Circuit Selector Switch (L-847)

A Circuit Selector Switch (L‑847) is a specialised switching device used in AGL power distribution systems to direct the output of a Constant Current Regulator (CCR) to one of several series lighting circuits (e.g., runway edge, centreline, taxiway, approach). It provides the flexibility to share a single CCR among multiple lighting loads, reducing the number of regulators needed and enabling backup or redundancy schemes.

The L‑847 designation comes from FAA Advisory Circular 150/5345‑5 (currently Revision B), which specifies the performance and construction requirements for circuit selector switches used in airfield lighting — a separate AC from the one covering the CCR itself (AC 150/5345‑10, the L‑828 Constant Current Regulator spec). The switch is typically installed in the equipment shelter between the CCR output and the individual circuit feeders. It can be operated locally via a manual handle or remotely via a motor‑driven actuator controlled from the ALCMS or a control panel. The switch ensures that only one circuit is energised at a time, preventing conflicts and overloading, and it provides clear visual indication of which circuit is selected.

The L-847 switch exists to let a single CCR serve more than one lighting circuit, avoiding the cost and equipment-shelter space of dedicating a separate regulator to every runway, taxiway, or approach circuit at airfields where usage patterns don't require them to run simultaneously. Its purpose extends to redundancy as well as economy: if a CCR normally assigned to one circuit fails, the selector switch can redirect a working spare regulator to take over that circuit's load, providing a backup path without requiring a fully duplicated CCR for every single circuit in the system. Ensuring only one circuit is energised at any given time is itself a core part of the switch's purpose, since accidentally connecting a CCR's output to two circuits simultaneously would create an overload and current-sharing conflict the regulator isn't designed to handle.

Circuit Selector Switches are installed in airfield electrical rooms, equipment shelters, or AGL power centres. They are used in various applications to optimise CCR utilisation and enhance system flexibility:

• Sharing a CCR Among Multiple Circuits: A single CCR can be used to power different lighting circuits at different times (e.g., runway edge lights during take‑off and taxiway lights during ground movement).
• Redundancy and Backup: In a redundant configuration, a standby CCR can be switched to any circuit if the primary CCR fails, ensuring continuity of critical lighting.
• Maintenance Isolation: The switch allows an individual circuit to be isolated for maintenance without affecting other circuits powered by the same CCR.
• Load Shedding: Non‑critical circuits can be deselected during generator operation to prioritise runway and approach lighting.
• Integration with ALCMS: Motorised selector switches can be remotely controlled from the ALCMS, enabling automated circuit selection based on operational needs or fault conditions.

The switch is installed in the equipment shelter or electrical room between the CCR output terminals and the individual circuit feeders it serves, mounted on a panel or rack within reach of the CCRs it's associated with so the interconnecting cabling stays short and manageable. Each circuit feeder wired to the switch is clearly labelled at the point of termination, since a mislabelled position is a direct route to energising the wrong circuit, a mistake the switch's own interlocking is meant to prevent between circuits but can't catch if the wiring itself was connected incorrectly at installation. Where a motor-driven actuator is fitted, its control cabling is run back to the ALCMS or a local control panel alongside other AGL control wiring, kept identifiable so a remote circuit-selection command can be traced to the correct switch in an equipment room serving several CCRs and circuits. The switch's position indicator, whether a mechanical handle position or an electronic status readout, is mounted so it remains clearly visible to anyone working in the equipment room, giving an immediate visual check of which circuit is currently selected before any related maintenance work begins.

The Circuit Selector Switch consists of a multi‑position rotary switch or a set of interlocked contactors that route the CCR output to one of several load circuits. The switch has a common input (from the CCR) and multiple output terminals (each connected to a separate series circuit). A mechanical or electrical interlock prevents more than one output from being connected at a time.

Key functional aspects include:

• Manual Operation: A front‑panel rotary handle with clear position indicators allows the operator to select the desired circuit. The handle is typically spring‑loaded to ensure positive engagement.
• Remote Operation: Motor‑driven versions incorporate an electric actuator (often with position feedback) that can be controlled via dry contacts, Modbus, or other protocols from the ALCMS or a remote control panel.
• Position Indication: The switch provides local visual indication (pointer on the handle) and remote position status via auxiliary contacts or a communication interface.
• Interlocking: Electrical and mechanical interlocks prevent simultaneous connection of multiple circuits, protecting the CCR from overload and avoiding damage to the switch.
• Load Break Capability: The switch is designed to make and break the series circuit under load (up to the rated current, typically 6.6A or higher) without causing arcing or damage, complying with relevant switching standards.

The selector switch sits between a single CCR and multiple candidate loops, so it is itself a control point rather than a passive component — ALCMS or an operator commands a position change, and the switch's own interlock (mechanical or electrical) is what prevents that command from ever connecting two circuits to one CCR simultaneously, protecting the regulator from paralleling loops of different impedance. Because only one circuit is powered at a time, any fault the CCR reports (voltage rise, no output) is unambiguously attributable to whichever circuit is currently selected — the switch's own position feedback is what lets ALCMS know which physical circuit a CCR-level alarm actually belongs to.

The switch is built around a set of heavy-duty contactors or a motor-driven multi-position switch mechanism, sized to carry the full series-circuit current (commonly up to 20A) without significant contact resistance, since even a small added resistance in a constant-current loop shows up as an unwanted voltage rise the CCR has to compensate for. Contacts and busbars are housed in a metal cabinet with clear internal segregation between the different circuit paths being switched, preventing cross-connection between loops that must never be energised together. Where switching is motorised, a geared actuator drives the mechanism between positions under control-cable command, with mechanical position-indication contacts wired back to the ALCMS so the selected circuit routing can be confirmed remotely rather than only by physical inspection. The cabinet includes cable gland plates for the CCR input and each of the output circuit feeds, with a separate low-voltage compartment for the control and position-feedback wiring.