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Earthing and Bonding Design for 690V AC Railway Tunnels

Mansour Abdelwahed

Dear IET Technical Team,

I am an IET member (MIET) currently reviewing the earthing scheme for about 5 km AC train 960 VAC tunnel supplied from two substations (each with separate earth electrodes, ≤5 Ω). Both substations are interconnected by two paralle

System Configuration Overview:

  • Each substation is equipped with its own earth electrode system designed to achieve a resistance of ≤5 ohms.

  • The substations are electrically interconnected via two parallel protective earthing (PE) conductors that run along the full tunnel length (5 km), ensuring both equipotential bonding and redundancy.

  • These PE conductors are intended to:

    • Interconnect both substation earthing systems,

    • Provide a continuous protective earth along the tunnel for all connected equipment (lighting, SCADA, signaling, etc.),

    • Bond all exposed conductive parts and metallic structures inside the tunnel.

  • I would appreciate the IET’s expert input on the following aspects:

    1. Is the use of only end-point earthing (via the substations) with continuous PE conductors across 3 km acceptable for a 690V AC system, assuming the conductors are adequately sized and bonding is done at regular intervals?

    2. Would additional intermediate earthing electrodes or equipotential bonding bars be recommended, especially to mitigate the effects of fault current return path impedance or potential rise under earth fault conditions?

    3. Are there any best-practice thresholds for voltage drop or rise along PE conductors during fault events in such long LV systems, particularly with respect to maintaining safe touch and step voltages in a tunnel environment?

    4. Which standards would best guide this setup from the UK or international perspective? (e.g., BS 7671 Section 542, EN 50122-1 for railway applications, IET Code of Practice for Earthing, or IEEE Std 80?)

Parents
  • +1

    690 volts AC is most unusual for a traction supply. Standard traction supplies include 25Kv, AC and 750 VOLTS dc

    690 volts AC generally implies a three phase, 4 wire system with 400 volts from any phase to neutral, and 690 volts between any two phases. Most traction circuits use an earthed return via the running rails, no easy way to do that from a 400/690 volt system with an earthed neutral.

  • 0 in reply to broadgage

    DC systems like London underground use voltages of order 630, with an offset earth. I'm not a railway guy either, but I have friends who are involved. My first observation is that any railway of any weight will consume  a lot of current at that sort of voltage. Have you made estimates for the likely traction current? One reason that  LV systems (sub kV) are usually live rail , and not overhead live wire, is voltage drop, and its also the reason the feeder transformers are less than a km apart on busy sections.

    You say your tunnel if 5km long ! 

    For example an 8-car train can use up to ~3600 MW of power (4200 amperes at your voltage more or less )  for motor currents  when accelerating hard. 
    Now if we consider how much voltage drop we might like at peak load mid-span, and indeed how much uplift if you have some kind of  regenerative braking, you very quickly need a substation every KM or so, or hopelessly large conductors, or more sensibly a higher traction voltage.
    (Again using London underground as an example, for a nominal 630V line, overshoot to 790 during braking was common, as were dips below 500V. In recent time, there has been a program of voltage uplift to 750V nominal and 890 during braking all new motors are qualified to run between  500 and 1000V Compared to normal voltage excursions seen in power distribution, where 10% droop is bad, the expected perturbations  are very large.)

    I'm worried that before getting tangled with the finer details of the earthing there are wider issues of sensible voltages and currents and the no. and spacing of the feed points to decide.

    In summary, 690V wont really go 5km, or even half of it, with any sensible conductor choices. You will need HV or at least MV , and suitable step-down  transformers, in the tunnel. These of course will each need their own earthing arrangements.

    Mike.

    Edit
    PS
    some facts and figures for track impedances in this Australian paper here https://railknowledgebank.com/TrackAsset
    - R, Rail Resistance: ~ 0.03 ohm/km (each steel rail)
    - L , Rail Inductance: ~ 2mH/km (1 mH = 1 x 10-3 Henry)
    - RE, Rail to Earth Resistance: ~ 100 ohm.km
    - RT, Track Resistance, rail to rail: ~ 200 ohm.km
    - C , Track Capacitance, rail to rail, ~ 1,000 pF/km

    I understand the figures in the UK at least are comparable.



