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e27002 aurora

On Mon, 30 Nov 2015 10:20:15 -0800 (PST), wrote:

On Monday, 30 November 2015 17:50:15 UTC, John Kenyon wrote:
On 28/11/2015 10:19, Clive Page wrote:
On 27/11/2015 15:41, Roland Perry wrote:
In message , at
06:25:25 on Fri, 27 Nov 2015, gordonT remarked:

"Overhead wire problems between City Thameslink and Kentish Town are
causing delays of up to 30 minutes to trains between London Blackfriars
and West Hampstead Thameslink. This is expected to continue until
16:00."

Don't worry, if they electrify the route it'll all be fine.

Oh wait...

Actually, if they installed 3rd rail electrification all the way north
to Kentish Town or, or maybe West Hampstead, that would solve a lot of
problems. I'm surprised nobody has thought of that before.

Dual electrification isn't just a case of sticking up more knitting or
fastening down some insulators and dropping another rail down.

What about the return path, and the signalling?

Exactly so. AC and DC electrified railways require different earthing and bonding arrangements. All electrified railways, unless they use a fourth rail system, return the current to the substation via the running rails. On a DC railway, these are not directly earthed; in other words they are not referenced to earth but are not completely insulated from it either. This arises because if a DC railway were to be earthed, a proportion of the return current would leave the running rails and attempt to return to the substation via the earth or any buried metalwork in the vicinity. By the process of electrolysis, a DC current leaving a conductor to flow through the earth will cause corrosion at the point it leaves the conductor.

So on a railway or tramway, the corrosion would be worst at a point remote from the substation, where the current is most likely to flow via earth. In addition, if there is any buried metalwork in the vicinity, eg water or gas mains, the current will flow along them and at the point where the current leaves the buried metalwork to return to the substation, it will corrode that as well. This phenomenon is known as "stray current corrosion" and can cause significant levels of damage if not mitigated against. (To take one example, I have seen one-inch diameter steel bolts of the type used to fix OLE structures to the ground broken in two as a result of stray current corrosion.) So the DC railway has to be insulated from earth, but not too insulated otherwise there is a risk that someone could receive an electric shock when they touch the rails, or a train, and earth simultaneously. It is for this reason, also, as mentioned previously, that the Underground went for fourth rail, as the
risk of stray current corrosion damaging the cast iron tunnel linings was too great.

On an AC railway, the traction return circuit has to be earthed for safety reasons and in any case, AC does not cause electrolytic corrosion. So at an AC-to-DC changeover, the running rails have to be earthed and additional mitigation measures put in place to minimise the incidence of stray current corrosion. These can include: the provision of additional rails bonded to the running rail to reduce the resistance of the return path via the rails (and thus "encouraging" the current to flow that way rather than via earth), which can be seen, for example, at Drayton Park; locating substations close to the changeover point (as is the case at Farringdon, Ashford, Acton Central and Drayton Park); and installing a mat of wire mesh underneath the tracks. In addition, on the AC side, isolating transformers are used to ensure DC return currents do not penetrate the AC-electrified system. The examples at Drayton Park can be seen immediately north of the station, whereas those at Acton Central
are about 1/4 mile north.

Farringdon/City Thameslink has a unique system which uses contactors at the new Ludgate Cellars substation (immediately north of City Thameslink) actuated by track circuits to switch the return circuit between the AC and DC systems. These were originally electro-mechanical but are now solid-state examples. This was used because there was no space to install isolation transformers, which have a large footprint.

It is this complexity that has meant that there is no dual electrified section at Highbury and Islington on the North London Line (there is a "dead" section between the AC and DC with insulated joints to separate the return systems) and there will not be a dual electrified section at Abbey Wood either.

The issue is less pronounced on multi-track sections, as the presence of multiple running rails all bonded together will reduce the levels of stray current corrosion to an acceptably low level. This is what presently happens at Ashford and between Euston and Camden.

Modern Traction Immune (TI) track circuits are designed to be immune to both DC and AC return currents; AIUI they are AC at a prime number frequency in the range 1.1-1.3 kHz to minimise the risk of beating and resonance (and thus false Clears) to As Low As Reasonably Practicable. Axle counters are not affected by traction return currents.

Thank you for your clear explanation.

Xposted to two urban groups because that is where voltage change overs
are common.

[email protected]

On Thu, 03 Dec 2015 10:19:28 +0000
e27002 aurora wrote:
On Mon, 30 Nov 2015 10:20:15 -0800 (PST), wrote:
On an AC railway, the traction return circuit has to be earthed for safety
reasons and in any case, AC does not cause electrolytic corrosion. So at an

What happens if something in the ground acts like a partial diode?

--
Spud

Charles Ellson[_2_]

On Thu, 3 Dec 2015 15:24:29 +0000 (UTC), d wrote:

On Thu, 03 Dec 2015 10:19:28 +0000
e27002 aurora wrote:
On Mon, 30 Nov 2015 10:20:15 -0800 (PST), wrote:
On an AC railway, the traction return circuit has to be earthed for safety
reasons and in any case, AC does not cause electrolytic corrosion. So at an

What happens if something in the ground acts like a partial diode?

You can still get some electrolytic corrosion with AC and any point
where different materials (especially if one is aluminium IIRC) meet
has potential (no pun intended) for a diode effect to enable/magnify
it.

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