Re: Near miss at Victoria (London)

["Chris Downs" <cvdowns@ozemail.com.au>]

In Sydney (and elsewhere) I thought the "air gap" was the space between the
two sets of overhead at an overhead anchorage/transition point on main lines,
that is, the span(s) where the finishing and the commencing overhead wires run
parallel and the pantograph will have simultaneous contact at some point.
Both wires are mostly electrically connected by additional conductors but
where the overhead is to be electrically sectioned (sub-stations, section
huts, certain outer-suburban crossovers) the two sets of wire are electrically
isolated (unless a pantograph is passing beneath) providing a gap of air
between the two sets of overhead.

The other type of section was initially provided by "continuous-feed
wood-break section insulators" (the diamond shaped overhead arrangements).
The wood was replaced by another type of insulator at some stage.  What I
think of as "ski jump" section insulators are now used.  I thought that prior
to ski jumps sectioning insulators being used the older type could be powered
"over" in switching or series only (notches 1 and 2), not in parallel or weak
field (notches 3 & 4).  Thus the ponderous starts from the up Illawarra Local
at Sydenham in single deck sets where two of these older type of sections were
encountered within about 400m of the train leaving Sydenham and drivers would
hold notch 2 for 500+m.  Notches  3 or 4, if used, would produce a very
noticeable buck as power was momentarily lost (the sections weren't so
continuous).  Regen. on the chopper cars encountered exactly the same problem
when braking across such sections.

I surmised (rightly or wrongly?) that the notch 2 restriction was lifted with
the introduction of the ski jumps sections.  (Although bucking is still
noticeable sometimes, the up approach to Glenbrook is a notable example where
the buck regularly knocks out regen. on V-sets.)

Sydney has no equivalent to Bizzie's 25kV arrangement (such as between Roma St
station and the Merivale Bridge or opposite Mayne Car Sheds) where the
pantograph traverses a lengthy dead section of contact wire rather than what I
think of as "air gaps".  Brizzie also has "ski jumps" but only for crossovers
and sidings.  Other mainline sections (not air gaps or dead sections) have a
more intricate version of Sydney's old diamonds that does provide truly
continuous power.

A question to those in the know:  In many thousands of km travel on Brizzie's
sparks I have never seen serious arcing between the pantograph and overhead as
I've seen in Sydney on occasions.  But my old man told me such arcing was
common on some trips he made under 25kV in the UK (loco hauled trains in all
cases I think).  Why is this so, overhead quality/age, higher current for
electric locos or?

Ta

Chris

jjjim <jjjim@bigpond.com> wrote in message
3A56AFF9.E35DE68B@bigpond.com">news:3A56AFF9.E35DE68B@bigpond.com...
> I would argue that :P
>
> The 25KV arc would stretch quite a distance , Elec units here have vacum
Breakers
> Which are set to break power to the train before hitting an airgap,  if not
the
> arc would be drawn right thru the air gap because of the High voltage.
where as i
> am led to belive sydney's DC trains dont have to have / need such a system,
could
> be wrong but
>
> David Johnson wrote:
>
> >
> >
> > I would suggest a breaker or knife switch opening 25kVAC at 50 Amps would
have
> > a much smaller arc than opening 1500VDC at 1000 Amps.
> >
> > --
> > David Johnson
> > trainman@ozemail.com.au
> > http://www.ozemail.com.au/~trainman/
> > ------------------------------------
> > These comments are made in a private
> > capacity and do not represent the
> > official view of State Rail.
> > C.O.W.S. Page 11.
>