The International Protection Rating consists of numerals and letters that indicate what the fuse box is rated to protect against. In cases where the fuse box does not protect against a specific type of hazard, the number or letter will be replaced by an X.
The fuse boxes are rated as being protected against liquid ingress, solid particles and body parts. These ratings provide a way for builders to determine whether or not the fuse box provides adequate protection against hazards to which it is likely to be exposed. For example, a fuse box with a liquid ingress protection rating of 4 would be suitable for any application where splashing water might be a hazard. It would not, however, be appropriate for use in an area where water jets would be an issue, which would require a protection rating of 5.
Likewise, letters that are added to the IPR code can denote whether or not the device provides protection against harm coming to humans. A level A, for instance, will prevent the back of the hand from coming into contact with circuits while a level B would mean that fingers are protected, meaning that the electrical fuse box would offer more protection.
Building codes and certain types of electrical circuitry sometimes requires that a specific level of International Protection Rating be provided by the fuse box itself. In other cases, the builder or designer may want to call for a certain level of International Protection Rating in anticipation of the fuse box itself being handled in certain ways or presenting specific hazards to the users of the fuse box.