Isolating Transformer

An Isolating Transformer (also known as a series transformer or lighting transformer) is a single-phase transformer used in Airfield Ground Lighting (AGL) systems to couple individual light fixtures to the constant current series circuit. Its primary winding is connected in series with the CCR output, carrying the full loop current (typically 6.6A or 20A). The secondary winding supplies power to a single lamp or LED module at a lower voltage (typically 6V, 12V, or 24V), matching the lamp's voltage requirement. This arrangement ensures that each light fixture receives the correct current regardless of the number of fixtures in the loop, while providing electrical isolation for safety and fault tolerance.

The transformer is designed to operate continuously at its rated current and is typically buried in a concrete pit or directly in the ground near the fixture. It is a passive, maintenance-free component with high reliability, but its failure can cause a complete open circuit in the series loop, affecting all downstream lights. Therefore, proper selection, installation, and periodic testing are essential.

The isolating transformer exists to solve a problem specific to constant-current series lighting: every fixture on a loop shares the same current, so there has to be a way to couple hundreds of individual lamps to that shared circuit without letting a single fault at one fixture take down the whole loop, or exposing maintenance staff to the loop's series voltage while working on an individual light. Galvanic isolation is what makes this possible — each fixture sits electrically separate from its neighbours even though they're all fed from the same primary current.

This isolation is also what allows the series circuit itself to function reliably in the first place: because current transfer, not voltage, is what matters in a series loop, a transformer can fail short or a fixture can be removed for maintenance without disrupting the loop's constant current to every other light. That fault-tolerance is the whole reason airfield lighting is wired this way rather than with a conventional parallel circuit, and the isolating transformer is the component that makes it work fixture by fixture.

Isolating transformers are installed at every light fixture position in the AGL series circuit. They are used in:

• Runway Lighting: Edge lights, centreline lights, threshold lights, end lights, touchdown zone lights, and approach lighting systems.
• Taxiway Lighting: Edge lights, centreline lights, stop bars, and guidance signs.
• Obstruction Lighting: Low, medium, and high intensity obstruction lights.
• Approach Lighting Systems: Sequenced flashing lights, barrettes, and crossbar lights.
• All AGL fixtures: Both elevated and inset fixtures require an isolating transformer for power and isolation.

The transformers are typically located in a handhole or pit adjacent to the fixture, or buried directly in the cable trench. They are rated according to the wattage of the connected lamp (e.g., 45W, 65W, 100W, 200W) and the primary current (6.6A or 20A).

Whatever its rating, an isolating transformer is installed at the fixture position it serves, either in a handhole or concrete pit set to grade, or buried directly in the cable trench, with the choice driven by ground conditions, fixture type, and maintenance access requirements. The primary winding is spliced into the constant-current series loop with an L-823 (or larger, for 20A circuits) primary connector, and the secondary leads run a short distance to the fixture's own connector, keeping the low-voltage secondary cable run as short as practical. Enclosure depth and cable routing are tied into the runway or taxiway centreline survey and set out to clear frost, drainage, and mechanical loading, while remaining shallow enough for routine access during fixture maintenance.

The isolating transformer operates on the principle of electromagnetic induction. The primary winding (connected in series with the loop) carries the constant current from the CCR. This current creates a magnetic flux in the core, which induces a voltage in the secondary winding proportional to the turns ratio. The secondary voltage is chosen to match the lamp's rated voltage at the rated current. For example, a typical 45W halogen lamp at 6.6A requires about 6.8V. The transformer steps down the loop voltage (which can be hundreds of volts depending on loop length) to the low lamp voltage.

Key functional aspects include:

• Galvanic Isolation: The primary and secondary windings are electrically separate, providing safety isolation between the high-voltage loop and the lamp.
• Constant Current Transfer: The transformer maintains the loop current on the primary side, and the secondary current is scaled by the inverse of the turns ratio, ensuring the lamp receives the correct current.
• Voltage Regulation: The transformer is designed to operate with minimal voltage drop and good regulation across the specified current range (2.8A to 6.6A for 5-step dimming).
• Short-Circuit Protection: In the event of a lamp or cable short on the secondary side, the transformer's impedance limits the fault current, protecting the CCR and the loop.
• Open-Circuit Protection: If the secondary is open (e.g., lamp removed), the primary impedance prevents excessive voltage rise, but the loop current still flows through the primary.

