London Underground/Overground traction voltage

[Danemouth]

I don't pretend to understand all the technicalities of this thread but have found it an interesting read.

 

Just goes to show the breadth of knowledge available on RMWeb!

 

Dave

[roythebus]

To add to what Gwiwer says, I'm not aware of any special arrangements at Richmond for stock transfer between the SW and NLL/DR sections (electrically) as the 4th rail is as he says bonded to the running rails on both sides of the station.

 

I'm not sure of the arrangements at Queens Park as I've never worked on that line, but do know that there's a regular Bakerloo train to Kilburn High Road to keep the 4th rail "clean" and to keep drivers' route knowledge. See the district Dave site as this was discussed on there recently.

 

As for the GN taking over the Northern city branch, I was at KX when this work was carried out and worked and walked the tunnels to Old Street. The tunnel was clear for a 31 as far as Highbury, an 08 reached Old Street albeit with a few scratches on the roof, but later in the works we regularly used the cut-down 501s converted to battery cars. These had an excellent turn of speed on the open section, one I worked achieved well over 60 mph on the Hertford branch!! But I digress.

 

The common thing for 313s taking the wrong route towards Turnham Green was to change ends, release the brake, and roll back onto NR metals!

[Trog]

The Neutral terminal in your own home wiring is at 0V. The only difference between that and the Earth terminal is that Neutral provides a low resistance return path whereas the Earth terminal does exactly that, connect to earth. NEVER TRY USING THE EARTH INSTEAD OF THE NEUTRAL OR VICE VERSA OR YOU RISK DEATH BY ELECTROCUTION. 

That's interesting considering that earth and neutral are connected together in a block below my electric meter.

[Dagworth]

That's interesting considering that earth and neutral are connected together in a block below my electric meter.

There are several different wiring systems for earth/neutral once it gets the other side of the meter. http://www.arca53.dsl.pipex.com/index_files/elect4.htm

 

Andi

[Gary H]

There are several different wiring systems for earth/neutral once it gets the other side of the meter. http://www.arca53.dsl.pipex.com/index_files/elect4.htm

 

Andi

Indeed there is, all explained here aswell- http://www.diynot.com/wiki/electrics:earthing_supply_types_and_bonding:earthing_arrangements

[96701]

Rule one. Never connect neutral to earth yourself. It depends on your supply type, and that belongs ton the supply authority.

[QWILPEN]

Rule one. Never connect neutral to earth yourself. It depends on your supply type, and that belongs ton the supply authority.

 

100% agree with this if you are not suitably trained / authorised do not interfere with any of the cables at or near the meter, if in doubt contact your regional distribution company( not nessesarily the same as your supply company).

 

Steve

Edited August 18, 2013 by QWILPEN

[Gwiwer]

 

To add to what Gwiwer says, I'm not aware of any special arrangements at Richmond for stock transfer between the SW and NLL/DR sections (electrically) as the 4th rail is as he says bonded to the running rails on both sides of the station.

 

 

There are frequent scheduled moves at Queens park and Gunnersbury and several timetabled daily at East Putney and Wimbledon where SWT empty cars are routed via Point Pleasant and what is now the District Line (but which was originally an L&SWR route) for pathing and driver route knowledge purposes.  I wasn't aware of the move at Kilburn but it makes sense as there would need to be familiarity with that move for emergency use.  To my knowledge there is no scheduled use of the NR / LU connection at Richmond.

 

On one of my very occasional visits to London I found the Bakerloo service disrupted by a stalled southbound train at Maida Vale.  I arrived at Queens Park northbound and was surprised to see tube stock in all four platforms.  We were decanted onto the adjacent train which proceeded to Harrow while ours terminated in the car sheds to the north of the station.  This train had come up from Kilburn High Road and the one in the southbound "Euston" platform was destined to run there to cross over no doubt to keep some semblance of service going.  With trains arriving every few minutes from the south the car sheds at both ends were full meaning reversal of southbound trains there wasn't possible.

 

By the time I had worked out what was going on and returned in the hope of taking some rare photos the southbound tunnel was open and trains were running again.  C'est la vie.

