What safety precautions shoul I take when using relays with 220 volts ?

I want to use a relay as switch for some 220 devices in my house , am going to set it up permanently inside the wall.
Are there any safety precautions that I should take ? or any parts I should add ?
I am using this modulo which has optocoupler isolation

You mean you can not get to it once it is in the wall ?
You should never make something like that.

Check the specifications for that module. I'm not sure it is designed for mains voltage.
What if a screw gets loose and a mains wire pops out the header ?

The basic rule is that the live metal parts(*) should be impossible to touch. So the back of the PCB needs a plastic shield
(unless its mounted against the inside of the box). Leads going into the screw terminals should be shrouded (heat-
shrink or rubber tubing) if possible, else the wire insulation should go as far into the terminal body as possible.

It would be best to place it in an earthed metal box, especially if there is any danger of the surroundings being inflammable.

If no earth available a totally insulated [flame-retardent?] enclosure is best - google "double insulated".

And remember that all mains devices must be fused to protect mains wiring.

Oh, yes cable-clamps and strain-relief where wires go into the enclosure to reduce chance of bare wires being accidentally
pulled out.

With your specific board you'll notice one end is mains, the other end is low-voltage - arrange the layout
accordingly and ensure mains wires cannot reach the low-voltage parts should they come adrift.

[ And of course check your local regulations... ]

(*) here live means live and neutral - the neutral wire can go to lethal voltage in a fault condition so is treated the same
as the nominal live wires.

Nope ,I'll be able to access it .
What I mean is that am gonna leave it there for a long time not just sth I'll try.

Thanks that was really helpful.

Make sure you do not exceed rated power. IIRC it's recommended to stay within half or less of the rated power when switching incandescent lamps. Inrush current also plays a factor depending on loads type.

Have a look here, it's an excellent document about relays published by Panasonic: Panasonic Industry Europe GmbH | Industry Sector Partner

Panasonic has nice document about Solid State Relay Cautions For Use,

Solid State Relay Cautions For Use, Panasonic

PDF version