    PPS
    As an old rule of thumb, maximum distances for economic power distribution balancing cable costs and losses against transformation costs  and losses set in at about 1 volt per metre. So while a 12V battery is fine on a car a few metres long, but a big lorry with trailer etc might be better with a 24V system.
    In terms of AC 230V single phase will go a few hundred metres, and 400V and split phase more or less double this as the drop in the neutral conductor can be greatly reduced, but to double that distance again needs really out-size wiring for the current and it quickly becomes cheaper to drop in a second substation.  11kV networks tend to have a span of no more than 10s of km, so 33kV is used in more rural systems, while 400kV and higher is needed to reach across a small country.

    Its not a perfect rule - one can of course throw copper (or aluminium)  at the problem to extend the reach slightly  further, and for small loads that is often done, and systems where we are only signalling but not really delivering power, like phones (50V, few km) and Ethernet (4Vp-p 100m, POE 50V/100m ), this is estimator method is way out  but this  does not look like a small load ;-) 

Reply
  • 0 in reply to broadgage

    DC systems like London underground use voltages of order 630, with an offset earth. I'm not a railway guy either, but I have friends who are involved. My first observation is that any railway of any weight will consume  a lot of current at that sort of voltage. Have you made estimates for the likely traction current? One reason that  LV systems (sub kV) are usually live rail , and not overhead live wire, is voltage drop, and its also the reason the feeder transformers are less than a km apart on busy sections.

    You say your tunnel if 5km long ! 

    For example an 8-car train can use up to ~3600 MW of power (4200 amperes at your voltage more or less )  for motor currents  when accelerating hard. 
    Now if we consider how much voltage drop we might like at peak load mid-span, and indeed how much uplift if you have some kind of  regenerative braking, you very quickly need a substation every KM or so, or hopelessly large conductors, or more sensibly a higher traction voltage.
    (Again using London underground as an example, for a nominal 630V line, overshoot to 790 during braking was common, as were dips below 500V. In recent time, there has been a program of voltage uplift to 750V nominal and 890 during braking all new motors are qualified to run between  500 and 1000V Compared to normal voltage excursions seen in power distribution, where 10% droop is bad, the expected perturbations  are very large.)

    I'm worried that before getting tangled with the finer details of the earthing there are wider issues of sensible voltages and currents and the no. and spacing of the feed points to decide.

    In summary, 690V wont really go 5km, or even half of it, with any sensible conductor choices. You will need HV or at least MV , and suitable step-down  transformers, in the tunnel. These of course will each need their own earthing arrangements.

    Mike.

    Edit
    PS
    some facts and figures for track impedances in this Australian paper here https://railknowledgebank.com/TrackAsset
    - R, Rail Resistance: ~ 0.03 ohm/km (each steel rail)
    - L , Rail Inductance: ~ 2mH/km (1 mH = 1 x 10-3 Henry)
    - RE, Rail to Earth Resistance: ~ 100 ohm.km
    - RT, Track Resistance, rail to rail: ~ 200 ohm.km
    - C , Track Capacitance, rail to rail, ~ 1,000 pF/km

    I understand the figures in the UK at least are comparable.



    PPS
    As an old rule of thumb, maximum distances for economic power distribution balancing cable costs and losses against transformation costs  and losses set in at about 1 volt per metre. So while a 12V battery is fine on a car a few metres long, but a big lorry with trailer etc might be better with a 24V system.
    In terms of AC 230V single phase will go a few hundred metres, and 400V and split phase more or less double this as the drop in the neutral conductor can be greatly reduced, but to double that distance again needs really out-size wiring for the current and it quickly becomes cheaper to drop in a second substation.  11kV networks tend to have a span of no more than 10s of km, so 33kV is used in more rural systems, while 400kV and higher is needed to reach across a small country.

    Its not a perfect rule - one can of course throw copper (or aluminium)  at the problem to extend the reach slightly  further, and for small loads that is often done, and systems where we are only signalling but not really delivering power, like phones (50V, few km) and Ethernet (4Vp-p 100m, POE 50V/100m ), this is estimator method is way out  but this  does not look like a small load ;-) 

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