Electrical Isolation

Discussion in 'London Underground' started by Deepgreen, 20 Apr 2015.

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  1. Deepgreen

    Deepgreen Established Member

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    A London Underground traction current question - if a train enters a section of track where the traction current has been discharged for engineering works, when the train bridges the gap between the 'live' and 'dead' sections, does it cause the 'dead' section to be 'livened up' while the gap remains bridged?
     
  2. Bigfoot

    Bigfoot Member

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    On 3rd rail land when the current is discharged earth straps are put down connecting both running rails to the juice to protect against accidental switch on/trains entering protected areas. When the gap is bridged by the train the current will immediately blow the breakers as the current is short circuited through the running rails.

    I would presume similar straps are applied to LUL lines.
     
  3. PermitToTravel

    PermitToTravel Established Member

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    My understanding is that the dead section will briefly become live, but a fuse inside the train will blow as current is not meant to flow from one pickup shoe to another.
    --- old post above --- --- new post below ---
    They have really cool short circuiting bars which can be laid across the track and connect both current rails to a running rail. I believe LU staff are trained to use them to switch the power off (if there's no other method available), which must be quite a sight!
     
  4. Mojo

    Mojo Administrator Staff Member Administrator

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    This, admittedly rather old, video (skip to about 4 Min in) may interest you

    [youtube]xumonIs52Lk[/youtube]
    --- old post above --- --- new post below ---
    The short circuiting device does not touch the running rails; it simply connects the positive and negitive rail. SCDs must be placed at both ends of the train.
     
  5. PermitToTravel

    PermitToTravel Established Member

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    Derp, of course that would be the case. Wasn't thinking, sorry
     
  6. Met Driver

    Met Driver Established Member

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    Most traction current section gaps are longer than the distance between the two sets of collector shoes on a single car, so these cannot be bridged (LU trains do not have bus lines connecting the shoes on multiple cars). Those that aren't can be bridged and the dead section would become live. That is why drivers are trained to motor or coast fully into a dead section if they are unable to stop in time.
     
  7. AM9

    AM9 Established Member

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    Would it not be better to have a floating section at each section gap that was only connected to a supply if the supplies on both sides were also powered. That way, any bridging could only make the floating section live, not the dead section. This would be similar although not quite the same as an OLE section break where there is an earthed section in the middle.
     
  8. Mojo

    Mojo Administrator Staff Member Administrator

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    That seems to be unnecessarily complex in relation to the current "solution" whereby three red lights illuminate before a section gap.

    I'd have to ask..?

    1. Where would this "floating section" be powered from? Would it be single end or double end fed?
    2. What happens if traction current in this "floating section" needs to be discharged, would you have a "floating section" to the "floating section?"
     
  9. Nym

    Nym Established Member

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    The SCD is not a method to discharge traction current, it is to prevent traction current being re-charged after it has been shut down from the LU PCC.

    Indeed in some sections, especially towards the end of the section the SCD can take a highish number of seconds to trip the supply, as the impedance of the supply rails and cables reduces the current flow, sometimes as far as to look like a train starting up.

    I wouldn't trust an SCD to take the juice off, I'd be getting it turned off, checking with a CRID (Current Rail Indicator Device) and only then putting down an SCD.

    It should also be noted that LU has significantly more switching operations than NR.
     
  10. AM9

    AM9 Established Member

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  11. Met Driver

    Met Driver Established Member

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    If the issue was of sufficient concern then they would probably work towards lengthening all the short gaps so that they couldn't be bridged.

    Use of an SCD to attempt to discharge traction current is permitted in an extreme emergency, when routine methods have failed, except when a discharge has been requested through the controller but they have been unable to confirm that traction current has indeed been discharged. There was an incident a few years ago (a one-under) where the controller had been unable to confirm that traction current was switched off because the power control room was unable to open the breakers at one end of the section. An SCD was subsequently placed which exploded after 20 seconds or so. The CCTV footage of the incident now features in training videos.
     
  12. Mojo

    Mojo Administrator Staff Member Administrator

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    I mean, what if the actual "floating" section had to be discharged?
     
  13. AM9

    AM9 Established Member

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    What I was trying to say was: if the 'floating' section was energised by switchgear when both adjacent (main) sections had power, and if either of those sections lost their power, the floating section would be disconnected (isolated) by that switchgear. The voltage would then fall to zero if a suitable permanent bleed resistor was fitted. If it was necessary to discharge it , i.e. force an erroneous feed to trip, (I can only forsee two causes),
    1) faulty isolating swithchgear and
    2) a train bridging from an adjacent main section.
    In the case of 1) the rails would need to be shorted with a SCD, - maybe difficult if a train was sitting over the length of the floating section's conductor rails, but if it was erroneoujsly being fed from one of the adjacent sections, the track on those would be accessible.
    Similarly, if the train was causing the supply to be present by bridging, the SCD could be deployed where the track was not covered. Like all complex systems, some failure modes would need specific details to assess the risks.
    I'm no expert on this, just musing on whether isolation could be achieved without interrupting normal running.
     
