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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?)

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  • 0

    There isn't really a technical team here, and I'm no expert, but I can perhaps open by asking a few of the more obvious questions...

    o Is it 690V (rather than 960V) (both are mentioned) - and more importantly is this used for traction (powering the trains themselves), or signalling, or just for other things (e.g. lighting, ventilation etc.)

    o What part of the world is this in (you mention both UK and US standards) - geographical areas can influence which standards are generally acceptable.

      - Andy.

  • 0 in reply to AJJewsbury

    Hi Andy,

    Thank you for your kind reply, and for taking the time to open the discussion.

    Let me clarify the setup and context in more detail:

    Voltage & Usage

    • The system voltage is 690V AC (not 960V — that was a typo, and I appreciate the catch).

    • This voltage is used for traction supply (i.e., powering the train motors directly), not just auxiliary systems.

    • The system is AC-fed, with two step-down transformers (likely 13.8 kV / 690V) at opposite ends of the tunnel, forming a redundant dual-end-fed system.

    Project Location

    • The project is based in the Middle East — specifically in Saudi Arabia.

    • However, the system a design that aligns with international best practices, including UK (BS 7671, EN 50122-1) and IEEE/IEC guidelines.

    Mansour

  • +1 in reply to Mansour Abdelwahed

    Railway traction and signalling are certainly out of scope of BS 7671 (so that's me out) - but some here do have some railway knowledge, so hang on...

    If I'm reading this right, there seems to be a suggestion that the same system is feeding both traction and auxiliary lineside equipment (e.g. lighting) - like I said this isn't my area, but that feels very odd to me, if that is the case. In the UK I'm pretty certain they're strictly segregated - with great care used when bonding between the earthing systems, especially where the rails are used for "return" traction current. Maybe with a relatively short line things won't be so bad, but still it feels odd.

    Just from an ordinary physics point of view, it's somewhere between very difficult to practically impossible to maintain safe touch voltages on an earthing system during a line-earth fault (at least with traditional TN/TT setups). The supply voltage continues to exist and is generally divided along the conductors - e.g. equal sized PE and line (in the sense of phase, not track) would imply half the voltage appears at the point of the fault. As the conductors generally have a relatively low impedance, adding extra electrodes (which typically have resistances of many Ohms) make little difference to the overall picture - often the main mitigation is to raise the voltage of the ground for a few metres around the electrode (hence reducing the voltage difference in their immediate area) rather than reducing the voltage on the earthing system in general. Most precautions against shock rely on limiting the duration of the fault (e.g.0.4s for 230V), rather than the magnitude of touch voltages. For railways (especially signalling) other approaches might be more appropriate (like unearthed (IT) systems, so first fault pose little shock hazard.

      - Andy.

Reply
  • +1 in reply to Mansour Abdelwahed

    Railway traction and signalling are certainly out of scope of BS 7671 (so that's me out) - but some here do have some railway knowledge, so hang on...

    If I'm reading this right, there seems to be a suggestion that the same system is feeding both traction and auxiliary lineside equipment (e.g. lighting) - like I said this isn't my area, but that feels very odd to me, if that is the case. In the UK I'm pretty certain they're strictly segregated - with great care used when bonding between the earthing systems, especially where the rails are used for "return" traction current. Maybe with a relatively short line things won't be so bad, but still it feels odd.

    Just from an ordinary physics point of view, it's somewhere between very difficult to practically impossible to maintain safe touch voltages on an earthing system during a line-earth fault (at least with traditional TN/TT setups). The supply voltage continues to exist and is generally divided along the conductors - e.g. equal sized PE and line (in the sense of phase, not track) would imply half the voltage appears at the point of the fault. As the conductors generally have a relatively low impedance, adding extra electrodes (which typically have resistances of many Ohms) make little difference to the overall picture - often the main mitigation is to raise the voltage of the ground for a few metres around the electrode (hence reducing the voltage difference in their immediate area) rather than reducing the voltage on the earthing system in general. Most precautions against shock rely on limiting the duration of the fault (e.g.0.4s for 230V), rather than the magnitude of touch voltages. For railways (especially signalling) other approaches might be more appropriate (like unearthed (IT) systems, so first fault pose little shock hazard.

      - Andy.

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  • 0 in reply to AJJewsbury

    Thanks so much for your thoughtful and detailed response — I really appreciate it.

    You’re absolutely right that BS 7671 isn’t meant for traction or signaling systems, and in our case we’re mainly following EN 50122-1 and IEC 61992 for the traction side, along with IEC 60364 for the auxiliary systems.

    Just to clarify:

    • This is a 690V AC traction system, and it doesn’t use the rails as a return path — the return current goes through dedicated cables, so we avoid many of the bonding challenges you’d normally have with DC rail systems.

    • The auxiliary loads (like tunnel lighting, SCADA, and ventilation) are powered from separate transformers and have their own distribution and protection. So we are keeping the traction and auxiliary systems electrically separate, but of course they’ll be bonded correctly where needed.

    You also made a great point about touch voltages. We know that earth electrodes don’t solve everything — especially over long distances — so we’re relying on proper PE sizing, fast fault clearance, and equipotential bonding throughout the tunnel. We are now planning to add intermediate earth rods, mainly to help with step voltage around access points.

    On the telecom side, we’re following TIA-607-E, which actually allows telecom systems to be bonded to the main power earthing system — as long as it’s done properly. This helps prevent voltage differences between equipment and improves safety, especially for sensitive systems like SCADA.

    Really appreciate your feedback — it's helping us stress-test the design and improve the outcome. Thanks again.

  • 0 in reply to Mansour Abdelwahed
    Mansour Abdelwahed said:
    On the telecom side, we’re following TIA-607-E, which actually allows telecom systems to be bonded to the main power earthing system — as long as it’s done properly.

    ISO 30129 (EN 50310) along with IEC 61000-5-2 would be the European approach. These would be required for EMC conformity, including the ETSI approaches. There are some key differences between the IEC approaches (IEC 60364 series, IEC 61000 series, and EN 50310/ISO 30129) vs the TIA standards - choose only the approaches that are compatible with both systems.

    There is also a potential issue with electrical safety if certain approaches to "grounding" in TIA are chosen without appropriate consideration of the safety approaches in IEC 61140 (and IEC 60364 series).

    This could affect the safety of individual items of equipment to relevant EU/UK standards.

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