Obscure Objects of Transport Beauty: third rail insulator pot

Be they ever so humble, there are certain objects from the world of transport which have an unexpected beauty of their own. We’ve already looked at the ubiquitous reflective road stud, so commonplace that its intrinsic attractiveness is generally overlooked. It’s time for another, which will be familiar to users of the rail network in southern England (and around Liverpool in the north-west of England), the London Underground, and other metros around the world.

It’s the third rail insulator pot:

Porcelain insulator pot. By Daniel Wright [CC BY-NC-ND 2.0]
Porcelain third rail insulator pot. By Daniel Wright [CC BY-NC-ND 2.0]. More pictures here

It was manufactured by Doulton (there’s a manufacturer’s mark on the other side) for the mainline railway network in southern England, and comprises a white porcelain base with a metal bracket on the top. To be exact, it’s a British Rail type 100, and according to the date cast into the metal bracket, was made in 1978. It’s a lovely object, all curves and shine; a buxom beauty cool to the touch, with a topping of steel. It’s one of those objects which just looks right, because it is right. It is a perfect marriage of form and function.

In real life, insulator pots aren’t usually well seen, so the beauty of these small objects tends to go unnoticed. Unless they’ve been recently installed, they are usually covered in a layer of brake dust, and often oil from passing diesel trains. So they become a shade of brown-black at one with the sleepers and ballast over which they sit; becoming invisible in plain sight.

Third rail insulator pots at Woking, Surrey, UK. By Daniel Wright [CC BY-NC-ND 2.0]
Third rail insulator pots at Woking, Surrey, UK. By Daniel Wright [CC BY-NC-ND 2.0]

Once they’ve been cleaned up, it’s not just me who recognises the artistic merits of a porcelain third rail insulator pot. Proper photographers like them too. There’s one in the online portfolio of Tristan Appleby (it’s here, and it’s a much better photo than mine. Pfft.)

Insulator pots perform a simple but essential function. They insulate the electrified rail which provides power to trains from everything else around it, except the ‘shoes’ on the train which are in contact with the rail to collect the electricity. Ground-level railway power systems have been used since early electric railways such as Volk’s Electric Railway in Brighton, UK. The VER opened in 1883 and uses a third rail laid between the two running rails to supply power to its trams. Ground level power supply was installed on London’s first tube railway, the City and South London, in 1890, and the Underground has never looked back since.

It’s not an obsolete technology either. Ground-level power supplies are still being installed in newer urban metro systems such as the Docklands Light Railway (London, UK, 1987) and the Copenhagen Metro (Copenhagen, Denmark, 2002) although the exact arrangements vary between systems.

What you wouldn’t want to use a ground-level power supply for is a large mainline railway network, with routes of over 100 miles long, and trains running at speeds of up to 100mph. It’s unfortunate then, that that is precisely the system used today across huge swathes of the railway network in southern England, and from which the insulator pot above originates.

The London and South Western Railway (LSWR) started installing third rail electrification in 1916, with the third rail supplying power initially at 600 V DC (direct current) and later 750 V DC. Following the merger of the London and South Western Railway and various other railways to form the Southern Railway in 1923, third rail electrification was extended. In doing so, the Southern Railway converted an earlier electrification system supplying 6,600 V AC (alternating current) through overhead wires, which had been installed by the London Brighton and South Coast Railway from London Victoria to various locations in south London, starting in 1909. Ironically, it is overhead wires and an AC supply that are the standard electrification system for the mainline railway today, although they are rated at 25,000 V AC. Technically, the conversion was arguably a backward step, but the much greater route mileage electrified with the LSWR’s third rail system meant that the Southern Railway settled on that as its standard.

Third rail systems have several problems when used on mainline networks, rather than the urban metros where they can generally be found today. For a start, at high speeds, it is hard to maintain reliable contact between the third rail and collector shoe on the train. When the shoe bounces off the rail, electricity can arc across the gap, causing damage to electrical equipment (and a bright blue flash too; it’s one of the signature sights of the third rail system in southern England). Low voltage third rail electricity supplies also require many more substations than the equivalent amount of track electrified by 25 kV AC overhead wires, because the low voltage needs more boosting. Third rail supplies are also vulnerable to icing up in the winter, or getting buried under leaves or snow. Mind you, overhead lines sometimes blow down in strong winds, so it’s not like third rail supplies are alone in suffering from bad weather.

