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Discussion > Questions for electrical transmission engineers

I came across a Met Office document "Safe Keeping. Protect your world with our climate science and services".

Essentially a sales document, with scary headings like: "Climate change is an urgent global issue that has already begun to impact upon every nation, influence the way we live and threaten our very survival".

One page is headed "Sourcing Energy" and mentions a study into "climate change and its potential impacts on the UK energy industry". It includes the following:

Q: Are the effects just limited to generating power?

A: No. Towers, cables, poles, conductors, insulators and transformers are all involved in transporting energy to our homes and businesses, and can be vulnerable to damage from wind, heat, flooding and lightning which are predicted to increase as the climate changes.

For example, outside the UK's cities cables hung between poles expand and sag in high temperatures. This means that cable 'de-rating' – reducing energy flow through cables to lower their temperature – will be needed more frequently as our climate changes to keep them a safe distance from the ground. (...)

My question to electrical transmission engineers:

- Is this true? Do overhead cables sag significantly more than normal when the weather is hot?

- Are overhead cables heated sufficiently by the current they carry to sag significantly more than when not carrying current?

- Is it plausible that extremely hot weather would require a reduction in current carried by lines to avoid excessive sagging?

May 27, 2013 at 10:11 PM | Registered CommenterMartin A

Martin, I can't answer your questions, but can correct the Met Office site. There are no "predictions" coming from the IPCC, where are they batting their predictions from?

May 28, 2013 at 7:09 AM | Unregistered Commentergeronimo

Yes, sag is a problem, as is the tension caused by the weight and contraction due to cold weather (i.e. it can break).

You can try this yourself. Piece of string/wire. Tighten it across two points. Mark the distance between the points. Add a small amount to this distance length and see the sag. The sag is pronounced even for small additions.

Do cables sag differently during the seasons? Yes.

Do they heat due to the electrical load? Yes

I am not an expert, do I believe any climate changes over the next 50 years will have any effect on the current engineering solutions? I doubt it.

Current cabling should be within the temperature and load ranges.

The challenge is that whilst you can alter the length of a cable between pylons. Making it too short could cause it to snap in winter.

Therefore you get into the territory of repositioning your pylons, which is an altogether bigger issue.

May 28, 2013 at 9:02 AM | Unregistered CommenterJiminy Cricket

I find it hard to believe that cables that can survive a temperature range from -10 to +30 degrees over the course of a year without snapping when under power load suddenly snap when it goes from -9 to +31 instead (an average temperature rise of 1 degree, which is not even postulated for the next 50 years anyway)

It's more scarum bunkum.

May 28, 2013 at 9:29 AM | Unregistered CommenterTheBigYinJames

The document is just a sales document, Climate Services for Dummies.

Like any Management Consultancy, the Met Office is just trying to show you that you have a problem you don't actually have and then sell the solution you do not really need.

The whole document is crap, but it works. And the more you charge for the services, the happier people seem to be.

But it is obvious the Met Office should not be trusted with actual science. All the big consultancy companies of the 1990's were broken up from their accounting/audit practices precisely because of the conflicts of interest.

The Met Office is using FUD to make money, whist also being the supplier of FUD facts.

Strange world big consultancy.

May 28, 2013 at 10:14 AM | Unregistered CommenterJiminy Cricket

The cabling engineers factor in a large margin for error, as currently there are surviving cables in much hotter countries like Saudi Arabia (which will not be getting hotter if CAGW is true as the equatorial region do not heat) this is just scaremongering.

May 28, 2013 at 10:21 AM | Registered CommenterBreath of Fresh Air

it is not the current infrastructure they are trying to sell services for. They want a piece of the Greentopia.

They will sell services (essentially "value-added" knowledge) to large companies as the green infrastructure is rolled out.

To the transmission companies, for a lat. and long. they will sell the likely temp range.

To power companies they will sell info to help them determine peak loads (when air con is on in the summer.)

They want a slice of the Greentopia that is all. Which is fine, but it is not fine if they are the guardians of the base data.

May 28, 2013 at 10:25 AM | Unregistered CommenterJiminy Cricket

As the temperature profile for a location changes over tens of years due to CAGW, the sag characteristics of the cable will change. As you cannot easily move a pylon then this has to be designed into either the cable length, or into the pylon itself with tensioners.

This is what they are highlighting I believe.

Of course if we have cooling based instead of warming then the cables will have a greater break problem not a sag problem.

I would hedge my bet(t)s :)

May 28, 2013 at 10:32 AM | Unregistered CommenterJiminy Cricket

Do cables sag differently during the seasons? Yes.
Do they heat due to the electrical load? Yes

This is obvious to anyone who knows about thermal expansion of metal and electrical resistance. But are the extra sags significant? Does hot weather and maximum load double the sag? Or increase it by just a few per cent?

I was hoping for some specific actual figures from an experienced CEGB or electricity board transmission engineer.

