Isolation Switch

An Isolation Switch (also known as an isolating switch or disconnect switch) is a manually operated device that provides electrical isolation of a section of an AGL series circuit. Unlike circuit breakers, which are designed for automatic fault protection, isolation switches are intended for deliberate disconnection to allow safe maintenance, repair, or modification of the lighting system. They provide a visible air gap when open, ensuring that the circuit is de‑energized and cannot be re‑energized unintentionally. Isolation switches are typically installed at strategic points in the AGL loop, such as at the entry to a runway or taxiway section, allowing maintenance crews to isolate a specific area without affecting the rest of the airfield lighting.

The isolation switch's purpose is to give maintenance crews a way to positively and visibly de-energise a section of an AGL circuit before they work on it, which is a fundamentally different job from the automatic fault-clearing that circuit breakers perform. Its visible air gap exists specifically so that a technician can see with their own eyes that a circuit is open, rather than trusting an indicator light or a breaker's internal state — a distinction that matters because it is what makes lock-out/tag-out procedures effective and prevents someone else from unknowingly re-energising a circuit while work is in progress. By allowing a single runway or taxiway section to be isolated without shutting down the rest of the airfield's lighting, it also lets routine maintenance happen without unnecessarily disrupting other operations.

Isolation switches are used throughout the AGL infrastructure:

• In CCR Rooms: Between the Constant Current Regulator (CCR) output and the series circuit feeder cables, allowing the CCR to be isolated for maintenance.
• At Lighting Circuit Branch Points: Where the main series loop splits into sections for different runways, taxiways, or aprons.
• At Airfield Boundaries: To isolate entire lighting circuits for security or emergency purposes.
• At Maintenance Access Points: Located in manholes or pits where technicians need to isolate a section for lamp replacement, cable repair, or transformer testing.
• In Substations and Distribution Boards: As part of the primary distribution system for AGL power.

Isolation switches are mounted at the points in the network where a section needs to be taken out of service independently of the rest — CCR output panels, lighting circuit branch points, substation and distribution board incomers, and maintenance access pits or manholes along the series loop. Panel-mounted units are fixed so the operating handle is accessible from outside the enclosure without exposing live parts, while field-installed units in pits or manholes are housed in a sealed, labelled enclosure that keeps the visible-break mechanism protected from moisture and debris. Location is chosen so each switch clearly isolates a defined, labelled section of circuit, letting a technician work on that section, a fixture, or a transformer without de-energising the whole airfield.

An isolation switch operates on a simple but essential principle:

• Manual Operation: The switch is operated by a handle or rotary mechanism that moves the contacts to the open or closed position. The mechanism is designed to provide a positive action, ensuring the contacts are fully engaged or fully disengaged.
• Visible Break: When open, the switch provides a visible air gap between the contacts, giving a clear visual indication that the circuit is disconnected. This is a key safety feature, as it prevents accidental re‑energization and provides lock‑out/tag‑out capability.
• Load‑Make/Load‑Break Capability: Isolation switches used in AGL circuits are typically designed to handle the series circuit current (up to 6.6A or 20A) and can be operated under load, though for safety they are often opened under no‑load conditions.
• Interlocking: Some isolation switches are interlocked with circuit breakers or other switches to prevent operation under unsafe conditions, such as opening under heavy fault current.
• Mounting: They are housed in weatherproof enclosures (IP65 or higher) for outdoor installation, or in indoor panels for CCR rooms and substations.

An isolation switch carries no automatic control logic of its own — its position is set manually and it does not respond to current, voltage, or a remote command in normal operation. Architecturally it sits inline in the power path, typically between a breaker and the downstream equipment it protects, giving maintenance staff a visible, lockable break point independent of the breaker's internal contacts. Where isolators are interlocked with an upstream breaker or with each other (a common arrangement in CCR rooms), the interlock is a mechanical or key-exchange permissive rather than an electronic control signal: it physically prevents the isolator from being opened or closed in a sequence that could leave a circuit unsafely live or de-energised. Some switches used in larger installations add auxiliary contacts that report open/closed status to a local panel or the ALCMS, but this is a status indication rather than a control function — the switch state is always changed by hand.

The switch mechanism sits inside a moulded enclosure — weatherproof glass-reinforced polyester or polycarbonate for outdoor installations, sheet steel for indoor panel use — with an external operating handle mechanically linked to the internal contacts so the blade position is visible or padlockable from outside the enclosure. Silver-alloy contacts are used at the moving blade and fixed jaw to resist arcing and pitting over repeated operations, and the air-break design relies on physical separation of the contacts, rather than a sealed medium, to establish a visible isolation gap that maintenance staff can confirm by eye before working on the downstream circuit. Cable entries at the base or side of the enclosure accept glanded conduit or direct-buried cable, and the enclosure includes a hinged, gasketed door or cover secured by captive screws to maintain its IP rating in outdoor service. A mechanical interlock or padlocking facility on the handle allows the switch to be locked in the open position during maintenance, satisfying lock-out/tag-out requirements for the series circuit it isolates.