With so many IP ratings, it is easy to be confused about their actual meaning. However, published by IEC or the International Electro technical Commission, their standard 60529 details all the ratings. IP ratings are also known as Ingress Protection Ratings or International Protection markings. They classify and rate the degree of protection that mechanical casings and electrical enclosures offer against dust, water, accidental contact and intrusion by body parts such as hands and fingers.
Typically, IP ratings indicate the protection level offered by the enclosure of a device. Two or three numbers in the rating indicate the protection level. Within the IP marking, the number in the first position indicates protection from solid objects or materials. The number in the second position indicates protection from liquids, including water. The number in the third position indicates protection from mechanical impacts. However, the third number is commonly omitted as not being a part of IEC 60529. For example, IP65 denotes protection from solid objects to the level of 6 and from liquids to the level of 5. The levels, ranging from 0-9, represent increasing amounts of protection from different solids and liquids, with the level 0 representing no protection at all from either contact or ingress.
For protection from solids, level 1 denotes protection from any object larger than 50mm. This could be any large surface of the body such as the back of a hand, but not offering any protection against a deliberate contact with a body part. Level 2 represents protection against objects of size greater than 12.5mm, but less than 50mm, such as fingers or similar objects. Level 3 represents protection against intrusion from objects of size between 2.5-12.5mm, such as tools, thick wires, etc. Level 4 represents protection against objects of size between 1mm and 2.5mm, such as most wires, screws and so on. A level 5 enclosure will protect against ingress of dust, but not entirely prevent it from entering. That means although the dust protected enclosure will offer complete protection against contact, it will not allow dust to enter in quantities that may interfere with the satisfactory operation of the equipment. If your equipment needs to be dust tight, only an enclosure with a level of 6 will ensure there is no ingress of dust including complete protection against contact.
The second number in the IP rating denotes protection from liquids. For instance, level 1 represents enclosures offering protection from dripping water. Level 2 denotes protection from dripping water when the enclosure is tilted by 15 degrees. An enclosure with a rating of level 3 will not allow the ingress of water sprayed on it, while a level 4 enclosure protects against water being splashed on it. If you want the equipment to remain protected against water from jets, you must go for a level 5 enclosure. If the water jets are really powerful, only a level 6 or above enclosure will help. However, if you expect equipment to work even when immersed in water up to a depth of 1m, you need to go for an enclosure with a rating of level 7. For equipment expected to work even beyond an immersion depth between 1-3m, only an enclosure with a rating of 8 is to be used.