Isolation of TN supplies.

[westward10]

There is not quite the same thing as "reverse polarity" as you can have:

• L-L swap leading to wrong rotation. This is not obvious without specific test (closer to SP reversal) but will likely trouble machines using 3P motors as fan direction reversed, etc, or
• N-L swap and then almost certainly see SP loads on the two other L-(L instead of N) voltages burning out!

The first could go undetected if you only have SP loads, but the 2nd is likely to be found out pretty quickly so not likely to be discovered on EICR, etc!

Not really sure how this is overly relevant to what I suggested.

 

[timhoward]

Yep, although I often see 5 cores used on motor circuits as the was on hand and neutral ends up parked in a connector.

In my first sparking era (late 90's) I was always told to run a neutral to the isolator, as the chances are whoever is using the machine will want a socket and a work light next week, especially in the case of lathes.
But if a socket was added and the N came into play we certainly didn't swap to 4 pole isolators!
I'd imagine RCD requirements prevent this arrangement these days anyway.

The only real benefit to TP+N isolation seems to be easier testing. At least I think it's easier to know that everything upstream is isolated and you aren't finding upstream N-E issues, especially when going through the IR permutations.