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Bakerloo Line beyond Harrow & Wealdstone
On Sat, 29 Nov 2008, Steve Fitzgerald wrote:
In message , Christopher A. Lee writes No. Current leaking to ground and causing electrolytic corrosion. Remember, there were already pipes etc under London before the tubes were built, and the tubes were lined with cast iron segments.. Tram and streetcar track had the running rails at minus 10 volts so that stray current leaked from ground to the rails instead of vice versa. LU's centre rail is I believe at minus 200 volts. Traction current is fed at +420v (positive) on the outside rails and -210v on the centre rail (negative) the sum of these giving a traction feed of 630v dc. They are 'loosely' tied to earth through resistances in the sub stations that feed the supply. Despite being an electrician in a previous life and now a driver, I struggle to get my head round 'loosely tied to earth' and what it means in real life. I recollect an incident a few years ago when the traction current had been discharged (turned off) due to person under a train. For complicated reasons, the centre rail had become live at +420v. This must be some new meaning of the term 'turned off' of which i was not previously aware! As you say, complicated reasons - but this sounds like the kind of thing that really, really shouldn't happen. Crumbs. tom -- This is your life and it's ending one minute at a time. |
Bakerloo Line beyond Harrow & Wealdstone
On Sat, 29 Nov 2008 18:33:29 -0000, wrote:
"MIG" wrote in message ... : That's interesting, because I was under the impression that there was : no danger from the middle track, but 200 V could presumably give a : whack if there was any current. Doesn't the live rai change back and forth between third and fourth position? I remember hearing that the live rail is always in the third position when in a station, because there is less of a risk of somebody getting fried, should they fall onto the tracks. But the positive can be in fourth position just before and after the station. No. The outside rail goes outside the opposite running rail from the platform in stations. |
Bakerloo Line beyond Harrow & Wealdstone
In message , Tom
Anderson writes I recollect an incident a few years ago when the traction current had been discharged (turned off) due to person under a train. For complicated reasons, the centre rail had become live at +420v. This must be some new meaning of the term 'turned off' of which i was not previously aware! As you say, complicated reasons - but this sounds like the kind of thing that really, really shouldn't happen. Crumbs. It had been correctly discharged but been re-fed in error from elsewhere. You will note that what should normally have been -210v had now become +420v. -- Steve Fitzgerald has now left the building. You will find him in London's Docklands, E16, UK (please use the reply to address for email) |
Bakerloo Line beyond Harrow & Wealdstone
On Sat, 29 Nov 2008 12:52:12 -0500, Christopher A. Lee
wrote: On Sat, 29 Nov 2008 09:26:38 -0800 (PST), MIG wrote: On Nov 29, 2:52*pm, Christopher A. Lee wrote: On Sat, 29 Nov 2008 14:06:54 -0000, wrote: "Charles Ellson" wrote in message .. . On Mon, 24 Nov 2008 20:32:20 -0000, wrote: Is a fourth rail really necessary out that way, however? It is if you want the same train to work both there and on the Underground. Was the 38 stock on Island Line modified to work only on third rail? What did that require, if that was the case? Modification to the same general standard as other SR 3rd-rail stock, returning current via the wheels and chassis instead of via a central pickup to a conductor rail. Older LU stock might also have required upgrading of cables if the insulation was not suitable for 660v (or 750v?) use; under normal conditions an LU train running on LU 4-rail track doesn't have any parts at more than 440v to earth but when running on sections of "hybrid" 3or4-rail the highest normal voltage to earth is 440V to earth. Under fault conditions (centre conductor rail earthed) on LU the 3rd rail voltage can rise to the full supply voltage and current stock has cable insulation already rated to cope. I always understood that they were really needed only for the tub sections of the tube, to help power flow into the motors. No the LU 4-rail power supply is intended to keep the traction current within the two conductor rails and not find its way back via other bits of metal with consequent damage. In case of flooding, perhaps? No. Current leaking to ground and causing electrolytic corrosion. Remember, there were already pipes etc under London before the tubes were built, and the tubes were lined with cast iron segments.. Tram and streetcar track had the running rails at minus 10 volts so that stray current leaked from ground to the rails instead of vice versa. LU's centre rail is I believe at minus 200 volts. That's interesting, because I was under the impression that there was no danger from the middle track, but 200 V could presumably give a whack if there was any current. There isn't much current if there's nothing in section. You used to see track workers hopping on and off the centre rail and walking along it. |
Bakerloo Line beyond Harrow & Wealdstone
On Sat, 29 Nov 2008 18:53:49 +0000, Tom Anderson
