High-voltage distribution line cable insulation question

[lichurbagan]

I am from a city of overhead cable chaos like this.

I often come across news of people including children being electrocuted by touching overhead transmission lines while playing in the rooftop, while picking up a ball from a tin roof etc.

I feel both sorry and confused when I read these news. I have always known that electrical wires have rubber insulations around their whole body. If that is the case in general, how can people die merely from touching a live wire? Aren't there supposed to be insulation? Do the accidents take place due to uninsulated cables or worn-out insulations?

[ahbushnell]

Transmission lines normally don't have insulation. 

[lichurbagan]

Transmission lines normally don't have insulation.

What's the reason behind that?

[IanB]

On overhead transmission and distribution lines the air itself is the insulation.

However, these high voltage lines are supposed to be suspended high in the air beyond the reach of people. They are at the top of tall towers or poles that cannot easily be climbed.

One case where people can come into contact with high voltage wires is on electric trains. Children sometimes climb on top of trains where they can touch the overhead electric wires.

[mtwieg]

Transmission lines normally don't have insulation.

What's the reason behind that?

A few guesses:
1. Cost. For >5kV lines, that would have to be quite a bit of insulation, increasing the price a lot. Plus that would mean much less wire per spool.
2. Insulation would make the cables a lot heavier, meaning you'd have to reinforce how you hang them.
3. Insulation would make cables run much hotter per the same current due to poorer thermal transfer.

Best way to insulate HV power transmission is to bury it. But of course that's not possible in many cases.

[lichurbagan]

Thanks to all for answering and explaining. It looks like, my country has no solution to this problem. Here Tx lines are not out of reach. Children are being electrocuted even when they are on the rooftop of a 2 or 3 storied building or climbing on top of a single storied tin shade house. In a city of overhead cable chaos, uninsulated cables are like death traps.

[themadhippy]

Best way to insulate HV power transmission is to bury it.

theres a slight problem with that idea that means hv cables need insulation before being buried

[Gyro]

I'm sad to say that, having looked at several images on the web (I'm sure (hope) they must represent extreme cases), the utility companies or authorities appear to have little or no concern for human safety being a priority. Mains wiring appears to mixed in a tangle with telecoms wiring, aside from the high transmission voltage risk. Presumably the power poles have step down transformers to step the high voltage transmission lines to mains voltage but they are obscured by cabling in any image that I can see.

It's hard to see how such a situation could be remedied without massive disruption. Installation of underground services, with substations providing step-down to buried mains voltage services from buried high voltage cable (transitioning from overhead pylons in the the suburbs where population density thins) is the solution, as in many other countries. It would be necessary to dig up the streets to install the cabling and then to disconnect each building in turn from overhead and attach to the new underground service. It wouldn't be possible to just replace the overhead services without disconnecting entire streets or blocks for a significant period of time - it's a case of turning the high voltage off and cutting away all of the tangled mess.

The closest I can think of to your current system is the residential wiring used in the US and other counties, where high voltage poles are run along residential streets, with step down transformers attached each and mains voltage strung to a few, or individual properties. Telecom services are run much lower down the poles and then strung in the same way. Spacing is at least 10s of feet between the poles and properties. No tangles, just straight runs.

Such a system has no place in a crowded city environment where buildings have encroached both in density and height - it's hard to identify where the blame lies, with the utility companies? poor enforcement of planning laws? Or with the building owners who have encroached on the safe spacing from the lines? Either way, it is going to involve great disruption, cost and the will to accept the pain of rectifying it.

[IanB]

Thanks to all for answering and explaining. It looks like, my country has no solution to this problem. Here Tx lines are not out of reach. Children are being electrocuted even when they are on the rooftop of a 2 or 3 storied building or climbing on top of a single storied tin shade house. In a city of overhead cable chaos, uninsulated cables are like death traps.

Unfortunately, it may be that the cables were there first, and then people erected buildings underneath them afterwards. Especially informal buildings like tin shade houses.

The other part of the story is education. To teach children from the earliest age that electric cables are dangerous and to stay far away from them.

[Zero999]

The picture attached to the original post appears to only show low voltage cables, which are all insulated. There is only a shock hazard if the insulation is damaged and the cable is carrying a hazardous voltage.

Note that low voltage generally means under 1kV, which is still dangerous but one has to be in physical contact with the conductor to receive a shock. High voltages can arc through the air, so don't have to be touched to present a shock hazard.

[themadhippy]

chiang rai's finest

[mtwieg]

Best way to insulate HV power transmission is to bury it.

theres a slight problem with that idea that means hv cables need insulation before being buried

Right, I didn't mean that burying the cable removes the requirement for insulation on the cable (if anything, might need even more expensive insulation). I meant buried cable is more safely insulated than hanging cable.

Thanks to all for answering and explaining. It looks like, my country has no solution to this problem.

Perhaps no technical solution. Policy is probably the better remedy, meaning regulations should be issued and enforced. In the US, I believe utility companies are held responsible for maintaining the power lines in a safe manner.

[lichurbagan]

Thanks again everyone. I'd like to see how an uninsulated high voltage transmission cable look like. How to identify them at a glance? If anyone has any close picture, please feel free to post.

[IanB]

Thanks again everyone. I'd like to see how an uninsulated high voltage transmission cable look like. How to identify them at a glance? If anyone has any close picture, please feel free to post.

In the picture posted by themadhippy above, there is a transformer sitting on the cross bar with uninsulated wires descending to it at the top.

Uninsulated conductors can usually be recognized in two ways:

1. They will be supported by insulators and attached to insulated bushings where they terminate. Insulators are the zig-zaggy things made of porcelain, glass or plastic.

2. They will be surrounded everywhere by a big air gap, and will not be touching any other wires or structures.

You can usually tell high voltage cables because they will look "scary", but all electric cables should be considered dangerous, insulated or not. You can never tell if there might be a fault, or a weakness in the insulation, or an uninsulated section where a connection is made to a transformer.

[IanB]

In the picture below, the wires suspended far apart in free air are uninsulated, but the cables descending in the lower left corner are insulated because they are all touching.

[ejeffrey]

Thanks to all for answering and explaining. It looks like, my country has no solution to this problem. Here Tx lines are not out of reach. Children are being electrocuted even when they are on the rooftop of a 2 or 3 storied building or climbing on top of a single storied tin shade house. In a city of overhead cable chaos, uninsulated cables are like death traps.

Overhead cables are cheap to install but they do need constant maintenance.  For instance, our power company checks tree growth near our overhead lines annually, and trims them if needed.  The permitting process for buildings requires demonstrating adequate clearance in all directions from medium voltage (> 1 kV) transmission lines.  Only low voltage (< 1 kV) goes from pole mounted transformers to buildings, and that is insulated.  The insulation here is not inspected as often as it probably should be, but it comes into a building via a mast that keeps the wires away from most things including people on the roof.

Another aspect of safety is having exposed metal structures like metal roofing panels securely bonded to ground.  This prevents the roof panels themselves from developing a dangerous potential due to contact with a live wire, although touching a live conductor while on the metal roof would still be dangerous.

I think the sad reality is that if you don't have a robust set of building codes with permitting and inspections, and liability for injuries caused by negligence, that dangerous conditions will be common.  Electrical safety is sufficiently non-intuitive that even well meaning individuals trying to build a safe system will frequently do the opposite, much less people who aren't thinking about electrical safety at all.  Buried power lines are a possible part of the solution, especially in dense urban environments, but they still leave plenty of possible dangers.  More to the point, they significantly increase costs, and that is normally the root cause.