Fantastic, thanks! Once again, a lot to digest there and very gratefully received. So there was some validity in compound catenary for higher speeds then? I remember (being from the Glasgow area) that the original MK1 from GLC to Motherwell was wired from the outset compound catenary in places, but never saw anything other than Class 303s (max 75mph); at least not until late 1973/74. The auxiliary catenary wire has since all been removed. Does compound catenary require more weights/force to keep it in tension and is it more susceptible to high winds? And if I can ask another question relating to the nuts'n'bolts evolution of the equipment... The flat steady arms of MK1 never made it to MKII/MKIII etc. Was there a problem with these distorting?
Compound catenary is there for two reasons (a) to provide an auxiliary current path but primarily to (b) control the uplift of the contact wire and hence current collection quality by minimising variations in OLE 'compliance' (sometimes called 'elasticity') e.g. mm/N of uplift, so that uplift is maintained as constant as it possibly can be in-span between points of registration, i.e. cantilevers etc. The alternative, which is cheaper, is to use a stitch wire at each registration point but this requires perhaps more adjustment and maintenance, cheaper still is pre-sag.
Mk1 is fine if you accept the limitations of its age, its cost (lots of Copper and Phosphor Bronze as well as fixings) and some of the reliability issues it has such as over boom pulley wheels. You could feasibly squeeze a reasonable amount of speed out of it in its super-tensioned guise - say 125mph for a single pantograph. The stumbling block might be the loading and unloading of the droppers which can be a fatigue issue with the 4mm stainless steel droppers - perhaps no more so than Mk3B to be fair. BR modified it in the 1980's going from 100mph to 110mph running on the WCML primarily by removing the auxiliary catenary wire, ditching the loop droppers and converting it to simple AT equipment, as well as super-tensioning it up to 11.3kN/8.195kN. In compound configuration it requires at at least two sets of tensioning devices (these are traditional balance weights): One for the contact wire and auxiliary catenary wire - using a compensating plate to obtain the correct tensions, and one for the primary catenary wire. As a flavour of the nominal tensions required:
Mk1 Compound AT (Late 1950s/Early 1960s) - Contact Wire: CdCu 107mm2 at 8.9kN, Auxiliary Catenary: CdCu 7/2.1mm at 3kN, Catenary: 19/2.1mm CdCu at 8.6kN
Mk1 Super-tensioned pre-sagged Simple AT (Mid 1980s) - CdCu 107mm2 at 11.3kN, Catenary: 19/2.1mm CdCu at 8.195kN
Mk4 Compound AT (Mid 1970s) - Contact Wire: CdCu107mm2 at 21.0kN, Auxiliary Catenary: AWAC 7/3.95mm at 11.0kN, Main Catenary: 19/3.39 AWAC at 24.0kN.
I've mentioned Mk4 (or BRB HSTT range as it is named in OLEMI) as this was BR's planned OLE range for future high speed lines (200km/h+) using a mix of Mk1 and Mk3A equipment, it was tested in a scaled configuration on the Old Dalby Test Track but never used in anger. Gary Keenor says that it was planned as a WCML upgrade for the APT-P which make some sense. BR had a habit of planning or optioneering 200km/h, 250km/h, 300km/h and even 400km/h versions of the APT (those numbers are correct BTW) so a supporting OLE system capable of these speeds makes some sense in terms of forward planning. Mk4 had potential for at least 155mph and the literature available suggests BR thought it could have been stretched up to 186mph. Certainly the tensions are in the ballpark for a high speed line comparable to the simple AT V300 OLE which SNCF uses, where the contact wire is at 20kN and the catenary is 14kN.
This shows Mk1 in its compound catenary configuration (note the loop droppers) and
this after conversion to simple configuration. As to whether it (compound OLE in general) is more susceptible to high winds - generally the higher the tension in a wire the less it is liable to blow-off and unwanted dynamic behaviours. There is also a design factor called the critical wave speed which this
paper explains. BR did quite a bit of work on simple (contact wire + catenary) OLE and it was reckoned that this would be just about acceptable up to 140mph for single pantograph operation, above that speed the greater uplift and need for a stitch wire meant that simple equipment would have been inadequate in terms of current collection quality.
The curved steady arms are there to provide clearance to the pantograph, as to why the straight Mk1 arms were not incorporated into Mk2 or Mk3 I don't have an answer for that I'm afraid. Mk3 has straight arms but these are cheaper crimped galvanised steel items so I would guess the cost of copper came into play. There are still a number of straight steady arms in legacy OLE ranges which can still be allocated, I'm not aware of whether there was an issue in terms of radial loads.