[roythebus]

The only "regular" moves at Richmond was an evening parcels working up till about 1985 then the stock transfer to the NLL when it was 2EPB units!

 

Prior to relaying in 1972 there was a double junction at Richmond from the LSWR to the DR/NLL which saw the occasional freight until they put a weight limit on Kew Bridge, 73s and 33s only!

 

There were also a couple of electrified sidings at Richmond behind the signal box which were out of use by about 1968 when Pioneer Coaches (remember them?) took over the goods yard.

Edited August 19, 2013 by roythebus

[adanapress]

I was once told that the L.T. Epping Ongar line was fed only from the Epping end -  from the sub-ststion there, and the voltage drop over the distance was such that the three car Cravens set then used,  when departing Southbound from Ongar, was indeed only getting about 240 volts.  I do know for sure that the start was extremely sluggish.

[SouthernMafia]

https://www.bbc.co.uk/news/business-45458222

 

Slightly related to this topic hence why its been dug up, but more so my question as to how these people did not get electrocuted if they're in the pit under a moving train. There's not much space down there and careful movement around the 4th rail when you're in shock can't be easy. They must have been very lucky...unless there's some fail safe available?

[melmerby]

https://www.bbc.co.uk/news/business-45458222

 

Slightly related to this topic hence why its been dug up, but more so my question as to how these people did not get electrocuted if they're in the pit under a moving train. There's not much space down there and careful movement around the 4th rail when you're in shock can't be easy. They must have been very lucky...unless there's some fail safe available?

The outer rail carries the higher voltage (+420v DC) and is against the tunnel wall side away from the pit, the centre rail will give you a shock but might not be lethal.(-210v DC)

The track is nominally 0v

 

Keith

[phil-b259]

https://www.bbc.co.uk/news/business-45458222

 

Slightly related to this topic hence why its been dug up, but more so my question as to how these people did not get electrocuted if they're in the pit under a moving train. There's not much space down there and careful movement around the 4th rail when you're in shock can't be easy. They must have been very lucky...unless there's some fail safe available?

 

Please remember that the running rails in theory play no part in the traction current arrangements. In other words you should be able to touch one of the current rails and the running rails without completing the electrical circuit. The only theoretical danger comes from if there is an earth fault present on the outer positive rail somewhere (say a cable rubbing on the underside of running rail causing the conductors the metal conductors to be exposed) where the running rail then becomes theoretically live at +420V.

 

Obviously the real world is not perfect and in reality touching the negative ail and the running rail may well give the person a bit of a shock - but this is extremely unlikely to do any harm whatsoever.

 

Again I remind people that railway traction supplies and signalling circuits are not the same as employed elsewhere - many are specifically designed NOT to have an earth connection for safety or electrolytic corrosion concerns meaning contact with them MAY (note the specific use of this term) not result in the harm you might expect.

Edited September 8, 2018 by phil-b259

[melmerby]

Please remember that the running rails in theory play no part in the traction current arrangements. In other words you should be able to touch one of the current rails and the running rails without completing the electrical circuit. The only theoretical danger comes from if there is an earth fault present on the outer positive rail somewhere (say a cable rubbing on the underside of running rail causing the conductors the metal conductors to be exposed) where the running rail then becomes theoretically live at +420V.

 

Obviously the real world is not perfect and in reality touching the negative ail and the running rail may well give the person a bit of a shock - but this is extremely unlikely to do any harm whatsoever.

 

Again I remind people that railway traction supplies and signalling circuits are not the same as employed elsewhere - many are specifically designed NOT to have an earth connection for safety or electrolytic corrosion concerns meaning contact with them MAY (note the specific use of this term) not result in the harm you might expect.

The traction voltages of +420v & -210v can only be quoted such if there is a reference point of 0v, that is earth.

Quote

"The traction current has no direct earth point, but there are two resistors connected across the traction supply. The junction point of the resistors is earthed, establishing the reference point between the positive and negative rails by voltage division. The resistors are large enough to prevent large currents flowing through the earthed infrastructure. The positive resistor is twice as large as the negative resistor, since the positive rail carries twice the voltage of the negative rail."

