Scotty, are they stealing copper conductors in UK? I really didn't expected it, I considered this as a poor countries specific! Actually in our country they "invented" a way of stealing aluminium conductors from 110 kV line under tension !
Somewhere in 90's it was a big problem for the utilities, but now this practice decreased - I hope the reason is that most of the "specialists" have made once a mistake.
By the way, during my visit to Ethiopia some years ago I saw many drums with ACSR left unguarded near to the road (new 132 kV line was in construction in that area). I asked colleagues from the utility and they said - No, thefts are not a problem, because there is not a system for buying up the scrap metal!
Well, I am sure when the progress come there they will face the same problem too ;-)
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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant