The system is used provide a guide to the degree of protection that a luminaire has against the ingress of solids and liquids.
The first number relates to solids and ranges from 0 –6. The ratings offer a logical progression from larger to smaller objects, so the lowest number (zero) indicates that there is no protection against touching live parts within the equipment. The most common rating for a light fitting intended for general interior use is IP20, which tells you that nothing larger than 12.5mm diameter can reach a live part when the equipment is fully assembled.
The second number relates to liquids and ranges from 0 –9. Unlike the natural progression assigned to the ingress of solids, the second number relates to styles of liquid ingress; drips, sprays, splashes, jets and immersion.