As leakage current flows along this path, it can carbonize the insulating material, creating a permanent conductive track that can eventually lead to a short circuit and an arc flash. The specific types of dust and contaminants present will vary depending on the industry and environment. Given that the short circuit would ignite some of the dust, this is a pretty bad position to be in. In NFPA 499 for instance referring to dust explosion problems such as coal they give a figure of 1/32 inch thickness and claim that this is when you can no longer see a white painted background clearly as a good indicator of when it is too thick. As to vaporizing and such, what? That's not really. Small changes—heat, smells, noise, or dust buildup—can indicate that a breaker is struggling long before it fails. Understanding these early clues is not just maintenance—it's electrical safety.
[PDF Version]