Re: Near miss at Victoria (London)

[jjjim <jjjim@bigpond.com>]

Lol i didnt realise it was you i was arguing with   trainman  ;-)

Hmm i guess you have a point with the DC , i kinda forgot DC theory when it comes to
arcing,  (kinda forgot we were talking DC in the first place lol)     Still 25Kv,  AC or
not, the inductive load would maintain the spark between cycles to some extent.  so a
knife switch opeing at full load (say 2000Kw ) would be a  bit hairy :p

JJJim

David Johnson wrote:

> jjjim wrote:
>
> > I would argue that :P
>
> Let the argument begin!
>
> > 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.
>
> A 25kVAC spark would jump 2.5 cms without any prior air ionisation.  A 1.5kVDC spark
> would be lucky to jump half a centimetre under the same conditions.
>
> A 25kVAC supply turns off 100 times a second, extinguishing its own arc.  A 1.5kVDC
> supply never turns off.  This is why some light switches will say 250VAC 10 Amps or
> 125VDC 3 Amps.  It is harder to stop DC flowing than AC.
>
> A 25kVAC load of 250 kilowatts would be pulling 10 Amps.  A 1.5kVDC load of 250
> kilowatts would be pulling 167 Amps, 16 times the current (1/16th the voltage).
> Remember, volts don't flow - Current does.  It is the Current that is hard to stop.
>
> >  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.