DaveNewcastle
Established Member
(I think we had this debate about a year or so ago - all the info probs still searchable on here if you're dedicated!)
The challenges are: Restricted Height clearance for OHLE, high conductivity of saltwater, arcing from OHLE to structure (produces power losses, current dips, metal fatigue and potential for circuits tripping out).
The structure of the bridge is a factor - each of the 3 "diamond" sections has 4 lateral cross pieces which are about 45degrees to the track. These are the obstacles to the adequate clearances above the envelope of a train (you can look at a youtube pic of a cab ride through the bridge to get an impression).
I'm not professionally involved, but I'd suggest that the solution could involve:
a) reducing the overall voltage by as much as the traffic will accept (linespeed is low, gradient is nil, so Wattage required should be achievable with greatly reduced voltage). That will reduce arcing.
b) placing a neutral section under each of these low diagonal cross pieces (so eliminating arcing to the bridge) the risk of standing a train on the bridge in a neutral section could be reduced to negligible - procedures to ensure continued movement under most fault and hazard situations will suffice. It wouldn't normally be helpful to stop traffic on the bridge even in acute fault conditions!)
c) Novel construction techniques in mounting the OLHE. These could include non-standard heights, new insulators and higher tension, maintained by a new tensioning system (and probably all three of these). Given the finite length of the bridge, and the well-established infrastucture for maintaining it, its not unreasonable to adopt exceptional mounting techniques.
d) Planning and driving policies which ensure that the total power demand over the span of the bridge is kept low (similar to existing policies which reduce combined the weight of passing trains on the bridge).
e) Automated Isolation of sections of OHLE when no traffic is present (to prevent the very considerable losses of power through insulators, arcing and other leakage paths). This is completely at odds with accepted railway engineering practice, and I understand why. But in the 21st century, it might be time to look for change in exceptional locations.
In summary - yes. I think it might be more work in getting the new standards approved and adopted than it would be in actually installing them.
Traditional gantries and cateneries will be fine for the long approach piers at each end. But the new OHLE suspension with greater insulation and multiple "neutral sections" and switching the OHLE power in response to a train's presence will require some new engineering input.
BTW Some of these issues also apply to the electrification of the Severn Tunnel, that's another discussion which crops up on here from time to time.
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