This question might be silly but I am sure I'm not the only one who wants to get sure before destroying something :)
I'd like to use the relay expansion to switch on/off a light bulb (for testing purposes). Therefore I would cut the electricity cable of a lamp into two and put the ends in the two sockets of a relay on the expansion.
Is the relay expansion designed to handle this amount of power (230V, 50Hz)? Is there anything I need to know to take care of before I start?
This is a very good question. And the answer is that most relay expansion boards are not designed so that they safely can be connected to the mains voltage without causing a safety hazard that could result in electric shock or a fire.
The reason in most cases is that the circuit board is made with too short distances between low voltage parts and mains voltage traces. To save money, most of the relay boards use relays that were never intended for this type of use. The relay may have been designed to be used where the low voltage circuitry is grounded, so that if a connection between live wiring and the circuitry that controls the coil, the low voltage circuitry would not end up at a hazardous voltage.
But with micro controllers for hobby use it is most of the time so that the circuitry is not connected to ground, and even if it was (through USB cable ground) the ground connection is not strong enough that it will be safe. So the relay boards that are going to be used for controlling mains voltage will look different. there will be large distances between AC power traces and the rest of the circuitry. THe relays will be different and a bit larger, with much longer creepage distances and with coil pins far away from power pins.
Still, most relay board manufacturers list the characteristics of the relay, not understanding that the circit board design must meet electical safety requirments to be safe. They invite users to connect the relay board to the mains voltage but their product is unsafe.
Another thing to remember is that there must be a fuse in series with the load, and the fuse rating must be low enough that if there is an overload the fuse will open before the circuit board traces or the relay contacts are damaged. A relay with 2A contacts, I would use max 1.5A fuse. The mains outlet may have 10A or even higher rating.