[...Best to assume both are hot.
I normally would assume that also, but I know that my house is wired properly.
One mistake was assuming that a manufactured molded cable would be wired correctly.
But I take the point made by the replies here that my other mistake was not fully isolating both power wires before handling the connected device.
I wouldn't trust an ordinary DPST toggle switch for isolation. ... fit a proper safety isolator switch
I will look into a isolator switch for this, but that would mean a rebuild.
As well as the RCD, the box has a standard DIN mount double pole circuit breaker.
I imagine that they do not suffer from the fault mode you described?
Do you know if such circuit breakers are designed to also be used simply as regularly operated switches?, ie: can it be used as an isolator switch?
Meanwhile I'll rewire both DPST switches to switch both live and neutral, and continue my usual practice of switching them both off when not in use.
They are large switches with screw terminals, I'll check the rating, but I doubt that current handling would be a problem anyway.
I normally would physically disconnect before intentionally touching the load, it was an accidental touch.
Of course, intent is irrelevant when you're dead...
If I do rebuild the variac box, maybe it should include some sort of a "neutral fault" alarm.
In any case, I will be checking active/neutral polarity much more often in the future.
The real problem here is your assumption about neutral being safe, never do that...
I do know that a fault can render neutral unsafe, (boy, do I know!) and that it's far better to switch both rather than active alone.
The fact that the OP hasn't understood such basic concept of the mains and is blaming the cable is very alarming. Please learn the basics and take care.
I do understand the basics. And that cable *is* faulty, not as bad as live and earth swapped, but still wrong. I'm sure it would be illegal to sell here in Aus.
Yes, I did make the mistake of not unplugging and/or fully isolating both active and neutral lines before handling connected devices, and nearly paid dearly for it.
At least I made the non-mistake of fitting an RCD, and I'm thankful for that.
To all posters: I know it is wrong to assume that neutral is safe. And despite that, I've been guilty of doing so.
The main purpose of my post is to alert people to the fact that manufactured cables that are intended to be polarized may sometimes be polarized incorrectly (for any given locale)