Hacker News

A train-sized tunnel is now carrying electricity under South London

• 51 Comments
• No flashy product launch, no app, just "we quietly replaced a 1960s failure point before it failed."

  By BrtByte 6 hours ago

• It’s not just a tunnel, it’s a game-changing lifeline

  By 9NRtKyP4 an hour ago

• I'm shocked it only took five years and only cost 4 month's worth of bomalian budget.
  Great job whoever was in charge.

  By swarnie an hour ago

• Anyone have any idea why the cables are arranged like this? https://8400e186.delivery.rocketcdn.me/articles/wp-content/u...
  What's the zig-zag pattern for, seems like a fair bit of extra conductor.

  By luma 9 hours ago

• This view is just very extreme, it is much less zig zag. It is just mounted to the wall at the high points and slack in between. Certainly there is also a reason for the exact amount of slack like thermal expansion.
  https://cdn.ca.emap.com/wp-content/uploads/sites/9/2018/04/l...

  By jo909 9 hours ago

• That’s just slack. You’re seeing a very long distance with extreme foreshortening in this image.

  By nkrisc 3 hours ago

• Do you mean the catenary [0]?
  [0] https://en.wikipedia.org/wiki/Catenary

  By bregma 7 hours ago

• First guess (may be wrong) is to manage thermal expansion/contraction constantly on a micro-scale.

  By probablypower 9 hours ago

• I would also add that there some slack for repairs.

  By nraynaud 9 hours ago

• My understanding is that it's mostly mechanical and thermal, not electrical cleverness

  By BrtByte 6 hours ago

• Maybe to reduce electromagnetic coupling? Seems they're offset a bit.

  By chadcmulligan 7 hours ago

• Is that a tandem bicycle? Cute.

  By Reason077 8 hours ago

• Yes. First time I see a tandem in such tunnel. Until know I just saw and used normal bicycles in such kind of tunnels.

  By _trampeltier 7 hours ago

• Tandem bike or a new SCP variant.

  By zeristor 7 hours ago

• > Anyone have any idea why the cables are arranged like this?
  I think that's just cables sagging, which is a requirement to accommodate thermal and seismic displacements.

  By locknitpicker 8 hours ago

• It looks like AI slop to be honest. My second best guess is that it could be arrayed transformers.
  I don't think a utility company in their right mind would allow workers to bicycle inside a tunnel powering the grid.

  By 1-6 an hour ago

• There are three reasons:
  * Cable thermal expansion under current load: https://www.ahelek.com/news/cable-thermal-expansion-and-its-...
  * The amount pictured is in excess of that required for thermal expansion. The excess is to have some spare length in case of modifications. For example if you have to replace the transformer and the terminals are not in the same location. You cannot extend a massive cable like that easily or without degrading its specs.
  * The sine wave pattern makes it into AC of course (/s)

  By defrost 9 hours ago

• I'm slightly confused by the "New Cross substation in Southwark" whilst the map immediately above clearly shows it in Lewisham[0][1].
  Best I found was a page talking about vertical boreholes on Old Kent Road (opposite Commercial Way which is just inside Southwark) but nothing about a substation there on any page.
  [0] Estimating off the National Grid map, it's roughly vertically centred between Lewisham and Greenwich DLR stations - absolutely not in Southwark!
  [1] I feel like I'm going mad - the number of pages I found whilst trying to find the exact location that said the same thing under the same map is honestly discombobulating.

  By zimpenfish 5 hours ago

• It’s easy to find on https://openinframap.org

  By fanf2 2 hours ago

• Impressive piece of work, first time I’ve heard of this.
  I had heard that tunnels were a good first step for rolling out super conducting cables, but that doesn’t seem to be a thing.
  Superconducting cables have progressed a lot. I’m assuming that setting up a cryogenic system to keep cables cool enough, in a confined space wasn’t thought to be worth it.
  The tunnels look tight enough, and boiling liquid nitrogen from a leak could cause asphyxiation I imagine.

  By zeristor 10 hours ago

• "I had heard that tunnels were a good first step for rolling out super conducting cables, but that doesn’t seem to be a thing."
  Yeah tunnels underground would be better for superconducting cables, but it is indeed not really a thing as the cooling and installing and maintainance would be waaaay more expensive, than just using higher voltage. Or if one really cares about the loss, use direct current - but we are talking aber very small distances here.
  If superconducting would be easy, we likely just would have fusion plants everywhere with no need for transporting electricity long distances.

