That's all very well, but on the 2391 time is short - and in the real world, time is money.
Consider this:
Whilst performing safe isolation of the installation you have your voltage tester in your hand -
so you might as well perform the same tests on the incomming side of the main switch and confirm
supply polarity.
Then, as you have the cover off the board anyway, disconnect the earthing conductor and measure
Ze
Re-connect the earthing conductor and measure your
PFC
These three tests have took you 5 minutes and you haven't moved.
Then onto your sequence of tests -
continuity,
ring final,
IR, Polarity
Now, really speaking, you should put the cover back on the board before re-energising. (HSE and all
that) - but that's okay, because your finished in there and shouldn't have to take it off again
On with the remaining live tests -
Zs,
RCD and the
functionals.
Job done.
Your not doing anything wrong - it's just more efficient