Hi, I'd seriously think about using optical isolation between the Arduino and the high-power stuff.
Could be relays like these: http://goo.gl/3mXu0 (Also see the schematics on their pages)
Or do you own with opto-isolator chips. See http://arduino-info.wikispaces.com/Popular-ICs and scroll down to the Opto stuff...
DISCLAIMER: Mentioned stuff from my own shop...