Knowing the difference between imperial cable and metric?

[7029 Dave]

About the only thing I've not still found in use

Last time I worked on it was the 80s, we rewired a few old big houses in Highgate London.

 

[Mike P]

The other problem with cable is when it was run in the cavity and the house then had cavity wall insulation. I have removed cable that was just copper no insulation!

 

[ExArmy]

Out of interest would you say a cable ever has a lifespan?
Let's say the imperial cable in this property is still in situ in another 50 years, and tests at 2Mohms?

Cable has a lifespan yes, but the word span just means extent of something from end to end, we don't know the maximum figure yet, it depends on external factors and how much power it's delivered in it's life. Just yesterday I did a board change and found a socket circuit wired in 7/029 with 11 sockets, evidence of a few alterations over the years, but still IR readings off the scale above 999Meg. I didn't inspect every backbox but the stuff I saw looked as good as it probably did when it was installed, likely around 20 years before I was even born.
other than green goo, and sometimes hard or brittle insulation from heat damage in the backbox from bad terminations, I've still yet to see an imperial PVC cable deteriorate like VIR does.
It's difficult to code it because you leave yourself open to "well what exactly is wrong with it, other than it's old? it still works". I might advise imperial cables be renewed when building work is done, but thats off the record and just my thoughts not something I'm putting in writing on a certificate. I don't think it would ever be a good idea to cover up old cables with insulation, plasterboard and new floor coverings as it's not worth the risk, plus the most common complaint of electrical installations that were wired in imperial cable are that there were never enough sockets by modern standards, so my advice is probably no surprise to them and they will want it rewired anyway when it's appropriate