Isolating Transformers & Series Circuits

Isolating transformers are the individual power-supply units that feed each AGL fixture (or small group of fixtures) from the primary series circuit. They are typically 1:1 or step-down transformers with a primary winding connected in series with the main loop and one or more secondary windings that supply the lamp or LED module at a lower, safe voltage — the exact secondary voltage depends on the connected fixture's wattage rating, since these transformers are specified by secondary current (typically 6.6A or 20A) and wattage rather than a fixed secondary voltage.

Series circuits are the backbone of AGL power distribution. A constant current regulator (CCR) drives a fixed current (usually 6.6A) through a loop of cable that passes through every isolating transformer in that circuit. Because the current is the same through all series-connected elements, every fixture on that circuit receives the same current, and thus the same intensity, regardless of how many fixtures are installed or how long the cable run is.

This combination — isolating transformers on a series circuit — is what makes AGL systems reliable, scalable, and easy to control. A single CCR can power dozens of fixtures, and a single fixture failure does not take down the whole circuit because the isolating transformer's primary winding remains a low-impedance path for the series current.

This combination exists to solve two problems at once that would otherwise be in tension with each other: an airfield needs every fixture on a circuit to receive exactly the same, consistent brightness regardless of its position in the loop, but it also needs a single failed fixture or transformer to never take down every other light sharing that circuit. A series circuit alone would guarantee the first but fail badly at the second — a genuine open circuit anywhere in a pure series loop stops current everywhere. Isolating transformers are what let the loop keep the uniform-current benefit of a series circuit while still tolerating individual fixture faults, because each transformer's primary winding provides a low-impedance path that keeps the loop current flowing even if that fixture's secondary side or lamp has failed.

The isolation itself is equally purposeful from a safety standpoint: each fixture sits electrically separated from every other fixture on the loop, so a technician working on one light, or a fault developing at one location, doesn't expose the rest of the circuit — or the people maintaining it — to that problem. This is what allows a single CCR to reliably drive dozens of fixtures spread across kilometres of airfield, with the confidence that the system as a whole will keep functioning even as individual components inevitably age, fail, and get serviced over the system's operational life.

Isolating transformers are installed at every lighting fixture position across the airfield, buried in below-grade pits or directly embedded with the fixture:

• Runway Edge and Centreline: Each edge light and centreline light has its own isolating transformer, typically mounted in a shallow pit adjacent to the fixture.
• Taxiway Edge and Centreline: Similarly, every taxiway light is fed through its own transformer.
• Approach Lighting: Even elevated approach light structures use isolating transformers, often housed in a base pit.
• Stop Bars and Guard Lights: These critical fixtures also use isolating transformers, often on separately switched series circuits.
• Airfield Signage: Illuminated signs are powered via isolating transformers in the same way as lighting fixtures.
• Primary Series Loop: The transformer primary windings are connected in series with the main cable, forming the continuous loop that starts and ends at the CCR.

The series circuit itself runs between the CCR and the furthest fixture, with the cable passing through each transformer's primary in a daisy-chain fashion (or sometimes in a loop with two feeds for redundancy).

Each isolating transformer is installed in its own shallow pit or base can immediately adjacent to the fixture it feeds, so the low-voltage secondary run to the lamp or LED module stays short while the primary winding sits in-line with the series cable passing through the pit from the previous fixture to the next. The transformer is positioned above any drainage sump in the pit and set on its mounting bracket clear of standing water, with the primary and secondary connectors made up using L-823 or equivalent watertight couplers before the pit lid is closed. Because the primary winding forms part of the continuous series loop, the transformer's physical position in the daisy-chain has to match the circuit design and as-built drawing exactly, so a technician tracing the loop later can rely on the documented order of fixtures matching what is actually buried in the ground. Where a circuit uses a loop configuration with two feeds for redundancy rather than a single daisy-chain, the two cable runs into and out of each pit are kept clearly identified during installation so the loop's redundancy is not compromised by a mislabelled connection.

The isolating transformer is the interface between the high-voltage, low-current series circuit and the low-voltage, high-current fixture. The primary winding is designed to carry the full series current (e.g. 6.6A) with minimal voltage drop, and its inductance presents a relatively high impedance to the series circuit, allowing the transformer to extract power while still passing the current along to the next transformer.

Key functional aspects include:

• Constant-Current Feeding: The transformer steps down the voltage (e.g., from several hundred volts on the primary to a level set by the connected fixture's wattage on the secondary) while stepping up the current, matching the fixture's requirement. The secondary current is proportional to the primary current divided by the turns ratio.
• Fault Isolation: If a fixture or its secondary wiring fails open, the transformer's primary remains intact, so the series current continues to flow. If a secondary short occurs, the transformer's primary impedance limits the current, preventing a total circuit failure.
• Intensity Control: Because all transformers on a given series circuit share the same primary current, adjusting the CCR's output current changes the brightness of every fixture on that circuit simultaneously — this is how intensity steps work.
• Electrical Isolation: The transformer provides galvanic isolation between the primary series circuit and the secondary fixture, improving safety and reducing the risk of shock or earth-leakage faults propagating through the system.
• Multiple Secondary Windings: Some transformers have multiple secondary taps to supply different fixture types (e.g., one for an edge light and one for a sign) from the same primary.

The isolating transformer has no electronics or command input of its own; its place in the system architecture is as the passive interface node between the CCR's series-current control loop and each individual fixture. Because every transformer's primary is wired in series with the same loop, the CCR's constant-current regulation is what actually governs brightness across the whole circuit — the transformer just steps that shared current down to a safe secondary voltage for its fixture, with no local intensity decision to make. This series-primary arrangement doubles as the circuit's built-in fault-isolation logic: an open secondary (a failed lamp) doesn't interrupt the primary loop, so the rest of the circuit keeps operating, while a shorted secondary is limited by the transformer's own impedance rather than collapsing the whole circuit — with any resulting anomaly still surfacing at the CCR as a voltage change against commanded current, exactly as it would for a cable or connector fault.

An isolating transformer is built around a toroidal or laminated core wound with primary and secondary copper windings, encapsulated in resin or, in older designs, housed in an oil-filled metal casing to provide electrical insulation and protect the windings from moisture during buried service. The encapsulated or oil-filled unit is sized to fit within a standard base can, with primary and secondary leads terminating at sealed L-823 connectors that mate to the incoming series circuit cable and the outgoing fixture cable respectively, keeping every connection point watertight to an IP68 rating. The insulation system between primary and secondary windings is rated to a high dielectric strength, since the transformer's core function is to isolate each fixture electrically from the rest of the series circuit so a fault at one fixture does not propagate along the loop. Class H or higher insulation materials are used throughout to tolerate the sustained thermal loading of continuous series-circuit operation, and the transformer's compact cylindrical or rectangular form factor is designed to sit alongside the fixture's connector and cabling within the confined space of a base can.