I've just come across a 50-ish-year old ring circuit where one cable at the CU is the usual 7/0.029" red/unsheathed/black Imperial T&E cable, but the other cable is red/transparent/blue, all three conductors (apparently) 7/0.029". I've never seen this cable type before - is anyone familiar with it, and if so, what's its story?
Sounds like the old three phase colours, red, white, and blue. Probably polythene insulation which is partially transparent resulting in the "white" core being more or less transparent.
Often used for 2 way lighting circuits. Came in both 3 core and in 3 core with earth, and of course twin and twin with earth. If compared to PVC, polythene insulation feels greasy and looks partially transparent. The copper of the red core can often be seen through the insulation.
;Polythene insulation enjoyed a brief period of popularity between the end of rubber insulation, and the general use of PVC.
The flat owned by my late grandmother and built in the mid 1960s used a mixture of polythene insulated cables and PVC types.
Produces a less lethal smoke than PVC, but does burn rather too well for comfort, is less lossy to RF, and apparently does not char in the way required to operate an AFDD...
If you have an offcut of insulation, PVC will sink in water PE will float.
The insulation floats and is more rigid than I'd expect for PVC. The transparent insulation shows no hint of white pigmentation. The R & B aren't faded in any way. I did think that maybe it was for 2-way switched lighting (without an earth), but it's a 2.9mm power cable.
For 3 phase without neutral or earth, in the days before earthing was what it is today, red white and blue would be power. The thing at the end of it would be what we now call class 0I (or in some older text 0A) and earthed via another route such as cable ducting or pipework.
Not allowed any more, but quite common in 1960 (not much before that or it would have been rubber).
Used like this with the second phase core as a full cross section earth is perfectly fine. I suppose you could feel the urge to add green yellow sleeving, I'd leave it, unless it is damaged in some way, its probably good for another 50 years if it keeps out of the heat and the sun.
