The overhead wires
in areas of multiple tracks were supported by a method called of
construction is called ‘head span’ wiring, this entailed using
cross support wires stretched between the masts. The catenery wires
being supported on the cross wires. The use of ‘head span’ type of
construction surrendered the use of an independent mechanical support
for each contact wire and this further reduced its impact on the
environment and also reduced cost.
The head span method was one of the key items which enabled the
cost of the electrification to be reduced by some 25% over the cost of
the electrification south of Weaver Junction, which used steel portal
construction. The head spans were usually prefabricated off site.
Added to this the two wire catenery used north of Weaver Junction
was designed to further reduce the cost of the electrification, and
without which it is unlikely the electrification would have got the go
ahead.
The 1966 line to Euston used mark 1 compound catenary with a
maximum permitted speed of 100mph. Experiments showed it ‘hogged’
making it higher mid span giving poor current collection. Whereas a
contact wire with some sag led to a design of simple catenary that
gave good current collection well in excess of 100mph.
The Mk1 catenery had cadmium-copper main and auxiliary wire and a
level cadmium-copper contact wire with copper wire droppers.
The sagged type was designated Mark 11 and consisted of hard drawn
copper catenary wire and a hard drawn copper contact wire with bronze
strand droppers.
By the time the west coast scheme was under consideration aluminium
conductor steel reinforced (a.c.s.r.) conductors were in use on
transmission lines and on other railways and tests were made on some 8
miles north of Crewe. Subject to modification this became known as
Mk111A and was adopted for the Weaver Junction - Glasgow scheme, it
showed a saving of £200 a mile over hard drawn copper and the use of
stainless steel droppers gave a further reduction of £110 per mile.
The Mk111A reduced the separation between the catenery wire and the
contact wire, simplifying the support design and with a reduction in
the height of support masts.
A number of the steel portal structures were used just north of
Fishergate Bridge; these span all the tracks, but were not the norm on
this project due to cost. This area contained some of the longest
spans of the whole project and the structures were needed due to the
track density and complexity. This type of structure was used
extensively on the Crewe, Manchester, Liverpool and London
electrification.
This
picture taken by Bill Ashcroft shows a northbound Class 104 DMU
leaving the former platform 4. The modifications to
Fishergate Bridge clearly visible, this work was to provide the
necessary clearance for the overhead traction wires. A new foot was
added to the existing girder beams and the original foot them removed.
In the background can be seen the brand new parcel platform and goods
lines.