Re: Near miss at Victoria (London)

["Michael McDonald" <michael.and.colleen.mcdonald@xtra.co.nz>]

"David Johnson" <trainman@ozemail.com.au> wrote in message
3A56C305.D48A5BCA@ozemail.com.au">news:3A56C305.D48A5BCA@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.

That may seem to be true, but it is probably a simplification.  Oneof the
critical factors will be "self-inductance".  Motors and electromagnets (and
transformer coils) have heaps, relatively speaking.  It is the collapsing
associated magnetic field that is reponsible for a voltage surge and the
consequent arcing.

I used to demonstrate this to school pupils.  Wire in series a 1.5volt
battery, a knife switch (so the pupils can see when the contact is being
made/broken), and a small electromagnet.  Take a lead from each side of the
switch, and get the pupil to hold one in each hand.  You close the switch.
"Tada!".  Pupil feels nothing (why should he/she - the voltage across the
leads is now zero!).  "Oh dear!" and casually open the knife switch.  At
least 98% of pupils then jumped and swore!

Now the point is, if you replaced the electromagnet with a length of
nichrome wire, so that the current was the same in both case, the pupil
shouldn't feel anything.

Cheers
Michael

> >  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.
>
>