Mains voltage

[JohnW2]

'Skin effect' is very small with 50 Hz, and therefore will not be a significant factor in relation to short lenths of cable, as in LV distribution networks.

The effect at mains frequencies is minimal, it only becomes important at RF frequencies.

Even though, as we've both agreed, skin effect is very small at 50Hz, maybe my qualification about "short lengths" is relevant - since, if you look at long-distance overhead HV (or should it be EHV?) transmission, rather than having single conductors they usually have groups of four in parallel - and I would suspect (although, as always, am not sure) that is to reduce the consequences of skin effect.

Kind Regards, John

 

[Harry Bloomfield]

rather than having single conductors they usually have groups of four in parallel

I wasn't aware of that, I did hear somewhere that they have a steel core for strength and alloy on the outside for conductivity - have you a link which confirms it?

I once queried how the pylon insulators worked, when ceramics don't work that well under tension - the reply was that they work under compression, but I've forgotten just how.

In today's paper they are suggesting we can expect rolling electricity blackouts, due the the gas shortages at peak demand times. They suggest 48% of generation is derived from gas. Best get some candle in stock and worry about your appliances which need mains power to work

 

[JohnW2]

I wasn't aware of that, I did hear somewhere that they have a steel core for strength and alloy on the outside for conductivity - have you a link which confirms it?

I'll see if I can find a link which gives an 'explanation' (of 'the reason') but, for starters ...

Kind Regards, John

 

[Harry Bloomfield]

I'll see if I can find a link which gives an 'explanation' (of 'the reason') but, for starters ...

A few pylons around here, but I don't recall seeing them like that - Is that a UK photo?

 

[JohnW2]

A few pylons around here, but I don't recall seeing them like that - Is that a UK photo?

I doubt that I would be able to find it again, so I don't know for sure. However, don't forget that I presented the picture to 'confirm' what I have seen with my own eyes (in the UK).

I'll see if I can find another which is definitely from the UK (and/ or a 'written description' relating to UK). I have a suspicion (but am not sure) that it's only done with the highest of transmission voltages.

Mind you, even if the pic I posted were not from the UK, it presumably serves to indicate that someone, somewhere in the world, sees the wisdom of such an arrangement.

Kind Regards, John

 

[Harry Bloomfield]

I'll see if I can find another which is definitely from the UK (and/ or a 'written description' relating to UK). I have a suspicion (but am not sure) that it's only done with the highest of transmission voltages.

Skin effect - Wikipedia

en.wikipedia.org

 

[flameport]

Skin effect at 50Hz become significant for conductors over about 250mm², which is why larger ones are typically flat busbars or multiple separated smaller conductors.

 

[SUNRAY]

It's not only a matter or being able to arrange them 'neatly'. To optimally (most efficiently) 'pack' (e.g. strands in a multi-strand conductor - or, indeed, cores within a multicore cable) they also ideally need to be symmetrical in all planes (which implies that they will fit within an outer circle, with all the outermost strands touching that circle) and to have minimal wasted space within the bundle of strands.

That is not the case with your 18-strand arrangement and nor, I think, for any of the others you illustrate, other than 3.

The first four numbers which are mathematically 'optimal' in these respects are 3, 7, 19 and 37 - which are, indeed, by far the most commonly seen numbers of strands in conductors.

Kind Regards, John

I assume you are ignoring the extremely common 16/0.2, 24/0.2, 32/0.2mm etc sizes then?

 

[JohnW2]

Skin effect at 50Hz become significant for conductors over about 250mm², which is why larger ones are typically flat busbars or multiple separated smaller conductors.

Yes ... or hollow conductors with round cross-section. I may have dreamed it, but I seem to recall that hollow (round) conductors are, or have been, sometimes used for 50/60 Hz transmission networks?

Kind Regards, John

 

[JohnW2]

I assume you are ignoring the extremely common 16/0.2, 24/0.2, 32/0.2mm etc sizes then?

I don't know how common they are, but was merely pointing out that the arrangements which seem to be the most common (for 'smaller numbers) are the first three 'mathematically optimal' ones - namely 3, 7 and 19.

Kind Regards, John

 

[SUNRAY]

I assume you are ignoring the extremely common 16/0.2, 24/0.2, 32/0.2mm etc sizes then?

I don't know how common they are...

Kind Regards, John

Extremely common, I bet you have significant quantities of 0.5, 0.75, & 1.0mm² flex in your life.

 

[bernardgreen]

but I seem to recall that hollow (round) conductors are, or have been, sometimes used for 50/60 Hz transmission networks?

I recall cooling fluid ( oil ) being pumped through the hollow conductors in underground cabling

 

[JohnW2]

Looking at the right-hand edge of the right hand overlay I would be thinking 1.5 times the bolt putting it at possibly 50mm2, but then the left end does look more 1.3 times and 35mm2.

Given unexpectedly clement weather today, I've attempted to get a better photo, in case you want to play with it. It looks as if the initial (relative to pole) bits of the main cable are a bit misleading, presumably with the cores not very tightly twisted together ...

Kind Regards, John

 

[JohnW2]

Extremely common, I bet you have significant quantities of 0.5, 0.75, & 1.0mm² flex in your life.

True. To be honest, once one gets up to fairly large numbers of strands, whether or not the number/arrangement is 'mathematically optimal' ceases to be very relevant. In particular, 'low' numbers of strands other than 3 and 7 could present some problems.

The case which started all this (19 strands) is pretty marginal (in terms of importance of the number of strands) - I was really expressing surprise that there should be a 'tolerance' in relation to the number but, as reds42 pointed out, it might be that, for whatever reason, different manufacturers consistently use different numbers.

I doubt that there would be any significant problem with 18 or 20 strands, but I would think that a manufacturer would probably stick to a nearby 'optimal number' unless there were a very good reason not to do so.

Kind Regards, John

 

[Harry Bloomfield]

I recall cooling fluid ( oil ) being pumped through the hollow conductors in underground cabling

Yes, and pumped through to pressurize them, to maintain insulation and keep out moisture.