IP in simple terms means Ingress Protection. The protection an item might or might not have from either solid objects such as dust, moisture, or both.
THE FIRST NUMBER characterises the level of protection from Solid Objects to tiny dust particles. Numbers 0 to 6 are used - the lower number indicating lesser protection:
0 No particular protection
1 Protected against a large part of the body such as a hand or from solid objects greater than 50mm
2 Protected against fingers or other object not greater than 80mm in length and 12mm in diameter
3 Protection against entry from tools, wires etc. that have a thickness of 1.0mm
4 Protection from entry from solid objects with a diameter or thickness greater than 1.0mm
5 Protection against the amount of dust that might interfere with the operation of the equipment
6 Dust Tight
THE SECOND NUMBER characterises the level of protection of an item's internals from various forms of moisture such as drips, sprays, steam or complete submersion. Numbers 0 to 8 are used again with the lower number indicating lesser protection:
0 No particular protection
1 Protection from dripping water
2 Protection from dripping water
3 Protection from sprayed water
4 Protection from splashing water
5 Protected against water jets
6 Protected against heavy seas
7 Protected from the effects of immersion*
8 Protected against submersion*
*Protected against submersion does not necessarily mean submersible. The submersion depth and/or time should be specified by the supplier and verified by the user as to a products' suitability to their requirement.
Using the example of an LED downlight therefore, how do we assess this item's suitability to be used in a bathroom?
We know we need water protection first and foremost but not against submersion as it's in the ceiling therefore 3, 4 and 5 would suit our needs. If we're talking about the possibility of a shower (spraying water) only can be considered. If not, a lower number ie 3 or 4 would suffice. Dependent on the latter, IP54 or IP65 would be the suitable level of protection for a bathroom.
Below is a guide for you to better understand the meaning of IP rating.
IP20, touch proof. Protection against solids up 12mm square, but no resistance to water.
IP44, limited water resistance. Protection against solids up to 1mm square and protection against water sprayed from all directions. IE resistant to splashes of water.
IP65, water resistant. Total protection against dust and protection against low pressure jets of water.
IP66, water proof. Total protection against dust and protection against high pressure jets of water.
IP67, submersible. Can be temporally submersed in water for short periods of time. From depths of 150mm to 1 metre.
IP68, submersible. Can be left constantly under water.