Streetlights for the third world

[treez]

Hi,
Supposing we want to put streetlights (40 x 40W units, each 30 metres apart) into a third world country.
We have to use aluminium overhead wiring, as copper gets stolen, and is too expensive.
It is powered from a local 240VAC generator, dedicated to this task.
Can i just use the single live wire to send along to the units in a daisy chain, and then connect each lamp neutral to the streetlight pole, which is obviously buried into the ground at its base...then i connect the neutral at the generator to the earth at the generator via an earthing rod.
(this is to save money on a neutral conductor)
Can this be done?

 

[wwfeldman]

It can be done.
But I don't think its a good idea.

If any of the connections to ground at any of the light posts fails, or becomes degraded, the light post is now an open live wire waiting to be grounded, perhaps by an animal, or a person.
What is the quality of the ground (that is, the earth), that it will make a good connection? What is the moisture and mineral content? Does it stay the same through the entire year? How long will it take for the aluminum in the ground to corrode, and the connection fail?

 

[treez]

Thanks, by "aluminium in the ground" you mean the lamp pole?
not a "good idea", but does it save money?

 

[wwfeldman]

I assumed, perhaps incorrectly, that the lamp post would be aluminum.
If the lamp post is wood, or concrete, then a wire has to be run down to earth to make the connection. I think most of my electrical connection comments and safety comments still hold.

Money is an issue, as you said. Aren't safety, ease of installation, ease of repair, and length of service also important?

You would need a real calculation of cost to know which version saves money.

 

[c_mitra]

into a third world country....

Notes from the third world.

Yes, copper wire is no no (it really gets stolen regularly).

But do you really save money by using earth ground as the return path?

The lamp posts are set in concrete and the return resistance can be excessive. If cost is a concern, you can use bare wire as the return (but that is also not recommended if some fault develops).

50/60Hz AC can also induce considerable corrosion at the post (where it meets wet soil). Right, AC does not prevent electrochemical corrosion (at low frequency).

Better option is to use thinner and thinner wire as you go further from the feed point. But physical strength is also a concern when you put them open on the poles.

I guess the cable cost is a sizeable fraction of the total bill but the total load is only 1.6kW and you are perhaps allowed 20% line loss but that is the reality.

My answer is a big NO for using the earth ground as the return path. You will get a bad name.

My suggestion? Use a dual core cable supported by a steel rope stretched between the poles.

 

[betwixt]

Certainly not safe for the reasons already given but I'm not clear on the 'neutral' return path. Are you saying that each pole is individually grounded and you want to attach a ground to the generator as well so the physical soil forms the return path or are you saying there is also a second conductor to carry the neutral return?

I'm worried that if a single overhead cable is used (presumably because of cost and terrain difficulties) you are implying there is no return wire at all. If that is the case, forget it right away, the ground resistance is likely to be magnitudes higher than acceptable and very variable according to ground composition and moisture level.

For Earthing purposes ONLY, it might be safe to ground at each pole but certainly it can't be used as the power conductor.

Brian.

 

[treez]

Thanks , we were advised that the earth as a sole return conductor would be conductive enough, and they wanted to do it like that to save money on wire, but i agree, its a bad idea.

I suspect that in one installation they had actually done it like that, as the reports we received of the performance, suggested that yes maybe there was a very high resistance in the wiring…or……to be more precise….no wire was actually used for the return!…..and the earth was perhaps used as the sole return “wire”.

In one other (different location) case, we suspect that the neutral was earthed at the generator…and that live and neutral wires were taken from the genny to each lamp, but that the neutral at each lamp was also earthed by connecting it to each of the metal lamp posts (which is staked into the earth).
The many lightning storms may have played havoc with this kind of setup.

 

[c_mitra]

In one other (different location) case, we suspect that the neutral was earthed at the generator...

It is a common practice to ground the neutral at one end. That will allow fault currents to go to earth. But the neutral should not be connected elsewhere once more to the mother earth!

Most high power generators are already three phase and a neutral is produced by using a transformer and the neutral is connected to ground. That is the common way to get three single phases out of a three phase supply. Often the neutral conductor is much thinner than the three phases.

Neutral IS not ground. The ground connection at every house is a safety feature and has no electrical significance.

 

[asdf44]

It is a known thing to use a single wire. I was aware of some high voltage long distance DC distribution that used it but there are other applications as well.

Seems like a tough situation if a customer is comfortable with it but you are not.

https://en.wikipedia.org/wiki/Single-wire_earth_return

 

[c_mitra]

I am just curious to know what you decided finally.

40X40W load (1.6kW) is not really high and at 220V the circuit current is not much.

If I assume (just a guess) that a 40W lamp actually runs at 90% efficiency, the total load will be 1.8kW.

If you add line losses (generally counted as 20%, but I am assuming 10%), the final load will be 2kW

That will be using about 9A of total current: let us say 10A (I love round figures).

10% line loss means 20V drop (assume the whole load is at the end) at 10A; line resistance of 2Ohm.

Total length of 30x40meters; 1200=1.2km; the cable cost is not going to be excessive.

Ground resistance over a length of 1km will be more than 10Ohm (but it depends on the locality: I live in a desert area and the water levels are around 50-70m below the surface).

 

[schmitt trigger]

Are those 240 volts line to line, or line to neutral?

If it is line to line, the calculated current will be higher.

Anyway, I will join the chorus....it is a bad idea.

By the way, in 3W countries I have seen electrical installations which give you the chills.
But they make do with whatever they can do, they are grateful they have electricity at all.

 

[c_mitra]

I guess the circuit will be looking something like this:

 

[treez]

Thanks, we agree that earth return is generally not adviseable.