  14. Mutant Lemming

    Mutant Lemming Established Member

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    Not any more but it did used to be in "Extreme Emergencies" in the days when we were a touch more blasé about traction current. Heard tales of the SCD getting welded to the track but seemed to work well enough when I've seen it done.
    I used to be called a scaredy cat for not pulling out overhead jumpers while the compressors were running let alone killing anything else that drew juice.
     
  15. Nym

    Nym Established Member

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    Yeah, we have interlocks with contactors on the new stock so we can't even arc up a shed lead any more, it's not that easy to do even when you don't have shed supply contactors.

    If you think about the Leicester Sq incident for a bit then you may come to a good reason why, considering the breaker would not trip, such a high level of damage was caused to the SCD, designed to withstand an I^2 t let through of a certain amount of Amps Sq. Seconds, as this is how most protection systems operate. Consider the very small amount of impedance between the breaker and track at LES.

    And yes, it is a bit scardycat not yanking out with the compressors on, the MAs draw nearly as much!
     
  16. AM9

    AM9 Established Member

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    Too many acronyms:
    LES, MA, PCC?

    Out of curiosity, what is the current curve of the breakers like, i.e. PSCC (Prospective Short Circuit Current) vs time?
     
  17. Mutant Lemming

    Mutant Lemming Established Member

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    As would the MGs
     
  18. Nym

    Nym Established Member

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    Leicster Square, Motor-Alternator, Power Control Centre.

    And it depends on the site as to what kind of protection settings are used.

    In addition to fault detection you also have the likes of inter-station tripping and tunnel telephone, etc.
     
  19. AM9

    AM9 Established Member

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    Thanks for that, - I might have eventually guessed MG and PCC but 'LES', that foxed me!
     
  20. Mojo

    Mojo Administrator Staff Member Administrator

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    Do you mean the Russell Square incident with regards to the exploding SCD?
     
  21. Mutant Lemming

    Mutant Lemming Established Member

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    Would the laying of an SCD by the guard in the Southgate fire incident have negated the necessity for him to repeatedly operate the tunnel telephone wires at intervals of 7 minutes or less ?
     
    Last edited: 25 Apr 2015
  22. Daniel

    Daniel Established Member

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    I don't know of the Southgate incident you are referring to.

    I am unaware of a 'necessity to repeatedly operate TT wires' at all.

    No matter how you discharge, the controller should ALWAYS be contacted.

    If the controller is unaware of the reason for discharge of a section, and has not been advised of any reason and making his/her own investigations 7 minutes after discharge, the controller will attempt to recharge traction current.
     
  23. Jonny

    Jonny Established Member

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    Are such indicators interlocked to the main signalling system, to reduce the odds of a train approaching one in the first place?
     
  24. Dstock7080

    Dstock7080 Established Member

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    Not at all, (not fitted with trainstops either) although passing one illuminated is regarded as a SPAD.
     
  25. Mutant Lemming

    Mutant Lemming Established Member

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    There was a fire in the centre of a six car 73 stock - because of the thick acrid smoke the guard defied the cardinal rule of not walking down a single track tube tunnel but with the fire and smoke saw no other option but to lead passengers that way. He frequently operated the tunnel telephone wires to ensure the controller wouldn't restore current and reduce the possibility of an oncoming train reaching them. In those days communication was poor and only the driver had a 'rat's coffin' with him. What the guard done in the situation was just about what anyone with any sense would have done - but would laying the SCD negated the necessity for him to frequently operate the tunnel telephone wires ?
     
    Last edited: 26 Apr 2015
  26. Dstock7080

    Dstock7080 Established Member

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    The Bounds Green fire was 11 August 1982 on the REAR car of an eastbound train.
    Full report here: http://www.railwaysarchive.co.uk/documents/MoT_WoodsGreen1982.pdf
     
  27. tsr

    tsr Established Member

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  28. Daniel

    Daniel Established Member

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    Just to highlight although it's been linked to above,mInhavent read the report and am just replying to what has been posted here.

    Okay, I can see what was meant now no what was intended by his actions. However, activating the TT wires frequently would not have prevented traction current being recharged. The controller/PCRO would not know that they had been operated a second time within the initial 7 mins, so if they had been activated again at the 5 min mark, a recharge would have still been attempted at the 7 min mark, some 2 mins later. After traction current had been recharged, however, using the TT wires would again have tripped traction current - provided they had not been over-ridden because of an assumed fault.

    Yes and no - but it's only confusing because the TT wires weren't being used fully anyway. The SCD's purpose is designed to, as the name suggests, create a short circuit, either tripping out the current feed or creating such an indication that it could be determined that an SCD had been laid, and current therefore requiring discharge. However, an SCD aims by no means a guaranteed method of discharge or assurance that a recharge won't occur.

    The only way of ensuring that traction current will not be recharged would be to speak to the service controller.
     
  29. Mutant Lemming

    Mutant Lemming Established Member

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    I think I did get a couple of incidents mangled in to one but it was an angle used to fight for the retention of guards on deep level tubes.
     
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