Circular connector dust caps are to provide environmental protection for circular connectors in both threaded and bayonet versions. Originally designed to meet military standards, they are used with rugged connectors in harsh environments.
Adding a circular connecter dust cap you your connectors can help you achieve an IP rating standard that might be crucial to achieve a safety standard for your circular connector. Dustcaps will help prevent the ingress of water, dust, sand and dirt get into your connector.
Circular connector dust caps protect and plug connectors and other wire harness components from physical damage. The cap works as a seal against particles, moisture and other possible contaminants.
An IP (ingress protection) rating is the rating backed by standard IEC 60529 to tell you how waterproof/dust-proof a component is. IP ratings are made up from two letters (IP) and two numbers. The numbers refer to different levels of protection, as seen below.
The first number relates to how protected against solids the item is while the second number is how protected it'll be against liquids.