In message , Clive
wrote:
They cope only with difficulty. In DC electrified areas AC track circuits
tend to be used to avoid interference and in AC electrified areas they
tend to use DC track circuits. As you can see there is no third way to use
in dual-electrified areas. I don't know if similar considerations apply to
axle counters.
I understand what you say, but surely there is a frequency that can be
used that isn't a harmonic of either the 25kV or the ripple on the dc?

Yes: it's usual to use 33 1/3, 83 1/3, and 133 1/3 Hz.

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In message , d
wrote:
Given all the harmonics produced by modern electronic power systems on trains
surely most track circuits would be pretty well filtered anyway?

It's the other way round: electronic power systems are required to
filter their output so that they don't generate frequencies likely to
interfere with track circuits.

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In message , Clive
wrote:
For example, third rail running rails need to be isolated from earth
to keep stray voltages away from underground pipes, but 25kV AC
running rails need to be grounded. Dealing with the interface is, um,
interesting.
Are you saying that traction current is the major problem when dealing
with the two different types (ac and dc)?

No, I'm saying that it's an issue.

More precisely, what I'm saying is that you can't just stick down some
third rail and say "that's it". You need to modify the existing AC
electrification to be compatible with dual-voltage. And, since the ECML
is currently not signalled with DC-immune equipment, you'd need to
replace it.

This is specific to the original suggestion: putting third rail from
Drayton Park to Finsbury Park. If we were talking about building a new
dual-voltage line from scratch, these issues would have been addressed
in the design.

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In message , d
wrote:
Some years ago there was a very good post on uk.railway explaining some
of this. For example, third rail running rails need to be isolated from
earth to keep stray voltages away from underground pipes, but 25kV AC

I didn't know that about 3rd rail, I assumed the rails were grounded. Doesn't
that mean that theres a potential risk of electrocution just from the running
rails?

Yes, if they're not correctly controlled by the distribution system.

I used to commute on the same train as an electrification safety expert.
He once explained to me that, with the original wiring plans for HS1, a
broken cable on the Northern Line could resulting in someone standing on
the platforms at Ashford International getting electrocuted.

http://groups.google.com/group/uk.ra...05866eb1d7b1b0
http://groups.google.com/group/uk.ra...53349c291a59a1

are good postings on the topic, but I can't find the one I was thinking
of originally.

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On Wed, 16 Feb 2011 19:41:23 +0000
"Clive D. W. Feather" wrote:
In message , d
wrote:
Some years ago there was a very good post on uk.railway explaining some
of this. For example, third rail running rails need to be isolated from
earth to keep stray voltages away from underground pipes, but 25kV AC

I didn't know that about 3rd rail, I assumed the rails were grounded. Doesn't
that mean that theres a potential risk of electrocution just from the running
rails?

Yes, if they're not correctly controlled by the distribution system.

I used to commute on the same train as an electrification safety expert.
He once explained to me that, with the original wiring plans for HS1, a
broken cable on the Northern Line could resulting in someone standing on
the platforms at Ashford International getting electrocuted.

Don't see how that could happen. Besides ,even if you had a continous 3rd
rail all the way from say mordon to ashford the voltage drop over that
distance would be significant, probably to the point where at ashford it
wouldn't be dangerous.

B2003

In message , d
wrote:
I used to commute on the same train as an electrification safety expert.
He once explained to me that, with the original wiring plans for HS1, a
broken cable on the Northern Line could resulting in someone standing on
the platforms at Ashford International getting electrocuted.

Don't see how that could happen.

Nor do I, but I'm not the expert.

Besides ,even if you had a continous 3rd
rail all the way from say mordon to ashford the voltage drop over that
distance would be significant, probably to the point where at ashford it
wouldn't be dangerous.

I doubt it's that simple. More likely you'd get some kind of induced
voltage in the overhead that set up a standing wave or a soliton or some
other thing that doesn't decay. But I'm just guessing here.

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On Thu, 17 Feb 2011 19:41:51 +0000
"Clive D. W. Feather" wrote:
I doubt it's that simple. More likely you'd get some kind of induced
voltage in the overhead that set up a standing wave or a soliton or some
other thing that doesn't decay. But I'm just guessing here.

*boggle*

B2003

On Wed, 16 Feb 2011 19:16:24 +0000
"Clive D. W. Feather" wrote:
In message , d
wrote:
Given all the harmonics produced by modern electronic power systems on trains
surely most track circuits would be pretty well filtered anyway?

It's the other way round: electronic power systems are required to
filter their output so that they don't generate frequencies likely to
interfere with track circuits.

I'm sure I remember something being mentioned about the northern line signalling
having to be immunised when the 95 stock was introduced. *shrug*

B2003

On Wed, 16 Feb 2011 19:18:44 +0000
"Clive D. W. Feather" wrote:
More precisely, what I'm saying is that you can't just stick down some
third rail and say "that's it". You need to modify the existing AC
electrification to be compatible with dual-voltage. And, since the ECML
is currently not signalled with DC-immune equipment, you'd need to
replace it.

I wonder if its possible to have a signalling system that doesn't use the
rails at all so avoiding all of these issues. Perhaps using magnetic
detectors or infra red beams or suchlike?

B2003

d wrote in article
...
On Wed, 16 Feb 2011 19:18:44 +0000
"Clive D. W. Feather" wrote:
More precisely, what I'm saying is that you can't just stick down
some
third rail and say "that's it". You need to modify the existing AC
electrification to be compatible with dual-voltage. And, since the
ECML
is currently not signalled with DC-immune equipment, you'd need to
replace it.

I wonder if its possible to have a signalling system that doesn't use
the
rails at all so avoiding all of these issues. Perhaps using magnetic
detectors or infra red beams or suchlike?

Possible, yes but interference between traction and signalling circuits
can still occur and electricity and magnetism cross-interfere while
infra-red beams would fail with the "wrong sort of rain (or snow)".
Fluidics, hydraulics, fibre-optics are all possible also.

--
Mike D