EN1078 is a European standard concerning the test methods and requirements for cycling, skateboarding and skating helmets.
This standard covers the following aspects: construction, including field of vision; shock-absorbing properties; fastening system features, including the chinstrap and tightening systems; markings and information.

Helmets are designed to protect your head from an impact. From just one impact. If you fall, even if it doesn't seem like the helmet is damaged, you need to replace it: it could be weakened or cracked in invisible places... and thus no longer provide protection.
You should also consider replacing your helmet if it has gotten old. After 5 years, certain materials start to age, especially if the helmet is stored in a cold and/or damp place.