Depending on external factors like dust, foreign substances, moisture, etc., the terminals should be equipped with a protection class xx. The DIN-norm 40 050 refers to the installation at the wall. 

IP 20:
Protection against the penetration of solid bodies with a diameter larger than 12 mm.
No special protection against the penetration of water. 

IP 44:
Protection against the penetration of solid bodies with a diameter larger than 1 mm. 
Protection against splashed water (from all directions). 

IP 54:
Protection against the penetration of hazardous dust deposits (dust-proof).
Protection against splash water (from all directions). 

IP 65:
Full protection against the penetration of dust. 
Protection against the penetration of jet water (jet-proof).  

If a terminal has no protection class, but is used for outdoor use, it must be equipped with additional protection (e.g. weather protection roof).