turns ratio and the secondary power rating — which powers the connected fixture(s).

Specific design features of the 20A variant include:

• Large‑Gauge Windings: Primary and secondary windings use very thick copper wire to minimise I²R losses and prevent overheating at the elevated primary current (20A vs. 6.6A).
• High‑Flux Core: The core is significantly larger to handle the high flux density and prevent saturation at 20A, using high‑grade grain‑oriented silicon steel laminations.
• Enhanced Thermal Management: The encapsulation compound is formulated with high thermal conductivity, and the transformer housing often includes cooling fins or a larger surface area to dissipate heat more effectively.
• Optimised Turns Ratio: Designed to deliver the required secondary voltage at 20A primary current, with custom voltages available for specific fixture types (e.g., 30V for 600W, 60V for 1,200W).
• Robust Short‑Circuit Protection: The transformer's leakage impedance limits secondary fault currents, but short‑circuit conditions at 20A are far more severe than at 6.6A, requiring immediate attention and protection.
• High Dielectric Strength: With greater power transfer, the insulation system is rated for higher voltage stress, typically exceeding 3kV primary‑secondary.

The 20A transformer occupies the same position in the control chain as any 6.6A-loop transformer, just on a different-capacity circuit: 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 20A loop to the corresponding fixed current. The transformer has no addressable identity of its own — it converts whatever current the loop is carrying at that moment into the voltage its fixture or fixture group needs, and cannot be commanded independently of the rest of the loop. Because a 20A circuit is reserved for the highest-load installations, a loop fault detected by the CCR's voltage monitoring can potentially affect a large extended approach lighting array, a very tall obstruction light, or a long high-density runway section in one event; the choice between a 6.6A and 20A loop for a given installation, and what that implies for the scale of a single fault, is covered in the loop current selection logic below.

The 20A-rated transformer is a substantially larger and heavier assembly than the 6.6A range, reflecting the heavier-gauge conductor its primary winding needs to carry the higher loop current: even at an equivalent wattage, the winding and core are not interchangeable with a 6.6A unit. Construction otherwise follows the same principle as the rest of the range — windings around a laminated core, fully encapsulated in a moulded shell that is vacuum-impregnated and back-filled with epoxy or polyurethane resin, which is what gives the unit its IP68 rating directly from the potting. Complete units run from roughly 25 to 50 kg depending on the power option (500W to 2,000W), so mechanical handling and lifting provision at the base can are planned for during installation. Insulation is specified to Class H, dielectric strength is higher than the 6.6A range at 4kV primary-secondary, and leads terminate in heavier-gauge sealed splice connectors sized for the higher fault current a 20A loop can present.