I have castlemilks and swales and hebrideans and I can assure, 100% that they will stay behind 3 strand wire, on a windy moor, for 3 months without escaping.
Simple rule - MAKE SURE THE Energizer is putting out 2Joule minimum.
A good hard shock and they dont even graze within 6 inches of the wire.
I find cheap energizers putting out less than 2Joule and the odd sheep will test the wire and get through, or push its head through and pull a wire.
The key is making sure the sheep KNOW wire hurts.
From a young age train them.
Most of my sheep wont even cross bale twine for weeks on end they're that convinced it will sting.
Good energizer.
GOOOD Wire. DO noty bother with rope or tape, get good low OHM resistance wire, and just make a 2.5ac block and move it round the field.
I get lots of short 3 month grazing winter lets and offers to tidy up arable fields, shooting moors, 25 miles in each direction and I can trust my electric fencing -
http://www.electricfence-online.co.uk/shop/electric-fencing/energisers-battery-powered/hotline-raptor-energiser-6v-12v-1081653.htmlSimilar to the one I use.
At 3 joules the sheep will get one shock and wont try again.