Cooker switch shorted against metal back box

[John D v2.0]

Looks like theory is right, but what surprises me is that the terminal screws are exposed to flat objects when tightened. I always use MK, so maybe they go above and beyond, but I thought even with the back exposed it was supposed to have a good enough IP rating to avoid fingers touching live parts.
On the plus side you have a decent earth and adequate fault protection which acted correctly.
Anyway, hopefully the learning point is testing before energising, use MK or other quality kit, and avoid removing accessories when they're live.

 

[ban-all-sheds]

avoid removing accessories when they're live.

And avoid energising them while they are still partially removed.

 

[ban-all-sheds]

I think what I had stupidly done was to turn the power on, without either fixing the switch to the back box, or pulling it well away from the box. Instead it was loose in the back box

What I'd like to know is why?

What was the purpose of turning the circuit back on before you'd fixed the switch in place?

 

[plugwash]

If a terminal of the switch can touch any part of the back box then the back box is too small.

From the OP's pictures it looks like the screw touched the side of the box, so a deeper box wouldn't have made any difference.

 

[Taylortwocities]

What I'd like to know is why?

What was the purpose of turning the circuit back on before you'd fixed the switch in place?

That is the process known as ‘Testing’

 

[Jackrae]

That is the process known as ‘Testing’

"Trial" rather than "Test".. The former indicates if things 'appear' to work, the latter ensures it's 'safe' to trial