The isolating transformer sits in the middle of the same chain that governs every fixture in this library: 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 passes that command to the Constant Current Regulator, which regulates the entire loop to the corresponding fixed current (one of 5 steps between 2.8A and 6.6A, or the 20A equivalent). The transformer's job is purely local translation, not decision-making — it has no addressable identity on the loop and receives no command of its own; it simply carries whatever current the CCR is driving through the loop at that moment and converts it to the voltage its fixture needs. This is why transformer-level faults surface as loop-level faults: the CCR monitors loop voltage against its commanded current, and a voltage rise signals increased impedance somewhere on the loop — an open-circuit lamp or a failed transformer look identical from the CCR's perspective, and sectionalised field testing is what narrows the fault down to a specific transformer. The physics of how the transformer itself behaves under a fault (rather than how that fault is reported upstream) is covered in the isolation and fault-response logic below.

As the baseline design covering the 45W–300W range, the isolating transformer is built as a single encapsulated, potted unit: primary and secondary windings around a laminated core sit inside a moulded shell that is vacuum-impregnated and back-filled with epoxy or polyurethane resin, leaving no internal cavity for moisture to enter. This potting is what delivers the IP68, submersible rating specified for the unit — there is no separate gasket or enclosure lid to maintain. Insulation is specified to Class F or Class H depending on the rating selected, and the unit's leads terminate in sealed L-823 splice connectors suited to direct burial and continuous immersion. In service the transformer sits in a base can alongside the fixture or fixture group it feeds, secured by the surrounding backfill rather than mechanical fasteners, with stainless steel or tinned-copper lead wires resisting corrosion at the connection points over the working life of the loop.

An airport isolating transformer is the part that connects each light to the series loop. Searches for isolating transformer airport lighting or airport lighting transformer usually mean this component. The same unit is used as a runway lighting transformer (also called a runway light transformer), as a taxiway light transformer and on approach and signage circuits. What changes between these uses is the rating and the connector type, not the working principle.

Isolating Transformer Primary Voltage & Secondary Voltage

The isolating transformer primary voltage is not a fixed supply voltage. The primary carries the loop current (commonly 6.6 A or 20 A), so the voltage across it is a small drop that depends on the transformer rating and the load. The isolating transformer secondary voltage is the voltage delivered to the lamp or LED module, and it depends on the lamp wattage at the rated secondary current. Always read both from the nameplate and manufacturer data, not from a general rule.

Connectors. An airport lighting transformer connector joins the transformer to the series cable (primary side) and to the fixture (secondary side). Use the connector type that matches the transformer and cable, and see the L-823 connector, primary plug and secondary plug pages.

How Is an Isolating Transformer Selected?

To choose a transformer, match the loop current (6.6 A or 20 A), the rated wattage to the lamp or LED load, the installation type (plug-in, buried or elevated), the connector type, and the standard or project specification that applies. Rating the unit too low risks overheating, and too high wastes loop capacity.

Airport Lighting Transformer Testing

Airport lighting transformer testing normally covers three checks, each compared with the manufacturer data, the previous record or the project specification. The resistance and insulation tests are listed here, and the ratio test follows below.

• Isolating transformer resistance test: measure the DC resistance of the primary and secondary windings. An open winding reads as an open circuit, and a shorted turn can read low.
• Isolating transformer insulation test: use an insulation tester between the windings and between each winding and earth, with the transformer disconnected from the circuit.

Never test a transformer that is connected to a live series circuit. Isolate and earth the circuit first, following the airport's permit and lockout procedure. A transformer tester can speed up field checks.

Isolating Transformer Ratio Test

Pass a known current through the primary, measure the secondary current and compare with the rated ratio.