  By lukan 9 hours ago

• > Or if one really cares about the loss, use direct current
  I thought direct current had higher transmission losses vs AC

  By ferfumarma 7 hours ago

• AC has higher losses over a transmission wire because of the changing magnetic field that it induces which creates losses.

  By tallowen 7 hours ago

• There's a cool effect where if you hold a fluorescent tube under a high voltage power line, capacitive coupling from the varying electric field causes it to light up. Some energy is continually leaking out via this route, the tube just reveals it. (Magnetic induction too)
  https://youtu.be/0D50Dcvzkr4

  By HPsquared 5 hours ago

• Tesla vs. Edison is back! :)

  By teo_zero 7 hours ago

• Not if it's really high direct current, like https://en.wikipedia.org/wiki/High-voltage_direct_current
  Problem is they need these https://en.wikipedia.org/wiki/HVDC_converter , for these https://en.wikipedia.org/wiki/HVDC_converter_station which are expensive. So it depends on circumstances, how and when its economically feasible to use HVDC, or not.

  By LargoLasskhyfv 7 hours ago

• From Gridlock to Grid Power: The Promise of Superconducting Cables
  https://ee.eng.cam.ac.uk/index.php/2025/09/22/from-gridlock-...
  An interesting article, I’ll download the IoP report and maybe read it.
  But it talks about doing the hard work to improve the Technological Readiness Level from 7 to 9. Although these cables need rare earths so might be problematic.

  By zeristor 8 hours ago

• The tunnels are just for ease of maintenance.
  https://en.wikipedia.org/wiki/London_Power_Tunnels

  By mr_toad 9 hours ago

• The way the regulator regulates capital returns incentivises overspending on lavish infrastructure works as a way to return more profits back to the shareholders.

  By everfrustrated 2 hours ago

• > The tunnels are just for ease of maintenance.
  The fact that the tunnels are 50 meter underground leads me to wonder if their main requirement comes from national security needs.

  By locknitpicker 8 hours ago

• London underground is prime real estate - to find a level and avoid
  * the recent new massive and extensive sewer tunnels,
  * the secret basement extensions of the ultra rich, multi story archival storage, vaults, etc,
  * the new underground tunnels (rail / subway for US readers),
  * old roman and other buried but still 'conserved' architecture,
  * crypts, graves, plague pits,
  * WWII UXB's etc. etc
  is a hell of a 3D needle to thread - there's > 2,000 years of urban layering in that small area.

  By defrost 8 hours ago

• And unmapped (and changing, inadvertently diverted) rivers

  By OJFord 7 hours ago

• There’s Probably simply too much city in the way on higher levels.
  If you wanna knock out the grid, hit the substations and power plants.

  By ant6n 8 hours ago

• I think tunnels like this are more about future-proofing access than betting on a specific technology

  By BrtByte 6 hours ago

• > Superconducting cables have progressed a lot. I’m assuming that setting up a cryogenic system to keep cables cool enough, in a confined space wasn’t thought to be worth it.
  Yeah, the cost isn’t worth it.
  Buying two transformers to step up the voltage on one end and step down the voltage at the other end is going to be several orders of magnitude cheaper than actively cooling cables to 20K for their entire length.

  By quickthrowman 4 hours ago

• I'm looking forward to the next Amendment to the 18th Edition's conduit-fill factors.

  By sowbug an hour ago

• Cables on overhead high voltage lines are mounted using stacks of ceramic insulators, but here they seemingly just sleeved in some protection and hang on a tunnel wall. Why is that?

  By nopurpose 9 hours ago

• Overhead conductors use air as the insulator. Underground cables use an insulating jacket. In the past it was really difficult to build cables with voltage ranges in the 10s of thousands of volts without additional complexity like a dielectric oil being pumped through the cable. I think modern dielectrics are significantly better though.

  By VBprogrammer 8 hours ago

• Modern cables with XLPE insulation can handle very high voltages without active oil cooling, here’s a 345/400kV rated underground cable assembly rated for 90C: https://assets.southwire.com/adaptivemedia/rendition?id=332f...

