When working with mains voltage, use an insulation transformer or
Ground Fault Interrupter (GFI or FI). Don't touch water taps or
heating pipes, keep one hand in your pocket.
The following table gives minimum distances you should keep from
high voltage:
up to 5kV:
1cm
up to 30kV:
5cm
up to 80kV:
20cm
up to 100kV:
50cm
These distances resulted from my experience and should keep you safe,
but no guarantee!!!
Keep your hand at the main power switch whenever possible. Have a
torch at hand in case the fuse blows (and it's night :-)
There is an excellent page that covers very extensively the dangers of working with HV and how to avoid them:
Chip Atkinsons Tesla coil page.