A train-sized tunnel is now carrying electricity under South London
24 comments
·December 20, 2025nopurpose
VBprogrammer
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.
jo909
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.
zeristor
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.
zeristor
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.
lukan
"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.
mr_toad
The tunnels are just for ease of maintenance.
locknitpicker
> 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.
defrost
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,
* 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.
ant6n
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.
luma
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.
jo909
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...
probablypower
First guess (may be wrong) is to manage thermal expansion/contraction constantly on a micro-scale.
nraynaud
I would also add that there some slack for repairs.
locknitpicker
> 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.
defrost
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)
metalman
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
jimnotgym
So Everywhere else in Britain it is 'too expensive' to put cables underground. Just goes to show how London centric this country is.
docflabby
London and the south east is the UK in economic terms. Its the only those 2 regions which are net contributor to taxes
https://www.ons.gov.uk/economy/governmentpublicsectorandtaxe...
zeristor
Manchester is burgeoning, still someways behind London.
It’d be super, smashing, great! for the cities to be far better connected together across the Pennines.
jasoncartwright
Sounds like you are trying to compare the many hundreds of miles (thousands?) of transmission cables needed to cope with the massive geographic change of generation sources to this ~20mi cable system.
There are many examples of how the UK is London-centric. This isn't one of them.
VBprogrammer
It sounds like sour grapes. London contributes nearly a third of the UKs tax income. It has a higher population than the whole of Scotland.
Not to mention that over ground wires are manifestly better in every dimension except for aesthetics.
This is a great example:
nkmnz
What are the costs of an underground cable in the cities you’re considering, expressed as:
1. Cost per kWh transmitted?
2. Cost per person served?
3. Cost per pound of GDP generated?
Please provide this for London and the other locations you have in mind.
ra
I know what you're saying, but maybe there is a tipping point when the economics is worth it.
Cool to see cycling down there - much safer than on the roads above.
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?