Functional Supply Points (FSPs) might not be the most visible part of a rail signalling system, but they’re one of the most critical. When signalling power is reliable, everything else tends to run smoothly. When it isn’t, the impact can be immediate and costly.
At a glance, FSPs can look broadly similar. In practice, the differences between FSP01, FSP02, FSP03 and FSP04 are significant — particularly when it comes to resilience, maintenance strategy, and how a system behaves during a power failure. Understanding those differences early can make a real difference to project outcomes.
The key takeaway
1. The Strathspey Railway: A Highland Gem
Each FSP architecture is designed for a specific type of operational environment. Choosing the right one isn’t about “more” or “less” capability — it’s about matching the design to the risk profile, asset age, and operational expectations of the route.
Below is a practical look at how each type works, and where it tends to fit best.
FSP01: Simple, single-end-fed architecture
FSP01 is the most straightforward of the four architectures. Power is supplied from a single source and distributed along the line using a daisy-chain arrangement.
This simplicity brings some clear advantages. With fewer components and a single feed, the system is easy to understand, install, and maintain. For routes with relatively simple signalling requirements and lower operational risk, this approach can be entirely appropriate.
However, the trade-off is resilience. If the supply is lost at the source, everything downstream is affected until power is restored. That doesn’t make FSP01 “wrong” — it just makes it important to use it in the right context, where the consequences of a loss of supply are understood and accepted
FSP02: Dual-end-fed with manual resilience
FSP02 builds on the same daisy-chain concept but introduces a second power feed, supplied from the opposite end of the route section.
The key difference here is redundancy. If one supply fails, the system can be manually reconfigured to restore power from the remaining feed. This adds a layer of resilience without fundamentally changing how the system is laid out.
For many projects, FSP02 represents a balance between simplicity and robustness. It allows teams to manage power failures without a complete loss of supply, while still keeping the architecture relatively familiar and cost-conscious. The reliance on manual intervention, though, means recovery times depend on access, response, and operational procedures.
FSP03: Automatic reconfiguration for higher availability
FSP03 takes the dual-end-fed concept of FSP02 and removes the need for manual intervention. It does this by incorporating an Auto-Reconfiguration System (ARS).
In the event of a supply failure, the system detects the issue and automatically switches to the healthy feed. This happens without the need for staff to attend site or carry out switching operations, which can significantly reduce downtime.
This architecture is particularly relevant on routes where availability is critical and disruption carries a higher operational or financial impact. While the system is more complex, the benefit is a faster, more predictable response to power failures — something that can be difficult to achieve through procedural controls alone.
FSP04: A practical answer to legacy installations
FSP04 is slightly different in purpose from the others. Rather than being a general architecture choice for new installations, it is designed specifically to address legacy SIN119 systems.
Many older installations were built to standards that no longer align with current requirements. Replacing them entirely can be disruptive, expensive, and sometimes unnecessary. FSP04 provides a way to modernise non-compliant switchgear while retaining much of the existing power system.
This makes it a pragmatic option for routes where asset condition, access constraints, or budget considerations make a full redesign impractical. It allows infrastructure managers to improve compliance and reliability without the knock-on effects of a complete system overhaul.
Why these differences matter
It’s easy to think of FSPs as interchangeable building blocks, but their architecture has a direct impact on how a signalling system behaves under fault conditions. Decisions made at design stage can influence response times, maintenance strategies, and operational risk for decades.
In some cases, a simpler solution is entirely appropriate. In others, investing in automatic resilience can prevent repeated disruption over the life of the asset. The important point is understanding the behaviour of each option — not just its headline features.
When FSP architecture is aligned with the realities of the route, the result is a system that does exactly what it needs to do, without unnecessary complexity or avoidable vulnerability.
For a more detailed overview of Functional Supply Points, including typical configurations and standards considerations, visit our FSP resource page:
https://www.rsprail.co.uk/products/rail-power/functional-supply-point-fsps/