What does the boiler limited space mean?

The limited space refers to a space which is closed or partially closed, relatively isolated from the outside world, with narrow entrances and exits, staff can't work in it for a long time, natural ventilation is poor, and toxic, harmful, inflammable and explosive substances or oxygen content is easy to accumulate. To define a limited space, it is necessary to meet the following three conditions at the same time, which are indispensable: the volume is large enough for people to enter completely; Or restrict the entry and exit; It's not designed to stay for a long time.