wrote: On Sat, 29 Nov 2008, Steve Fitzgerald wrote: In message , Christopher A. Lee writes No. Current leaking to ground and causing electrolytic corrosion. Remember, there were already pipes etc under London before the tubes were built, and the tubes were lined with cast iron segments.. Tram and streetcar track had the running rails at minus 10 volts so that stray current leaked from ground to the rails instead of vice versa. LU's centre rail is I believe at minus 200 volts. Traction current is fed at +420v (positive) on the outside rails and -210v on the centre rail (negative) the sum of these giving a traction feed of 630v dc. They are 'loosely' tied to earth through resistances in the sub stations that feed the supply. Despite being an electrician in a previous life and now a driver, I struggle to get my head round 'loosely tied to earth' and what it means in real life. Doing it by memory from an old issue of "Underground" the resistor network is thus :- POS RAIL - ~400ohms - EARTH - ~200ohms - NEG RAIL Each resistor has a relay across it. If the power supply fails both relays drop out causing an alarm; if either conductor rail is earthed then one of the relays will drop out and bring up an alarm to indicate an earth fault (if both rails are earthed then you have a short circuit anyway). Transient earth faults can be caused by windblown metallic debris so IIRC the alarms will be reset up to three times in succession before someone is sent out to investigate. The "loose tie" to earth is an electrical consequence of the detection system and a design method of limiting the maximum conductor rail voltage under non-fault conditions. Purely "floating" (nothing connected to earth) systems can lead to such things as fault voltages which are the sum of two or more supplies if the wrong wires come into contact. I recollect an incident a few years ago when the traction current had been discharged (turned off) due to person under a train. For complicated reasons, the centre rail had become live at +420v. This must be some new meaning of the term 'turned off' of which i was not previously aware! As you say, complicated reasons - but this sounds like the kind of thing that really, really shouldn't happen. Crumbs. One train (or two?) bridging two sections ? It doesn't need to be a "good" connection to make a conductor rail lethally live. If it isn't the same incident ISTR there was also at least one incident involving incorrect closure of the switches (or failure to re-open one/them) between two adjacent sections. |
Bakerloo Line beyond Harrow & Wealdstone
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Bakerloo Line beyond Harrow & Wealdstone
On Sat, 29 Nov 2008 14:06:54 -0000 someone who may be
wrote this:- Was the 38 stock on Island Line modified to work only on third rail? Undoubtedly. What did that require, if that was the case? Leaving aside the incidentals, in a four rail system the traction electricity flows from one conductor shoe via a cable to the motor and then via another cable to the other conductor shoe. The traction current is thus confined to its own separate circuit and will not interfere too much with other circuits. In a three rail system the traction electricity flows from the conductor shoe via a cable to the motor and then via another cable to a rotating contact on the axle, from where it flows to the wheel and to the running rail. Thus the traction electricity is sharing the running rails with any other circuits and arrangements have to be made to stop it swamping these. In a nutshell, to convert the train from one system to the other the central conductor shoes are removed and the cable connected to newly installed rotating contacts instead. These contacts are very much like the commutator of a "traditional" electric motor but there is only one segment. Then what is the problem with Bakerloo line trains continuing further north, assuming that they don't require any modifications? They don't require any modifications (other than in the past those already mentioned by others to do with insulation if they are older trains). In these sections the central conductor rail is not energised but is rather connected to the running rail which is used for traction return. A "BR" train will "see" the traction voltage between the (outside) conductor rail and running rail and work. A "LT" train will "see" the same traction voltage between the outside and central conductor rail and also work. The difference is that on these sections the voltage difference is between +600 odd volts and zero, while on "LT" lines the voltage difference is between +400 odd volts and - 200 odd volts. The voltage difference used to drive the motors is the same, but the potential to the earth is different on the two systems. I have deliberately used round numbers for the voltages, rather than the nominal ones. -- David Hansen, Edinburgh I will *always* explain revoked encryption keys, unless RIP prevents me http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54 |
Bakerloo Line beyond Harrow & Wealdstone
On Sat, 29 Nov 2008 09:26:38 -0800 (PST) someone who may be MIG
wrote this:- LU's centre rail is I believe at minus 200 volts. That's interesting, because I was under the impression that there was no danger from the middle track, A dangerous impression. If there was no (electrical) danger from the central conductor rail then it would not be mounted on insulators. Insulators are a give-away that something is energised. -- David Hansen, Edinburgh I will *always* explain revoked encryption keys, unless RIP prevents me http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54 |
Bakerloo Line beyond Harrow & Wealdstone
wrote in message On hybrid sections (e.g. East Putney to Wimbledon) the 4th rail is at +630v. That would be the third (positive) rail. Otherwise the 'third rail' stock wouldn't work too well... Paul S |
Bakerloo Line beyond Harrow & Wealdstone
On Nov 30, 11:14*am, David Hansen
wrote: On Sat, 29 Nov 2008 09:26:38 -0800 (PST) someone who may be MIG wrote this:- LU's centre rail is I believe at minus 200 volts. That's interesting, because I was under the impression that there was no danger from the middle track, A dangerous impression. If there was no (electrical) danger from the central conductor rail then it would not be mounted on insulators. Insulators are a give-away that something is energised. Piecing it all together, I suppose it adds up, in that if a neutral rail was to be used, it might as well the the running rails or something earthed. So the fact that there's a special rail on insulators means it can't be neutral. Never really thought it through though. |
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