Wessex signal heads renewal - a bespoke solution

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A Network Rail project to replace filament lamps with LED signal heads on the Wessex route became complicated when Amey Consulting discovered a simple like-for-like replacement would not be possible.

Occasionally the most straightforward engineering project can throw up unexpected obstacles. This was the case when Amey Consulting identified a problem with flashing LED supplies, requiring a proactive approach to designing a robust solution acceptable to the client.

Network Rail wished to renew dozens of ageing signals at various sites along the Wessex route, improving performance by replacing the standard filament heads with Dorman LED signal heads. Amey was contracted to design, supply, install and test the new heads.

On paper, the Dorman LED signal heads were a straightforward swap for the filament lamps. Aware that working with dated infrastructure can be problematic, the Amey design team took care to avoid a ‘one size fits all’ approach, carrying out site surveys to ensure a bespoke solution for each individual signal. However, the inability of the new LEDs to reproduce the flashing effect provided by the existing filaments required a novel approach to the design and installation.

The signal heads require a flashing phase to warn train drivers of an approaching turnout at a junction. With filament lamps, this could be achieved by turning the current on and off at the required rate. However, since this would put a strain on the lamps, causing them to blow after a certain number of cycles, lamp life is prolonged by ‘pulsing’ between high and low current. The lamp brightens and dims, providing the flashing effect perceived by train drivers without interrupting the flow of current through the filament.

During the design phase of the project, the Amey Consulting designers identified that when the filaments were replaced with LEDs, the reduced current in the ‘off’ phase would not significantly affect the brightness of the LED. The new signals would therefore remain steady, rather than flashing, if the existing pulsing power supply was retained.

Having fully investigated the issue, Amey Consulting designers developed a solution for the wiring scheme to ensure the LED would flash as required – a fix supported by a Network Rail Technical Instruction. The proposed alteration was made during the initial design phase and incorporated in the final design submitted for commissioning.

The good collaborative relationship established between Amey Consulting and their Network Rail counterparts enabled Amey Consulting to clarify the rationale for the functionally equivalent design. The proposal was accepted for commissioning, resulting in a successful outcome for all parties.

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