Crash,
Just one point of note:-
If you take a power tool apart you will usually notice an R/C or inductive suppression device fitted across the on/off switch. This is there to reduce the arc (which occurs during switching) and provides a fair degree radio frequency suppression. If you encounter any problems, with computer glitches etc, using your relay board then would be a good idea to fit one of these suppression devices across the contacts of each relay.
(SSR’s, on the other hand, make use of the 'zero crossing point' of the a/c waveform for their on/off switching points, this reduces the spike / noise / arc to an absolute minimum and no other forms of suppression are usually necessary).
Tweakie.