Track feeder cables play a critical role in supplying electrical power to the third rail network, which is essential for the operation of electrified railway systems in the South East and North West (Merseyside) of England. These high-capacity cables ensure a consistent and reliable power supply to the third rail, which in turn powers electric trains. Any failure, degradation, or fault in these cables—whether due to ageing infrastructure, weather conditions, mechanical damage, or electrical faults—can result in significant service disruptions.
In heavily trafficked areas like London and the surrounding southern rail network, where a substantial portion of train services rely on third-rail electrification, a power failure can have far-reaching consequences. In the London area, such an event could impact up to 75% of all train services in the southern region and 25% of all rail traffic nationwide, affecting thousands of passengers, delaying commuter and freight movements, and leading to widespread congestion across the network. Given the criticality of these cables, regular maintenance, monitoring, and upgrades are essential to prevent failures and ensure the smooth operation of rail services.
Design for reliability – Health and Safety considerations
During 2019, Tratos was approached by a Network Rail (NR) HQ (Technical Authority) for advice or thoughts on problems they were experiencing with their Aluminium Conductor Track feeder cable supplied to Network Rail Specification NR/PS/ELP/21101 (Tratos, at that time, had not provided any cable to this specification).
NR uses the above-referenced track feeder cable to provide electrical power for the third rail network in the South West and Merseyside. Numerous cases have already been identified where severe corrosion and deterioration of the aluminium conductor have led to damage to the sheath and, eventually, an electrical fault, causing overheating and failure of the sheath.
Other than the commercial cost of downtime, train cancellation, and replacement costs, the consequences of this are the safety element of the number of people and total person-hours having to investigate and eventually replace or repair these cables while working trackside.
This is not an insignificant issue and is likely to continue regularly, in particular if the cables are replaced by the same type. Hence, in early 2021, an NR engineer gave us two samples of the affected cable for investigation and advice.
After an investigation by our technical team, including external analysis by the University of Salford, it was discovered that the aluminium conductor showed signs of degrading to a white powder in places, causing the tape covering and, ultimately, the sheath to become brittle, swell, crack, split, puncture, or even burst.
Analysis of the powder indicated the presence of aluminium hydroxide, which indicated that H2O had been infiltrating the conductor.
The water is unlikely to be pure rainwater, and as this cable is laid trackside, it will likely contain impurities (this is stated in the Network Rail Specification)
As Aluminium is more electronegative than steel, aluminium will become the sacrificial anode in a galvanic corrosion system, where the contaminated water will act as an electrolyte, and the presence of the 750V DC will increase the rate of corrosion.
In one sample the deterioration had reached the point where a major electrical failure had occurred resulting in a puncture of the covering with associated overheating and burning of the rubber sheath.
As one of the three samples was still fitted with a lug for connecting to the rail, the opportunity was taken to examine it for possible signs of water ingress into the cable. Although the heat shrink (?) tubing applied over the lug appeared to be adhering to the lug and cable, the level of adherence was not uniform, and once slit, the tubing could be pulled away from the lug and cable. It also appeared that the ends of the tubing had been taped.
Tratos Solution
Tratos developed a new version of track feeder cable, NRS/2/ELP/23002/02, using specially formulated materials to resist the effects of water ingress and sheath damage.
The conductors are supplied with a water-blocking material. Then two covering layers of crosslinked material are applied in extrusion in two closely adherent layers, forming a compact, homogeneous body that fits closely over the separator tape. This two-layer covering provides the optimum balance between electrical and mechanical properties.
The cable was rigorously tested, particularly for long-term wet ageing. We also tested for use in tunnels and met the fire safety classification requirement of B2ca-s1a-do-a1 as defined in BS EN 13501-6 – CPR.
Tratos believes that this product has some significant and demanding issues to overcome, in particular, the successful application of water-blocking to the conductor sectors. It potentially has massive safety implications for NR due to its expected longevity compared to the existing specification.
We at Tratos cannot tell you exactly how many hours of trackside work will be saved by utilising this product over its predecessor, but we do know that thousands of metres are involved. One of the key safety aims of the rail industry is to reduce manpower on the track, and there is the added benefit of reduced downtime, maintenance costs, and train delays.
