The problem
The soffit height of the footbridge is 4.448m above the rails at its lowest point. In order to achieve a basic clearance of 370mm, the contact wire would need to be 4.078m which is below the minimum permissible design contact wire height of 4.240m. The absolute minimum height of the wire is 4.165m as specified in Group and Network Rail standards.
As the bridge is a Grade II listed structure, civils intervention at the structure is costly and would involve a large amount of negotiating and seeking permission from heritage authorities and local councils. Lowering the track is problematic as here is already a reduced ballast depth at a nearby structure which would be impacted by lowering the track through Dewsbury station.
Useful experimental studies
During design development at the AIP stage, it was raised that experimental surge arrestors had been installed at Cardiff Intersection Bridge. The purpose of a surge arrestor is to limit any over voltage caused by events such as lightning strikes or fault currents for example.
Studies carried out by the University of Southampton proved that using surge arrestors along with secondary insulation and GLS insulating paint alongside standard Network Rail bridge arms resulted in clearances being reduced to 0mm without flashover with the arm physically touching the insulated coated surface without any collapse of voltage or detriment to performance.
Developing a solution
In light of this new information, Amey Consulting’s design team on TRU set about establishing the maximum contact wire height that would also provide the maximum clearance to the soffit of the bridge to provide optimal performance.
A new process using VCC (Voltage Controlled Clearance) One Pager forms was introduced which set the parameters for all interfacing disciplines. This document was instrumental in developing the proposals as the tolerances and requirements of each discipline could be recorded in one place. Although onerous and at times frustrating, the VCC one pagers were instrumental in developing the solution.
Following a period of establishing the ideal OLE position of the contact wire, a series of integration discussions were held to determine the optimal track position. As wholesale track lowering is not straightforward at this location, the only permanent way variable that needed to be taken into account was the track maintenance lift allowance (TMLA) which is nominally around 100mm. This was reduced to 50mm after some negotiation with the track CRE, PE and RAM, OLE tolerances were also reduced to the bare minimum resulting in a clearance of 52mm from the wires to the soffit which takes into account the uplift of the wires as a train passes underneath.
Agreeing the proposal
The final hurdle in finalising the proposal was gaining Network Rail acceptance for what is still an emerging technology that is only being used in one location on the UK rail network under experimental conditions.
As the Network Rail Project Engineer for OLE was heavily involved in the development of the VCC One Pager process, gaining their acceptance was relatively straightforward. A series of workshops were held with RAM representatives to explain the process, explain the tolerances used and how they had been determined in order to gain their acceptance.
Outcomes and benefits
The design proposal demonstrated that Overhead Line Equipment could be installed beneath Dewsbury Station Footbridge without costly civils intervention work to lift the soffit of the bridge. As the bridge is a Grade II listed structure, consents would also have been difficult to obtain.
The approximate cost of installing surge arrestors, secondary insulation and GLS insulating paint is in the region of £50k. The estimated cost of the bridge works to achieve a compliant electrical clearance is in excess of £2m. In addition to this, the Carbon saving is significant in that there is no requirement for cranes, MEWPs and various other items of machinery required to carry out the modifications to the footbridge. The Surge arrestors are to be installed as components of the OLE system and will not increase cost or carbon emissions significantly over what is already planned.