Re: Near miss at Victoria (London)

[David Johnson <trainman@ozemail.com.au>]

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.