Regardless of whether signs are used, the standard requires that the employer inform employees exposed to the hazards posed by permit spaces of the existence, location, and danger of those spaces.
Barrier failure resulting in a flood or release of free-flowing solid or liquid. Note: OSHA recommends that operators check with the instrument's manufacturer for the acceptable tolerance ranges.
Work activities may introduce hazards not present initially. Employers must certify that this training has been provided. Self-rescue by the worker is more difficult. They often get overlooked because access to them is rare.
There are many industry sectors that carry out work involving confined spaces. The key thing to remember here is that employees must be informed of the hazards posed by confined spaces.