Earthing and Bonding Design for 690V AC Railway Tunnels

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
  • Where is the project?

    IEEE standards cannot be mixed with BS, EN and IEC standards, although generally BS, EN and IEC standards are aligned in their approach.

    You would be advised to follow relevant railway standards and specifications. Many of the issues are likely to be outside the scope of BS 7671 directly.

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

    I don't think the IET as an organisation can or would respond directly in that manner?

  • Hi Geoff,

    Thank you for your helpful response.

    You're absolutely right on both points:

    1. I do appreciate that the IET as an organization doesn’t formally respond to individual technical designs — and my wording may have unintentionally implied otherwise. My intention was to gather expert opinions from the IET community, not to seek any official endorsement or sign-off.

    2. Regarding standards, I fully agree — mixing IEEE with BS/EN/IEC inappropriately can create inconsistencies, but the system a design that aligns with international best practices, including UK (BS 7671, EN 50122-1) and IEEE/IEC guidelines.

    The project is based in Saudi Arabia and involves a 5 km underground train tunnel fed at 690V AC from two substations, one at each end. Both substations have:

    • Their own local earth systems (≤5 Ω)

    • Two parallel PE conductors running the full tunnel length for equipotential bonding

    • All tunnel equipment and metallic infrastructure bonded at regular intervals

    There are currently no intermediate earth rods or meshes inside the tunnel. I'm exploring whether this is acceptable from a voltage rise and touch potential perspective, or if distributed earthing would be recommended based on international practice (i.e., from EN 50122-1 or comparable European metro/rail projects).

    Thanks

Reply
  • Hi Geoff,

    Thank you for your helpful response.

    You're absolutely right on both points:

    1. I do appreciate that the IET as an organization doesn’t formally respond to individual technical designs — and my wording may have unintentionally implied otherwise. My intention was to gather expert opinions from the IET community, not to seek any official endorsement or sign-off.

    2. Regarding standards, I fully agree — mixing IEEE with BS/EN/IEC inappropriately can create inconsistencies, but the system a design that aligns with international best practices, including UK (BS 7671, EN 50122-1) and IEEE/IEC guidelines.

    The project is based in Saudi Arabia and involves a 5 km underground train tunnel fed at 690V AC from two substations, one at each end. Both substations have:

    • Their own local earth systems (≤5 Ω)

    • Two parallel PE conductors running the full tunnel length for equipotential bonding

    • All tunnel equipment and metallic infrastructure bonded at regular intervals

    There are currently no intermediate earth rods or meshes inside the tunnel. I'm exploring whether this is acceptable from a voltage rise and touch potential perspective, or if distributed earthing would be recommended based on international practice (i.e., from EN 50122-1 or comparable European metro/rail projects).

    Thanks

Children
No Data