- The normal catenary sag in normal weather.

- The cable temperature rise under max load and the corresponding additional sag.
(I had always imagined that, for a given cable, acceptable transmission power loss set a limit to the current, rather than extra sag due to thermal expansion of the cable)

- What is the additional sag in extreme hot weather.

- What safety factor is used in engineering overhead cables.


May 28, 2013 at 10:43 AM | Registered CommenterMartin A

You think with all our Big-Power funding, we'd have someone here to answer your question :)

May 28, 2013 at 10:50 AM | Unregistered CommenterTheBigYinJames

If the solution designed today for a specific location has a life of 40 years, and the design parameters change due to load profiles and climate, then you can only answer the question "is the solution valid in 40 years" by looking at the modelled predictions.

So I have no problem with those statements in the brochure.

Maybe there are long spans across gorges or rivers or motorways where these things become critical.

It is about the change. I see no problem asking the question. How you determine the answer, and what weight you give Met Office data, is another question entirely. A qualified engineer may just say in 10 seconds, "of course not", but he still answered the question.

May 28, 2013 at 10:58 AM | Unregistered CommenterJiminy Cricket

One of the big fears in business is when your once all conquering USP (Unique Selling Proposition) starts to wane.

Normally it is increased competition that removes the "Unique". However the Met's case is in itself unique as it is their own observational data that is removing the compelling need for their products.

This could prove to be a very difficult situation for the MO's management as they have taken the following business mantra as literally as possible:-

"Before you can begin to sell your product or service to anyone else, you have to sell yourself on it."

May 28, 2013 at 11:09 AM | Registered CommenterGreen Sand

The transmission companies will have some nice sexy planning programs.

It will give the height and distance between pylons. All we need to know, is temperature range one of the parameters. Do you design differently for the Isle of Skye than for Bournemouth based on their temperature profiles?

I would guess yes. But I have no idea.

May 28, 2013 at 11:13 AM | Unregistered CommenterJiminy Cricket

A quick bit of Googling tells me that most power lines are steel-cored and that the coefficient of thermal expansion of steel is 0.000013 metres/metrelength/degK.
From the back of my fag packet:
Say cable length between pylons = 300m.
Expansion for 1 degree temp rise = 300 x 0.000013 = 0.0039m.

I rather doubt that 4 mm is anything to worry about.

May 28, 2013 at 12:03 PM | Registered Commentermikeh

But it is not the degree rise... and as expansion joints on rail tracks show, it is an issue.

It is the daily interval min-max and the shifting of that of over decades for one location.

Plus of course the electrical load is part of the temperature profile.

May 28, 2013 at 12:54 PM | Unregistered CommenterJiminy Cricket

"Before you can begin to sell your product or service to anyone else, you have to sell yourself on it."
May 28, 2013 at 11:09 AM Green Sand

But, on the other hand:

“It is difficult to get somebody to understand something, when their salary depends on their not understanding it.”

May 28, 2013 at 1:54 PM | Unregistered Commentersplitpin


You'd think with all our Big-Power funding, we'd have someone here to answer your question :)

Big-Power now is it? It'll be Big-Capitalism next. People making money out of things ordinary folk freely choose to buy, refusing handouts from the statist elite. It's disgusting and it's what funds us, that's all we know. But where all that massive money ends up is a mystery.

While my own cheque is still in the post I say great thread Martin. I'm gonna assume the overhead cables sag problem is complete nonsense. What is truly extraordinary is how much nonsense is built upon lower levels of nonsense in the climate area. Not turtles but turkeys all the way down?

The only analogy I can find is the opposite I see in my own field of software, where the deep and beautiful ideas of Alan Kay influenced the little known Jean-Marie Hullot, whose Interface Builder for the now-defunct NeXT Cube was gratefully used by Tim Berners-Lee to invent and prototype the original World Wide Web from 1989 onwards. I wrote a bit about that remarkable story in Keep It Simple, Stupid for Sublime Magazine in 2007. It's from this stream of enterprise and accident that Objective-C has now become the third most popular programming language in the world according to Tiobe. Who would have predicted that ten years ago, let alone thirty, when I first heard the language described by one of its inventors, Brad Cox, at an early conference on object-oriented programming in London?

But that for me is great thinking and design laying a foundation for its corollaries. What we have in climate is not only the opposite but a menace to so many other goods we have come to enjoy in what is still, not least due to Tim, a remarkably free world. So brick by brick, power cable by power cable, it has to be taken apart.

May 28, 2013 at 2:39 PM | Registered CommenterRichard Drake

Objective-C is the programming language we are forced to use by Apple for writing apps, which explains its sudden popularity in the last few years. It's a complete pig to use, by all accounts (I haven't tried) but if you want to write iOs apps you have to use it.

I'll be sure to look out for low swinging power cables on the way home, though.

