IIP ratings are the classifications used to express the degree of protection from dust, water and impact that are given by housings and electrical equipment. This is a European standard and the details can be found in BS EN 60529: 1992
(IP Code) initial number shows the degree of protection against impact & dust
First number
0 No Protection
1 Protection against solid objects over 50mm (e.g. hand)
2 Protection against solid objects over 12mm (e.g. finger)
3 Protection against solid objects over 2.6mm (e.g. tools and wires)
5 Protection against dust and limited ingress (no harmful deposit)
6 Total protected against dust
Second number shows the degree of protection against water
0 No protection
1 Protection against vertically falling drops of water
2 Protection against sprays 15 degrees from the vertical
3 Protection against sprays 60 degrees from the vertical
4 Protection against sprays from all directions, limited ingress permitted
5 Protection against low-pressure jets of water in all directions, limited ingress permitted
6 Protection from all strong jets of water, for use on ship decks, limited ingress permitted
7 Protection against temporary immersion between 15cm and 1m duration tests of 30 mins
8 Protection against long periods of immersion under pressure
For Example an IP67 housing is completely protected from dust and can be temporarily immersed in water