In Europe, you must make sure equipment meets the requirements of the EMC directive, the low voltage directive and the machine safety directive, amongst others.

Almost ALL electrical and electronic items must be tested and/or certified as compliant before being "placed in the market" - this phrase is usually taken to disallow testing for prototypes and amayeur projects, though it would probably not be a defence to say that "its only an amateur project", when your equipment caused a A330 to drop from the sky. It also means that you CAN bring in non-CE compliant equipment for your personal use perhaps, but you could never legally sell it, even privately.

To a certain extent how you do the testing is up to you: You must show "due diligence" in the design and manufacturing process, which means that you should work within the prescribed limits laid down in the several EN standards mandated under the directives. Your "due diligence" must also show that the units comply with these limits - this might mean you send the equipment to a test house and let them apply the mandated tests, or you might do it yourself in house. Again you "due diligence" must ensure that the levels that you test are calibrated and traceable in some manner to national standards.

Note this "due diligence" phrase. If you CAN'T PROVE you were competent, and that you did everything, as far as you could, in the right way, you can face unlimited fines and a prison sentence ! There HAVE been successful prosecutions, initiated by trading standards authorities, and I think at least one prison sentence.

Steve