Neon / Mains Tester Screwdriver – Good or bad?

Grenage

I'd like to add that isolating a circuit, or even turning off the main switch is no guarantee of safety.

I discovered a socket in my parents house that was connected to a lighting circuit.

At my last house I found that one of the sockets on a party wall was connected to the neighbour's ring.

Always. Test.  It takes seconds and the potential consequences of not doing so are someone's life.

FreeBear

Grenage said: I'd like to add that isolating a circuit, or even turning off the main switch is no guarantee of safety.

Indeed. It is just possible that some little s**t has been messing with the isolator switch and it no longer works. Came across that on an industrial machine tool. Even although the isolation switch said "OFF", it was still live (3-phase to boot). A 2" steel clue bat to the aforementioned little s**ts head educated him on the errors of his ways.
It is also possible that the contacts inside an isolator switch might be welded together. Unlikely, but if it had been used to switch high currents, a possibility.

Section62

Grenage said:

At my last house I found that one of the sockets on a party wall was connected to the neighbour's ring.

^This.

For DIYers learning from how-to-do-it videos on youtube it is easy to fall into the trap of imagining there is one consumer unit with a main switch which will isolate everything and all that needs to be done is to turn off the big red switch.

When you've seen a lot of other people's work close up you eventually learn there is no limit to the daft things that people are willing to do without appreciating the significance and/or consequences of what they've done. (FreeBear's isolation switch is another good example)

These threads often go down the route of criticising the professionals who chip in with how it should be done correctly, or there will be digs about pedantry.

But electricity can kill, and at normal mains voltage we have no inbuilt sense which allows us to detect it.  Therefore the use of the right tools and techniques is vital to keep people safe.