  By quickthrowman 4 hours ago

• Yeah, the wires in the new London tunnels are XLPE. Despite being first used in the late 60s it took a long time to be commonly used. Though much of the infrastructure around is still very old.

  By VBprogrammer 2 hours ago

• Cost, mainly
  The cost of oil insulated cables that can do 132kv is about £900 a meter. Whilst there are HV cables that exist on the outskirts of london, they are much rarer in zones 1-3.
  I assume that the cost of pylons with raw cables is much much cheaper. The problem is planning permissions, physical clearance. planning permission and now one wants to live near HV cables (that they know of. There are a bunch of 33kv cables buried outside posh people's houses in zone 5, and a bunch in canals.)

  By KaiserPro 8 hours ago

• Overhead high voltage conductors are not insulated with a coating, probably for many reasons but certainly for cost and heat dissipation.
  That means the path through the air to some conducting materials needs a certain distance, and that even when wet or iced over or whatever can happen up there.

  By jo909 9 hours ago

• Overhead lines need big ceramic stacks because the air is the insulation. In tunnels, the insulation is in the cable itself, and the tunnel just provides structure, cooling, and controlled geometry

  By BrtByte 6 hours ago

• The tunnel is too small to use air as an insulator so they use cable assemblies with multiple layers of insulation.
  Here’s some high-voltage cable spec sheets that show a cross-section of the assembly for voltages above 69kV: https://www.southwire.com/wire-cable/high-voltage-undergroun...

  By quickthrowman 4 hours ago

• essentialy no choice in putting infrastructure underground as the cost's and delays in putting a power corridor right of way through is unthinkable, they will almost certainly be useing old established locations for transformer substations that have the required set backs and other services, which must from time to time, come to the frenzied attention of developers, agast ,that they cant relieve someone of this "vacant" land

  By metalman 8 hours ago

• So tunneling isn't just an engineering choice, it's a planning survival strategy

  By BrtByte 6 hours ago

• "In total, the £1 billion London Power Tunnels 2 (LPT2) project, which began in 2019, spans 32.5km across seven South London boroughs from Wimbledon to Hurst."
  In spite of devolution and the so called "levelling up" programme for other parts of the UK, London obviously continues to be heavily subsidised by the rest of the UK.

  By nephihaha 7 hours ago

• "London obviously continues to be heavily subsidised by the rest of the UK"
  This is a farcical comment. Were you being sarcastic? The tax revenue from London massively subsidises the rest of the UK. The investment happens in London because you can guarantee it will make a return, and quickly.

  By EmbarrassedFuel 6 hours ago

• The real reason London is rich at all is because it was a trading depot with the continent. It made money from goods leaving England, and entering England. Later on, like Paris, it became wealthy off running an overseas empire, and when that empire vanished it turned to nearer territories.
  London has centuries worth of investment from everywhere else based on that. That money has stayed there, and money is spent constantly on infrastructure which helps it make more money. Contrast this with Liverpool, Cardiff or Belfast which suffered decades of decline for various reasons and a fraction of the investment.
  If the capital had been moved to Liverpool back at some point in the Middle Ages, then that would have remained a wealthy city instead of becoming a basket case in the eighties. The presence of the civil service and government alone would have kept Merseyside wealthy, and would have made it a huge tourist centre. Bigger than now, and even that was mostly to do with the Beatles.
  By the way, the state funded Wembley refit cost more than the construction of the Scottish Parliament. Guess which one got all the negative press?

  By nephihaha 6 hours ago

• Let me put this another way. If I got given a very well paid civil service job, I would end up paying a lot of tax in return. And if someone paid for my house to be renovated and build the best utility and transport connections, then the value of it would go up.
  And if mass media continued to promote my area continually then the value of my home would also go up. I would get given higher wages to cope with the increased cost of living there. We would get more tourists visiting my area, and firms and non-doms would set up there because of the positive image.
  Much like London.

  By nephihaha 6 hours ago

• You’re only explaining potential reasons why London is generating higher value.
  Which is still implicitly accepting the fact that London does create more value that then goes to the rest of the UK rather than the reverse.

  By hshdhdhj4444 3 hours ago