Nevertheless, southern England’s third rail power supply is considered so antiquated that the UK government has authorised the national railway infrastructure owner Network Rail to replace it on the line between Southampton and Basingstoke with 25kv AV overhead supplies, as part of an ‘electric spine’ railway that will then continue to Reading, Bedford and Sheffield along previously unelectrified tracks. The Southampton-Reading conversion is being viewed as a pilot project to judge whether it is worth converting the rest of the third rail network to overhead line supply. You’d think that with huge portions of the UK railway network still completely unelectrified, the government would have better things to do than spend money re-electrifying an already electrified railway, but there you are. That’s the way things get done here. Or don’t, in the case of the remaining unelectrified lines.

For now, though, the humble third rail insulator pot still has an important role to play in supporting the third rail around southern England and on the other 750 V DC third rail system centred on Liverpool; Merseyrail.

Insulator pots are very simple in concept. The porcelain glazed base is the actual insulator (non-conductive to electricity), and embedded into the top is a metal bracket which holds the actual third rail. The insulator sits on special fittings attached to the railway sleepers. Simples. Manufacturers have included Doulton (see one of their beautifully engraved advertisements from 1932 here) and Bullers. Doulton’s descendant company Allied Insulators still supplies porcelain insulators. There are two main designs – a standard one (like the one in the picture) and a flatter one. The flat ones are used at the ends of stretches of third rail, where the rail dips down towards the ground to ensure that as collector shoes come into contact with the third rail they do so smoothly, and don’t get knocked off.

The designs have varied a little over a time, and sometimes glass was used instead of porcelain (see a Pilkington third rail glass insulator here). London Underground’s insulator pots have smaller brackets than the mainline ones and look a little, well, the best word I can think of is ‘dinkier’.

Two London Underground insulator pots supporting the third and fourth rails in a deep level tube station. By Jess J [CC BY-NC-ND 2.0] via this flickr page
Two  porcelain insulator pots supporting the third and fourth rails in a London Ujnderground deep level tube station. By Jess J [CC BY-NC-ND 2.0] via this flickr page

Early insulators on the London Underground (before it was the London Underground at all, really) were rather less elegant, and can still be seen in situ at the closed Aldwych station. The London Underground is a happy hunting ground for admirers of porcelain insulator pots because the network uses a third and fourth rail to supply its trains with electricity, so it needs double the number of insulators per route mile than the mainline railway above ground.

All the above excitement is without even beginning to look at insulator pots outside the UK. The New York City subway and elevated railway used rather attractive-looking brown insulator pots, as can be seen at the Electric City Trolley Museum in Pennsylvania.

Needless to say, technology has caught up with the porcelain insulator pot. While aesthetically pleasing, the porcelain insulator pot has several drawbacks. For a start,they’re brittle. Drop one when you’re installing it and it will break. They’re very heavy, which makes transporting large numbers of them more difficult. They also (according to an electrical engineer I met when I visited the Woking Railway Electrical Control Room) explode from time to time. Yes, you did read that right. Apparently, if the insulation the porcelain provides isn’t perfect, or the porcelain develops a crack, stray currents can make their way from the third rail to the ground, through the insulator. The result is a loud bang, and bits of porcelain blasted around the local area. It’s clearly not ideal in terms of safety for track workers, passing trains, or even lineside badgers…

At least two companies now make plastic versions of the insulator pot. Allied Insulators, as well as supplying the porcelain insulator pots, also supplies plastic ones (having contracted Rutland Plastics to do the moulding, as you can see here on the latter’s website). A different plastic insulator pot is available from REHAU’s (here). If they don’t have quite the tactile charm of the old glazed porcelain versions, they are still visually intriguing objects. They are also speedier to manufacture, lighter, adjustable in height to ensure consistent contact with the third rail as it wears, and they’re recyclable. That’s progress for you.

