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

Hi Mansour,

Thanks for sharing all the details — I really learned a lot just by reading your post and the discussion that followed.

I'm not an expert in traction systems or tunnel design, and I'm still learning myself, but I’d like to share my thoughts based on my understanding and what makes sense to me from a safety perspective.

From what I know, having earth connections only at both ends of a 5 km tunnel sounds a bit risky — mainly because in case of an earth fault somewhere in the middle of the tunnel, the return path impedance might be high, and that could lead to a voltage rise along the PE conductors. That could create unsafe touch voltages, especially if someone is near metallic equipment or structures.

So, in my opinion, adding intermediate equipotential bonding bars or supplementary earth electrodes (even if they have a few ohms of resistance) could help reduce the risk — especially at access points or areas where people might be present. It may not reduce fault current a lot, but it can help equalise potentials and improve safety.

Also, I believe the standard EN 50122-1 is the best fit here, especially since it directly covers railway and tunnel earthing. I’ve read that BS 7671 doesn’t apply to traction systems, so it might only help for auxiliary systems like lighting or SCADA, as you mentioned.

Again, I’m still learning and may not have the full picture, but I hope this perspective adds something useful to the conversation.

Best regards,
Mudassar

Mudassar said:

From what I know, having earth connections only at both ends of a 5 km tunnel sounds a bit risky — mainly because in case of an earth fault somewhere in the middle of the tunnel, the return path impedance might be high, and that could lead to a voltage rise along the PE conductors. That could create unsafe touch voltages, especially if someone is near metallic equipment or structures.

Have a look at BS EN IEC 61936-1 and BS EN 50522 for earthing of the HV systems ... the combined system may well have to achieve much lower earthing resistances than you are currently considering to avoid problems simply bonding the two systems together.

Yes, EN 50122-1 for traction, but also consider there are other standards for signalling that may apply. BS 7671 (along with local national requirements) could be used for LV power for auxiliary supplies, power to SCADA, telecomms, security, drainage pumps and other utility supplies, etc.

There are particular EMC requirements for equipment in a certain distance of the rails, even if they are not part of the rail systems. 

Really appreciate your detailed response — and especially the references to BS EN IEC 61936-1 and BS EN 50522. I have to admit, those weren’t on my radar before, and they seem quite relevant, particularly given the complexity of bonding multiple systems in a tunnel environment. I’ll definitely be digging into those.

Your point about achieving lower earth resistance than just “less than 5 ohms” is well taken. In a 5 km tunnel, with critical systems and potential high fault currents, relying solely on endpoints for earthing might be technically compliant in some cases — but from a risk and safety point of view, I’m starting to think it might fall short, especially when considering touch and step voltages during fault conditions.

Also, I find your mention of EMC considerations quite thought-provoking. We often think about earthing and bonding primarily from a safety and fault-clearing perspective, but EMC often sits in the background — and in a rail tunnel where you've got traction power, sensitive SCADA, telecoms, and maybe signaling all coexisting, ignoring EMC could introduce operational headaches or worse. I suppose that’s where IEC 61000-5-2 and EN 50121 family might start to come into play?

And yes, BS 7671 is sometimes underestimated in these scenarios — while not intended for traction systems, its role in auxiliary system design is critical. Perhaps the challenge is more about integration — how we ensure clean separation where needed, but also safe and intentional bonding where required?

In short, this is proving to be a lot more layered than just sizing the PE and throwing in a couple of rods at each end!

Curious to hear how others have approached similar long tunnel systems — especially with mixed-use voltage systems and shared infrastructure. Has anyone come across specific mitigation strategies that worked well (or didn’t)? Maybe distributed earth mats, or even zoned equipotential sections?

Thanks again for your insight. These contributions are really helping shape my thinking.

Mudassar said:

Your point about achieving lower earth resistance than just “less than 5 ohms” is well taken. In a 5 km tunnel, with critical systems and potential high fault currents, relying solely on endpoints for earthing might be technically compliant in some cases — but from a risk and safety point of view, I’m starting to think it might fall short, especially when considering touch and step voltages during fault conditions.

Yes ... also perhaps worth considering traction return and signalling system needs though.

Is the traction power AC or DC, and what voltage level?

Mudassar said:

but EMC often sits in the background

Not on a railway - as you will have noticed, the BS EN 50121 series is quite extensive in its coverage, and covers more than just traction systems, but also lineside services and equipment, communications etc.

Mudassar said:

zoned equipotential sections?

Often used for large infrastructure, even inside large buildings (like airport terminals).

I would also throw BS EN 62305 into the mix for lightning protection, especially if the railway has overhead traction power, and/or any overhead cables. It will certainly cause issues between two ends, and end and centre, of the tunnel. Technicians have been killed working on telecomms cabling in tunnels during lightning storms!

From what I know, having earth connections only at both ends of a 5 km tunnel sounds a bit risky — mainly because in case of an earth fault somewhere in the middle of the tunnel, the return path impedance might be high, and that could lead to a voltage rise along the PE conductors.

Have a play plugging some numbers into Ohm's Law. Often, unless the PE conductor has a significantly higher impedance than the line conductor, the voltage difference along the PE doesn't change that much.

For (greatly simplified) example, for a fault close to the transformer - say L and PE are both 0.01Ω each, so 0.02Ω earth fault loop. With Uo of 690V you'd have a fault current of about 34.5kA - sticking that into V=IR, you end up with 345V dropped along the line conductor and 345V rise along the PE.

For a fault much further away, say we're looking a 1Ω each for L & PE - so 2Ω loop, the fault current would now be just 345A - plugging that in Ohm's Law for each of the 1Ω conductors gives (yet again) 345V across both L and PE conductors!

Of course if the L and PE are of differing c.s.a.s or differing materials, the balance can change - if the PE happens to have a lower impedance, the "problem" of voltage along the PE can actually get better with distance rather than worse.

   - Andy.

From what I know, having earth connections only at both ends of a 5 km tunnel sounds a bit risky — mainly because in case of an earth fault somewhere in the middle of the tunnel, the return path impedance might be high, and that could lead to a voltage rise along the PE conductors.

Have a play plugging some numbers into Ohm's Law. Often, unless the PE conductor has a significantly higher impedance than the line conductor, the voltage difference along the PE doesn't change that much.

For (greatly simplified) example, for a fault close to the transformer - say L and PE are both 0.01Ω each, so 0.02Ω earth fault loop. With Uo of 690V you'd have a fault current of about 34.5kA - sticking that into V=IR, you end up with 345V dropped along the line conductor and 345V rise along the PE.

For a fault much further away, say we're looking a 1Ω each for L & PE - so 2Ω loop, the fault current would now be just 345A - plugging that in Ohm's Law for each of the 1Ω conductors gives (yet again) 345V across both L and PE conductors!

Of course if the L and PE are of differing c.s.a.s or differing materials, the balance can change - if the PE happens to have a lower impedance, the "problem" of voltage along the PE can actually get better with distance rather than worse.

   - Andy.