VacuumVoodo gives a good summary of product safety testing. A CE mark is mandatory for all products shipped to the EU. The cost of obtaining a CE mark is very high, $10k - $15k per product in the USA through certified testing labs. Apparently less costly elsewhere in the world. The paperwork requirements are hell. Every component in every product must be documented as to materials used, flame retardantcy tests, toxicity, and so forth. We have one shelf of a large bookcase devoted entirely to component test data ? really should get this into a virtual format.
UL is a different animal. This is a private, non-profit testing lab. While there are no federal laws requiring UL testing of audio gear (that I?m aware of), many private and public entities do require some kind of testing mark on commercial and industrial products. This can be true of concert venues and city offices. Even Los Angeles County has their own product safety testing labs.
It?s in a manufacturer?s best interest to design to UL standards ? for audio, this would typically be UL813, and possibly others..
http://ulstandardsinfonet.ul.com/scopes/0813.html
For voltages under 48V, design requirements are fairly straightforward. UL is most concerned with higher voltage/power, such as power supply primaries, grounding, risk of fire, and mechanical safety. Even for home brew projects, I think it?s best to get in the habit of building to UL requirements. And for a manufacturer, it is essential. As a rule, CE is a bit stricter than UL, so designing to CE standards will usually satisfy UL requirements, though there are some exceptions.
JL