Best seperation gap between a live 240 and an earth?

hi
wondering if someone could help me please, whats the best gap for seperation between a live 240v ac and an earth plain to stop the electrons jumping to earth., is there an equation maby?

It rather depends on what the gap is made of

ok i see,

a, free air, i see on switch mode supplys they use about 3mm would that be sufficient?
b, pcb
thanks

I think the pcb spec is 4mm

regards

Allan

Google the words
creepage and clearance on mains equipment
For all the gory details and calculations / regulations

Thanks mike