6.6A Isolating Transformer

The 6.6A Isolating Transformer is a single‑phase, fully encapsulated transformer designed for use in 6.6A constant‑current AGL series circuits. With a primary current rating of 6.6A, this is the most common and widely used isolating transformer in airfield ground lighting systems worldwide. It is available in a comprehensive range of secondary power ratings — from 15W for low‑intensity signage to 500W for the most demanding approach lighting and obstruction applications.

The transformer provides essential galvanic isolation between the high‑voltage series loop (primary) and the low‑voltage lamp circuit (secondary), ensuring safety, fault containment, and the ability to remove individual fixtures without interrupting the series current. Its robust construction, fully encapsulated design, and adherence to international standards make it the backbone of AGL power distribution.

The 6.6A isolating transformer exists as the standard building block of the entire AGL series-circuit approach, because 6.6A is the current level most runway and taxiway lighting circuits worldwide are designed around. Its purpose is to make that constant-current architecture work at the individual-fixture level: taking the shared 6.6A loop current and stepping it down to whatever voltage each specific lamp or LED module needs, while keeping every fixture electrically isolated from its neighbours.

Because it spans secondary ratings from 15W to 500W, this same fundamental component can serve everything from a low-intensity taxiway sign to a high-intensity approach light on the same underlying circuit philosophy, which is why it functions as the backbone of AGL power distribution rather than just one option among several.

The 6.6A isolating transformer is the default choice for the vast majority of AGL applications, including:

• Runway Lighting: Edge lights, centreline lights, threshold lights, end lights, and touchdown zone lights on precision and non‑precision runways.
• Taxiway Lighting: Edge lights, centreline lights, stop bars, and clearance bars.
• Approach Lighting Systems: ALSF‑1, ALSF‑2, MALSR, and other approach lighting configurations.
• Obstruction Lighting: Low‑, medium‑, and high‑intensity obstruction lights on structures of all heights.
• Illuminated Airfield Signs: Mandatory instruction signs, information signs, direction signs, and destination signs.
• Airport Beacons: Rotating beacons and heliport beacons.

The transformer is typically installed in handholes, concrete pits, or direct‑buried enclosures adjacent to each fixture, using standard L‑823 primary connectors and secondary cabling. Its widespread use makes it the most commonly stocked and replaced AGL transformer.

Being the default series-circuit transformer, the 6.6A unit is installed at essentially every fixture position along the loop, in a handhole, concrete pit, or direct-buried enclosure sited immediately adjacent to the light it feeds. The primary winding is spliced into the series circuit using L-823 primary connectors sealed against moisture, with the secondary leads run a short distance to the fixture's own L-823 connector. The enclosure is set to finished grade or just below it, deep enough to clear frost and mechanical loading but shallow enough for the primary loop cable to maintain a consistent bend radius as it passes from one transformer to the next along the runway or taxiway edge.

The 6.6A transformer operates on the principle of electromagnetic induction. The primary winding is connected in series with the 6.6A constant‑current loop. The primary current creates a magnetic flux in the core, which induces a secondary voltage proportional to the turns ratio. The secondary voltage is selected to match the lamp's rated voltage at the rated current — for example, a 45W lamp at 6.6A requires approximately 6.8V, while a 150W lamp requires approximately 22.7V.

Key performance features of the 6.6A design include:

• Optimised Turns Ratio: Designed to deliver the correct secondary voltage at 6.6A primary current, ensuring maximum light output at full intensity.
• Low Core Loss: High‑grade silicon steel laminations minimise no‑load losses, improving efficiency across the entire dimming range (2.8A to 6.6A).
• Excellent Voltage Regulation: Maintains secondary voltage within ±5% over the full current range, ensuring consistent light output at all brightness levels.
• High Thermal Capacity: The fully encapsulated design provides effective heat dissipation to the surrounding soil or concrete, with power ratings up to 500W.
• Robust Short‑Circuit Protection: The transformer's leakage impedance limits secondary fault currents, protecting the loop and minimising the risk of CCR tripping.
• Galvanic Isolation: Complete electrical separation between primary and secondary circuits ensures safety and prevents fault propagation.

The 6.6A transformer occupies the same position in the control chain as any isolating transformer, regardless of the loop it sits on: 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 (one of 5 steps between 2.8A and 6.6A). The transformer has no addressable identity of its own — it converts whatever current the loop is carrying at that moment into the secondary voltage its fixture needs, and cannot be commanded independently of the rest of the loop. Because 6.6A is the standard loop current used across the vast majority of AGL installations, a fault detected by the CCR's voltage monitoring typically points to one transformer among many similar units on a runway, taxiway, or sign circuit rather than to a large specialised array; the choice between a 6.6A and a 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 transformer core and windings are fully encapsulated in a resin compound, typically epoxy or polyurethane, cast within a cylindrical or oval-section housing to exclude moisture entirely and achieve the IP68 submersible rating required for direct burial. The primary and secondary windings are wound on a laminated silicon-steel core, insulated to Class F or Class H depending on the power rating, with the encapsulant also serving to dampen vibration and provide mechanical protection against soil movement and impact during installation. Primary leads terminate in factory-moulded L-823 connectors, sealed splice connectors designed for continuous immersion, allowing the transformer to be spliced directly into the series circuit loop without a separate junction enclosure; secondary leads are similarly sealed and sized to the connecting fixture's cable entry. The unit sits within its own base can, an L-867 or L-868 base recessed into the ground beneath or beside the fixture it serves, with a drainage provision at the base of the can to prevent water pooling around the connectors. Larger power ratings use a proportionally larger encapsulated body and heavier-gauge windings to manage increased heat dissipation, and the exterior casing is moulded from a UV-stable, chemical-resistant polymer to withstand prolonged exposure to soil chemistry, de-icing fluids, and fuel spillage over decades of service.