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

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.

Mansour Abdelwahed said:

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?

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.

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

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

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.

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.

Mansour Abdelwahed said:

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

Usually, you can't mix IEEE and IEC standards to provide "best practice" I'm afraid ... they have provisions that do not align.

The only way, would be to develop your own specification to choose the best practice you'd like to adopt, and at the same time iron out any issues where the standards do not align (or have requirements that preclude the other approach).

Mansour Abdelwahed said:

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

You cannot adopt IEC 60364 series or BS 7671 for traction supplies, they are out of scope.

It is generally the approach that separate supplies are provided for traction, to SCADA/lighting/pumps/safety systems/general power

You might also need a separate signalling supply.

The railway system standards from UK and EU might be the best approach.

HOWEVER, some of the decision might depend on which standards it has been determined to use for traction and signalling systems ... for example, USA approach to those might mean you are best-advised to follow US approach for power (although this might not always align with National codes for LV power in the Middle East which often align with the IEC approach, e.g. similar to BS 7671)

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.

Well noted many thanks You're absolutely right that IEC 60364 and BS 7671 are not applicable to traction systems — we’re only applying them to the auxiliary tunnel services (lighting, SCADA, etc.), which are segregated from the traction system by design. while 

• EN 50122-1 for traction earthing and bonding,

• IEC 61992 for AC traction systems,

Your point about choosing a consistent approach based on regional or national traction/signal system standards is spot-on. Since Saudi Arabia generally follows IEC-based codes, that’s likely the core path for LV systems, but we’ll avoid importing incompatible methods from IEEE or US traction practices unless fully justified.

Thanks again for helping me clarify the technical framing — this is extremely helpful.

Mansour Abdelwahed said:

but we’ll avoid importing incompatible methods from IEEE or US traction practices unless fully justified.

One of the issues to address might well be with respect to read-across of fire safety.

My experience of the region is that fire safety is often specified to USA (NFPA) standards ... however, this is a tricky situation for electrotechnical installations, because NPFA approach requires UL certification, and NFPA installation standards (including National Electrical Code) for electrical installations etc. ... However, as discussed, the preference is for the European / IEC approach for electrical safety and electrical installations, and the two are not always compatible.

I fully understand this is not easy.