Earthing and Bonding Design for 690V AC Railway Tunnels

Question

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, &le;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 &le;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&rsquo;s expert input on the following aspects:

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?

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?

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?

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?)

broadgage · Accepted Answer

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.

AJJewsbury · Answer

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.

gkenyon · Answer

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)

Earthing and Bonding Design for 690V AC Railway Tunnels

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