Re: More 'PIDS'

["Alex Borodin" <joneses@primus.com.au>]

Matthew Geier <matthew@mail.usyd.edu.au> wrote in message
7j532t$53m$1@metro.ucc.usyd.edu.au">news:7j532t$53m$1@metro.ucc.usyd.edu.au...
> In article <3755EB04.3924CEF3@ozemail.com.au>,
> David Johnson  <trainman@ozemail.com.au> wrote:
> >Alex Borodin wrote:
> >
> >> Indeed, I would suggest that the systems in Melbourne and Sydney are
prone
> >> to EMF not because of the CRT's receiving too much EMF (plasma screens
would
> >> have a similar problem in that a great enough EMF would cause the
phosphor
> >> dots to light up and flicker)  but because of the video transmission
> >> method.. No, the EMF (even in Sydney or Melbourne) is not great enough
to
> >> affect a CRT at that distance.
> >
> >At Como, every time a train goes through, the Television loses reception,
and
> >the picture turns 5 degrees and changes colour.  It looks like the
deflection
> >magnets are being turned on the back of the picture tube.

You can get similar effects by playing with the HSync and VSync signals, and
Chroma (or RGB if you are using them)... if you distort them, you can get
quite similar effects indeed. But you could be right - it might be the
magnetic effect on the CRT itself if it isn't suitably shielded.

>  I suspect maybe the EMF is large enough. Sydney and Melbourne are 'low'
> volage DC systems that draw large currents as a matter of course. QR is
> high voltage AC. QR fault currents could be less than Sydney running
currents.
> (I dont know the numbers....)

We use 11kV AC. And you could be right... I shall ask around some of the big
brains around the place to see what the answer is...

>  The fact that Sydney/Melbourne is DC and not AC probably make the
problems
> worse, as the repeated fields with same direction of flux probably
magnetise
> things that should not be magnetised....

You could be right, (there are a lot of variables) - it's worthy of
investigation.

>  Of course stupid design can make all this worse.... :-)

Definitely. The monitors used make a big difference. Most standard PIDS type
monitors (say Barco or Teac) have built in Mag shielding and degaussing.
They're very robust items.

Alex Borodin
Software and Systems Engineer
Queensland Rail