In open air, the overhead contact wire is made from copper and is suspended from a catenary system supported by either lineside posts and cantilevers or gantries (portals). The wire is kept under tension to resist the upward force from the train’s pantograph.
In the tunnel, where space is limited, the contact wire is instead clipped into and supported by an extruded aluminium beam that runs the length of the tunnel. This, in turn, is suspended from the roof of the tunnel using insulators, so the high-voltage beam assembly, which is all energised at 25kV, is insulated from the tunnel by both the insulators and the air gap between it and the tunnel lining.
The aluminium structure of the entire support assembly oxides shortly after assembly by a natural process. That oxide layer is stable and protects the raw aluminium underneath. However, in the Severn tunnel, engineers found that something was attacking – ‘eating’ – the oxide layer. This exposed the bare aluminium, which then oxidised again. The process kept repeating, slowly consuming the aluminium structure.
One theory was that an anaerobic bacteria was responsible. This lives in wet environments and sometimes gives trouble on oil rigs. For this reason, the tunnel was deemed unsafe to electrify on a permanent basis.
Network Rail and overhead line manufacturer Furrer+Frey approached the problem in several different ways.