The wiki page for the Pi relay board has this notice in red:
[color=red]Place 2 layers of electrical tape on the top of the Arduino’s usb connector. This will prevent the relay shield from making contact. Do not operate voltage more than 35V DC.
The specification says 15A at 35V DC/ 250V AC.
- Can the board be used for 250V AC, and
- does it really need 2 layers of tape?