 

With all the muck & grunge you get around tracks the chances that there is leakage between the running rails and ground (hence the 0v earth point) is quite high, possibly sufficient to cause a shock to humans.

Dangerous current to humans is in the order of 10s of mAs. I assume signalling doesn't rely on such low currents.

 

Keith

[phil-b259]

The traction voltages of +420v & -210v can only be quoted such if there is a reference point of 0v, that is earth.

Quote

"The traction current has no direct earth point, but there are two resistors connected across the traction supply. The junction point of the resistors is earthed, establishing the reference point between the positive and negative rails by voltage division. The resistors are large enough to prevent large currents flowing through the earthed infrastructure. The positive resistor is twice as large as the negative resistor, since the positive rail carries twice the voltage of the negative rail."

 

With all the muck & grunge you get around tracks the chances that there is leakage between the running rails and ground (hence the 0v earth point) is quite high, possibly sufficient to cause a shock to humans.

Dangerous current to humans is in the order of 10s of mAs. I assume signalling doesn't rely on such low currents.

 

Keith

 

Naturally the traction system must be earthed eventually - but the point remains that unlike domestic based wiring no earth connection is provided at point of use so to speak. Thus from the perspective of the incident reported in the news the traction system is earth free and people can touch a running rail and a power rail and not have 1000s of amps flowing through them!

 

That is the key point - if you actually manage to bridge the two traction supplies (3rd and 4th rail on LUL or 3rd and running rails on NR) you will get 1000s of amps flowing through you and that is going to kill 99% of olk who try it. By contrast the number of people who will die as a result of 1 amp going through them is more like 1%. I don't doubt that if the shock happens to occur at exactly the right moment in time 10mA can stop someones heart - but that is being rather pedantic when talking about the general public coming into contact with railway traction supplies.

 

As for you last point, actually muck and gunge is a very good insulator and is why accidentally brushing the sides of a dry conductor rail in dry condition with your shins in thin trousers normally has no effect! or why it is necessary to scratch the surface of metal covered in such a build up with your meter probes to get a good reading. Naturally in wet conditions things are different but even then a certain level of insulation is present. Similarly Ice is a very good insulator despite consisting of H2O - a substance that readily conducts electricity when in its liquid state.

 

The current required to drive signalling relays is usually less than 1amp (the exceptions being the drive to point machines etc) while track circuit currents do not usually exceed a couple of amps.

Edited September 8, 2018 by phil-b259

[Trog]

 

As for you last point, actually muck and gunge is a very good insulator and is why accidentally brushing the sides of a dry conductor rail in dry condition with your shins in thin trousers normally has no effect! or why it is necessary to scratch the surface of metal covered in such a build up with your meter probes to get a good reading. Naturally in wet conditions things are different but even then a certain level of insulation is present. Similarly Ice is a very good insulator despite consisting of H2O - a substance that readily conducts electricity when in its liquid state.

 

No need for muck and gunge you can touch the clean metal of the conductor rail top with your fingers, and provided that you are not also touching something that is earthed and the weather is dry your boots are sufficient insulation to stop you getting a belt. Used to write on the possie rail with chalk, and the most painful thing that happened when a stick broke was bending your finger nail back on the top of the rail.

[melmerby]

1000s of amps flowing through you? That's just plain wrong.

 

You need to check your ohms law!

 

Keith

[jim.snowdon]

There is a difference between the 650/750V third rail electrifications and the third+fourth rail system used by London Underground (and others in the past), in that the running rails on the former are not deliberately earthed. They are, though at or about earth potential by virtue of the multitude of leakage resistances between the rails and the earth; they are what is described in engineering circles as "earthy". On the LU system, one rail is, as far as I ever understood it, is deliberately earthed; it is not, though, part of the traction circuit. It is one reason for there being a requirement in NR standards for insulated rail joints to be provided between the two systems where they meet.

 

Jim

[phil-b259]

1000s of amps flowing through you? That's just plain wrong.

 

You need to check your ohms law!

