Why Series Circuits Are Used in Airfield Lighting

In a conventional parallel system, the voltage at each fitting depends on cable length, loading and voltage drop, so fittings far from the source can receive a lower voltage and produce less light. Over long airfield cable routes with hundreds of fittings, keeping every fitting under the same electrical conditions is difficult. The result can be non-uniform light output — unacceptable on an active runway or taxiway.

In AGL, the objective is not merely to make the lights visible. The objective is to provide consistent visual guidance to pilots.

In a series constant-current circuit, the same primary current flows through every isolating transformer in the loop, and each lamp or LED fitting is fed its design operating current from its transformer secondary. For fittings of the same type this provides consistent operating conditions, more uniform light intensity, predictable brightness across the circuit, and better control of the overall lighting system.

This uniformity is critical because pilots depend on visual consistency during approach, landing, taxiing, and takeoff. Visible variations in output along a lighting pattern can contribute to false depth perception, misjudgment during approach, and confusion during low-visibility operations.

A bright–dark–bright pattern is not simply a lighting defect. It can become a safety concern.

Airfield lighting circuits can extend over several kilometres. Runways and taxiways cover large areas, and the electrical system must provide predictable performance from one end of the circuit to the other.

• Long cable runs can be fed from a single CCR, because current rather than voltage is what is regulated
• Voltage drop along the cable does not change the current through the fittings, unlike conventional parallel distribution
• Stable system performance across the circuit
• Consistent operation over long circuits, within the CCR’s rated output voltage and the cable insulation rating

This makes the series-circuit approach particularly suitable for large airfields, where lighting uniformity must be maintained end-to-end.

Another major advantage of the series architecture is that system behaviour becomes more predictable and measurable — circuit current stays controlled when the connected load changes (within the CCR’s rated range), and CCR monitoring (output current, open-circuit and earth-leakage indication) together with insulation-resistance testing and trending supports early detection, maintenance, and fault-finding. Series loops are normally unearthed, so a first earth fault can often be detected and alarmed without necessarily extinguishing the lights, depending on the CCR’s protection settings.

For an AGL maintenance team, the electrical system provides useful indications when a fault develops, rather than behaving like an uncontrolled conventional lighting network.

An important principle of AGL engineering is simple: failures will happen. The objective is therefore not to assume every component will operate perfectly forever, but to design the system so individual failures can be contained while the remaining lighting continues to perform its required function.

The series circuit works together with isolating transformers installed at the individual light fittings. Each transformer separates the primary series circuit from the secondary that feeds the lamp or fixture. If the lamp or secondary fails open, primary current keeps flowing, so the rest of the circuit stays lit; and the series-circuit voltage is kept away from the fitting itself, which also improves safety for personnel. Serviceability limits for lamps-out still apply, and the transformer does not protect against faults on the primary cable.

A fault on the primary cable — an open circuit, for example — will still take that circuit out of service. For this reason lighting patterns are normally divided between several series circuits (for example, alternate fittings or alternate rows on different circuits), so the loss of one circuit leaves the pattern recognisable rather than dark.

The isolating transformer is not an independent accessory. It is an essential part of the series-circuit architecture.

The real strength of an AGL series circuit comes from the combination of the series circuit and the Constant Current Regulator, which controls the circuit current and provides the required operating levels for the connected airfield lighting system.

Because the circuit depends on current continuity, opening the primary of an energised series circuit can produce dangerous voltages. Always isolate, lock out and prove dead under the aerodrome’s permit-to-work before working on any part of the circuit.

Because the circuit operates as a controlled constant-current system, lighting output can remain consistent across the complete circuit — fundamentally different from ordinary building lighting, where fixtures are normally connected in parallel to a voltage source.

Conventional parallel distribution is convenient for many commercial and industrial applications, but airfield lighting has different requirements — long cable routes, large numbers of fittings, critical visual guidance, continuous operational requirements, and strict requirements for consistency and reliability. A conventional parallel arrangement can experience variations in voltage and load conditions along the distribution network. Series circuits have trade-offs of their own — a primary-cable open circuit takes the whole circuit out of service, and the circuit carries high voltage at constant current — which is why circuit splitting, monitoring and strict isolation procedures are part of AGL design and maintenance.

For AGL, the priority is not electrical simplicity. The priority is uniformity, reliability, fault tolerance, and predictable performance.

The ultimate purpose of the electrical architecture is not the circuit itself — it is the visual information delivered to the pilot. Runway edge lights, centreline lights, touchdown zone lights, taxiway lighting, approach lighting, and other visual aids work together to create a visual reference system.

During good visibility, small variations may be difficult to notice. During fog, rain, night operations, low visibility, and critical approach and landing phases, the importance of predictable visual guidance increases significantly.

That is why AGL electrical engineering and photometric performance cannot be considered separately — the electrical system must support the required visual performance of the lighting system.

• Series circuits are a deliberate safety choice, not a legacy design — they exist to provide uniform, predictable light output
• A constant-current circuit keeps every fitting under the same operating conditions, helping avoid false depth perception and misjudgment
• Because the CCR regulates current, cable voltage drop does not change fitting current over long circuits (within the CCR’s rated voltage)
• Circuit behaviour is predictable and measurable, which supports fault detection and preventive maintenance
• Isolating transformers contain lamp-side failures so the rest of the circuit keeps operating; splitting patterns across several circuits limits the impact of a primary-cable fault
• In aviation, consistency and reliability outweigh the convenience of conventional parallel wiring