Third rail insulators at Woking Railway Electrical Control Room. On the left are the standard and flat versions of the porcelain insulator pot. On the right is one of REHAU's all-plastic versions. By Daniel Wright [CC BY-NC-ND 2.0] via this flickr page
Third rail insulators at Woking Railway Electrical Control Room, September 2013. On the left are the standard and flat versions of the porcelain insulator pot. On the right is one of REHAU’s all-plastic versions. By Daniel Wright [CC BY-NC-ND 2.0] via this flickr page

I can’t help thinking that Network Rail could make a useful income stream by selling to members of the public redundant porcelain insulator pots as it converts the Southampton to Reading line to overhead electrification. They’re beautiful objects to have around the house, supporting plant pots, propping open doors, not to mention providing conversation starters allowing you to recount everything above. What I can’t guarantee is that the person you’re speaking to will be there at the end of it.

How to find third rail insulator pots

If you want to see a British third rail insulator pot, the British railway network in the south-east, south of the River Thames, is mostly electrified on the third rail system. I shouldn’t have to say so, but don’t trespass and don’t approach – the live rail can kill. Insulator pots  can be seen quite satisfactorily from a station platform while you’re waiting for a train. Lines included in the Merseyrail franchise which covers Liverpool and the wider Merseyside area, are also electrified with the third rail system. Network Rail is increasingly moving to plastic insulators as track is renewed, so your opportunities to see porcelain ones will diminish. London Underground has insulator pots supporting its third and fourth rails.

further reading

Porcelain third rail insulator pots are still manufactured by…

  • Allied Insulators (here)
  • BHW Ceramics (here). BHW has a picture of its range. It’s delightful
  • Overhead Line Fittings (UK) Ltd (here). Overhead Line Fittings actually has a downloadable pdf catalogue solely comprising porcelain and glass insulators. It is full of the most delectable objects.

…and possibly other companies too. I’m happy to correct the above list if anyone thinks they have been missed off.

Finally, I should probably point out that this entry hasn’t been sponsored or endorsed by any of the companies mentioned.

correction, 30 January 2014

This entry originally suggested that the plastic insulator produced by Rutland Plastics was that company’s own product. Allied Insulators pointed out that the plastic insulator is in fact one of their own products, with Rutland Plastics contracted to undertake manufacture. I’m happy to make that clear, and have amended the entry accordingly. Apologies for any confusion caused.

18 thoughts on “Obscure Objects of Transport Beauty: third rail insulator pot

  1. yes they are great I’ve 2 1 from the central line Doulton and an AI from the NLL Type 140. I have no idea what the difference between a type 100 and 140 is? I think the last new ones are on the Jubilee line extension they look bespoke. As for the supposed advantages of plastic the height of the pot should not be a problem, as the pot and the rail should be on the same surface the sleeper. The weight is non problem, which means the only advantage is that they don’t explode which I suspect is massively exaggerated. The plastic ones are ugly and of course cheap.

    perhaps the pandrol clip for a future post?

    1. On the Type 100/Type 140 differences. I have a Type 120 in my collection, and I notice that the distance from the bottom of the insulator to the flat part of the bracket where the live rail rests is 120mm. On my Type 100, the distance is 100mm. I don’t have a Type 140, but I wonder if the distance is 140mm?

  2. I just got back from my typical Sunday afternoon stroll and discovered what looks to be an ‘Insulator Pot’, but it is made of glass. I have seen the ceramic ‘pots’ but never one made of glass, so I decided to do some ‘googling’ of glass insulators, which brought me to your very informative blog.

    1. Lucky you. I believe they’re very rare now, and I’ve never seen one. It’s a lovely idea and I imagine they look rather spectacular (assuming you’re into that sort of thing). I’m not sure whether or not they would be more fragile than porcelain ones, if they were made of solid glass, probably not.

      1. It sounds lovely. If I were you, I would clean it up and put it on display in the house somewhere. The glass ones are rare and old. But all the insulator pots are quite fabulous objects, and you can use them as excellent doorstops, paperweights, plant pot holders…

    2. This sounds like it is a Anchor type insulator which were installed midway along each length of conductor rail to prevent the rail moving,the number installed depended on the length of rail

  3. Have just got my very own Southern Insulator Pot from the workmen who are upgrading our local line. A true thing of beauty. I just hope the wife likes it.

    1. Oh dear. There ought to be a society for people like us who are made happy by railway insulators. In terms of photo, you can upload to Flickr or similar and then post a link as a comment, or if you email me a pic I’ll post it as a comment.

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