 

 

 

Last time I looked Ohms law said power = Voltage multiplied by Current

 

I have been told that such is power required a fully loaded 12 car EMU with all mod cons like air con easily pulls several hundred amps when drawing away from a stand due to the low voltage of 750V used by conductor rail systems. In the morning peak at places like East Croydon there are many trains doing this and as such the circuit breakers in the substations must be able to withstand a current flow in the thousands of amps. As such they are more than capable of remaining intact should the load be a human body bridging the positive and negative legs - which by extension means a human body could well have thousands of amps flowing through it for a brief period.

 

Circuit barkers in such places are there to protect the electrical infrastructure NOT PEOPLE.

[jim.snowdon]

It is important not to confuse electrical power (Watts) with electrical current (Amps). Ohm's Law is V=I*R, or Volts = Amps times Resistance (ohms). Power = Volts times Amps.

 

The resistance of the human body can vary a lot, but 500 ohms is a recognised value. Applying Ohm's Law, the current that would flow through a person touching the conductor and running rails would be 750V/500A, or 1.5A. That may not sound much, but it is rather more than the human body is designed for. Once current is flowing, the resistance of the body reduces non-linearly.

The important point, in the context of railway traction supplies is that the normal currents drawn by 750V DC trains are in the thousands of amps (as you state); the current flowing through the human body is lethal but rather smaller. In simple terms, the substation will hardly notice the load, and the circuit breakers will not trip.

 

Jim

[melmerby]

Although the actual internal body resistance is quite low, allowing for skin resistance, I think you will find that in normal conditions 500 ohms is on the low side.

However the actual current flow required to affect the heart is quite low.

At 10mA you will be affected, at 100mA likely you will die. (paradoxically however above 200mA you may survive!)

 

Keith

[Wickham Green]

Once upon a time there was the potential - if I may use that word - for third/fourth rail conflict at New Cross and New Cross Gate as well as other locations already mentioned ............ today with 'progress'  it's all third rail but there's no connection whatsoever between OvergrounD and the rest of the network at New Cross.

[PatB]

I'm reminded of the unfortunate mouse which got into our washing machine and bridged the mains terminals. When I finally discovered the source of the smell of hot electricity both its front end and back end looked rather surprised, but of its middle portion nothing remained.

[009 micro modeller]

When NR took over the Drayton Park - Moorgate route it was re-engineered from 4-rail to 3-rail but in doing so only it was returned to its original configuration since the GNR electrified it thus long before LT took it over. Those tunnels therefore accommodate class 313 vehicles readily.

The Great Northern & City Railway is an interesting one. The company was independent but initially supported by the GNR, who were going to use it for through running in the way that it is used today. Relations between the companies deteriorated and the GN&C ended up terminating at the underground station at Finsbury Park, rather than the GN one. It was not third rail but an unusual 4 rail system in which both conductors were outside the running rails, one on each side. If you google it there is an LURS PDF article about the GN&C, including pictures showing the original stock and electrification system. From reading the article I was slightly unclear as to whether the change to the normal LU 4th rail system took place immediately after the Metropolitan bought the line in 1913 or not until later when the switch to using small size tube stock was made. I seem to remember there are bits of redundant 4th rail between the running rails on this line as well, no insulators but still joined together as on the Watford DC lines.

[Nearholmer]

Folks

 

have a google for "Conduction of electrical current to and through the human body" by Fish and Geddes to understand how electricity harms people, and to understand that we aren't simple linear resistors that can be analysed using a simple application of Ohm's law.

 

the only factoid that I would add to the discussion of the LU traction power supply is that it is indeed operated at 420-0-210V (nominal) using resistors to act as potential dividers, and that it operates in sections, large areas in which the rectifiers are operated in parallel, each of which has voltage monitoring systems so that either pole being connected to earth by less than the resistance of the potential divider is detected. once detected, any fault is then 'chased', by switching the circuits, and by a process of elimination can then by located and dealt with. the 420-0-210V division originated in the relative sizes, and creep age resistances of the insulators - the system wasn't deliberately 'pinned' until after a series of nasty incidents caused by uncontrolled earth-fault current flow led to the provision of the potential dividers and detectors. I've never been able to work out precisely when the 'pinning' took place, despite nearly thirty years close involvement with the system, but I think it was immediately before or after WW2.

 

Kevin

Edited September 11, 2018 by Nearholmer