Chris Walker, Electrical & Plant Engineer at Mott MacDonald, explores the issues facing major electrification projects…
The world of electrification has gained some limelight in recent years with major electrification projects being commissioned. The miles and miles of steel and copper zigzagging through the country are certainly the main attraction, paving the way for new rolling stock and a better-quality railway for the public. However, a less obvious aspect is the effect on existing lineside infrastructure, including stations.
The goal of introducing electrification on routes is an improvement to the public, but it also introduces a risk of harm when things go wrong. Behind the scenes of electrification is the safety aspect, specifically earthing and bonding. Very few will even notice its implementation, as the aim is to provide compliant earthing and bonding systems without disruption to the public and avoid disturbing station aesthetics. No two stations are the same, especially when you consider the surrounding environment, which presents a challenging design ethos.
These challenges encourage the delivery of bespoke design packages for stations, that create not only compliant earthing and bonding designs, but designs that have safety at the forefront for both the public and authorised personnel. With the expansion of electrification throughout the UK, the role of station earthing and bonding within design deliverables has evolved. With passenger numbers on the rise and station owners cramming more technology onto the platforms to aid customer experience, the implication of de-wirements is now more prevalent than ever.
Gone are the days of dark station platforms barely lit, platforms are now awash with neat lines of lighting and CCTV columns with dazzling displays of customer information service screens. These visually impressive assets now must be considered as part of the earthing and bonding design process. The need for bonding relies heavily on where assets are in relation to the overhead contact line zone, the zone in which a broken overhead contact line would not exceed. Before any design works can even be initiated, a detailed assessment of the station is needed. Identifying elements of the station which lie within the zone allows designers to establish a baseline of bonding required.
The basic principle is to provide a common bonding approach, so that under fault conditions, faults would be cleared in the allowable time (i.e. 200 milliseconds). Along with this, any dangerous rise in touch potentials which could lead to electric shock are required to remain under certain limits set by the various standards. To achieve this, assets identified as requiring bonding are all connected to the station main earth terminal via a network of cables subtly routed in cable containment to achieve the overall goal.
Stations cannot be considered wholly autonomous, as some interface heavily with outside infrastructure. The overarching principle of bonding is to provide a safer environment; however, bonding can introduce risks which previously were never present. By bonding elements of the station, undesirable touch potentials can be transferred outside of the station boundary during fault conditions. This risk is particularly prevalent in areas such as station carparks where fencing and assets are shared.
Earthing and bonding is not a simple ‘bond everything’ approach, but more a delicate balancing act of assessing risk and safety. The knock-on effect of bonding can be seen well beyond the platforms if not considered, so where do you draw the line? This can be the most difficult aspect faced by engineers during design works. If the risk cannot be eliminated, physical separation is sometimes the only option. This is widely achieved with the installation of non-conductive fence sections or structures made of glass reinforced plastic or similar approved products. In some cases, even coating structures with a specialist non-conductive paint layer.
Not every station requiring remedial works has the luxury of all the ‘mod-cons’, with dated and sometimes sub-standard facilities. These stations cause a variety of issues around poor and dated infrastructure and in some cases listed structures. Workarounds must be considered for such situations, with solutions tailored for safety whilst also protecting the uniqueness of such structures. Aesthetics also pose a unique design problem; station operators are often dubious about having their surfaces ripped up for the sake of one or two cables, simply to be left with a less than appealing tarmac scar strew across all platforms. And who can blame them?
The variety and challenges brought forth by stations provides an interesting spin on the traditional earthing and bonding systems. As more infrastructure is crammed into a smaller space and the outside world slowly encroaches ever closer to electrified Network Rail infrastructure, the role of earthing and bonding will continue to play a vital and more dominant role in design deliverables.