adrianlee:
Yes I think arduino nano has 500ma max current input fuse through usb
No, an Uno/Mega has an (auto)resetable polyfuse.
A Nano has a tiny 500mA backflow protection diode (that burns when overloaded).
Relay link is a useless sales pdf. No real info there.
And they have hidden the most important side.
Were does it say the board is mains (230AC) rated?
Post a diagram.
Leo..