May 28, 2013 at 2:59 PM | Unregistered CommenterTheBigYinJames

JC; I used the expansion for 1 deg rise because that is the order of magnitude change which is "attributed" to AGW. So the MO comment about increased sag is complete twaddle.
Someone with specific knowledge - and better maths than me - could probably work out whether the minute extra sag due the the cable expanding would be compensated by the vertical "growth" of the pylons.
The comparison to rail tracks is a bit apples and pears. The tolerances have to be much finer as the rail is constrained. With today's long welded sections expansion obviously can be an issue.

May 28, 2013 at 3:40 PM | Registered Commentermikeh

TBYJ: I know, I know. People who are unfamiliar with Objective-C dislike it, as is true with all X where people are unfamiliar with X. I certainly viewed it as a kludge in 1983, adding Smalltalk syntax to C, and assumed it would not last long. Then in 1986 I saw Jean-Marie Hullot demo a version of Interface Builder on the Mac in a object variant of Lisp. We talked about making this more commercial and were debating the target language when Steve Jobs (or an underling) also spotted Jean-Marie's work and he took the trip from Paris to Silicon Valley to join NeXT. He told me at a conference later - probably the European Conference on Object-Oriented Programming in Nottingham in 1989, which I helped to organise - that they'd considered C++ but it wasn't dynamic enough for what he wanted to achieve so he reluctantly agreed on Objective-C. (Smalltalk or Lisp were not considered viable for the ordinary programmer at that point by others in NeXT.) When Apple took Jobs back and integrated this work as part of Mac OS X the Objective-C was retained. In 2007 the initial story was that all apps for the iPhone would be in HTML, CSS and JavaScript. But that very quickly changed as the need for 'native' access to all the new accelerometer goodies became clear. Thus a series of quick-and-dirty choices were part of what I called the enterprise and accident that have made the world a better place - and Objective-C lives on far longer than either Jean-Marie or I could I'm sure have imagined in 86.

Nevertheless, the beautiful aspects of Interface Builder were admired by the likes of Richard Drake and Tim Berners-Lee, leading Tim to have a NeXT cube at CERN in 1989, on which all the prototypes of the Web were produced. It gave us something to chat about when we met at MIT seven years later.

All this may be slightly off topic for this thread :) The point was that good ideas have a subtle, knock-on effect and this can finally lead to breakthrough and blessing for the masses. Note that Kay, Hullot and Berners-Lee made precious little money for their brilliant efforts. Marc Andreessen and Jim Clark of Netscape sure put that right - as have a few others after that, I gather. And that's the kind of stuff, the process of real invention, that politicians will never, ever be able to foresee. But given a set of ideas saturated in stupidity they can do enormous damage. There again, we're still here and the pennies seem to be dropping.

May 28, 2013 at 4:21 PM | Registered CommenterRichard Drake

So the MO comment about increased sag is complete twaddle.(...)
May 28, 2013 at 3:40 PM mikeh

Well, that's what it smelled like to me.

I'd still be interested to know the actual figures from real practice.

I wonder if the MO was talking about lines at the 132 kV - 400 kV level or at the 11 kV - 33 kV level. If it was rubbish, it hardly matters, I suppose.


Objective-C has now become the third most popular programming language in the world according to Tiobe.
May 28, 2013 at 2:39 PM Richard Drake

Gosh, has it really? Years ago (~1986), I suggested to the group I was working with (that wanted to get cracking with OO programming) that we should use Objective C. But we went for C++. I had always assumed that Objective C had been lost without trace.

May 28, 2013 at 4:38 PM | Registered CommenterMartin A

Well I think thermal expansion from climate is moot anyway.

The cables are aluminium/steel and can reach temps between 120-220 C depending on electrical load.

So in this case CAGW is the driver due to demand load.

May 28, 2013 at 4:38 PM | Unregistered CommenterJiminy Cricket

Great stuff Martin. I'm not sure I trust the Tiobe stats to the fourth significant figure but it's way up there these days. And if you're interested in who else is eligible to join us in the guild of ancient object programmers, look for the Object Pascal in this CV.

May 28, 2013 at 4:44 PM | Registered CommenterRichard Drake

Say cable length between pylons = 300m.
Expansion for 1 degree temp rise = 300 x 0.000013 = 0.0039m.

May 28, 2013 at 12:03 PM mikeh

Or, taking JC's 200°C temperature rise under maximum load (I'm guessing ambient of 20°C), that would mean ~0.8m extra sag for 300m spacing between pylons.

I'd have guessed that 0.8m was a fraction of the normal variation of sag due to random variations in cable weight, tensioning, pylon installation. In other words negligible.

May 28, 2013 at 5:40 PM | Registered CommenterMartin A

do you also need to factor in the replacement cycle for these cables - unless they are built to last indefinitely? If they are replaced every 50 (?) years, then they can be replaced by cables with new planning assumptions - problem solved.

May 28, 2013 at 7:12 PM | Unregistered Commenterdiogenes