A confined space is not defined by size. It is any place that is substantially enclosed and where there is a reasonably foreseeable specified risk: fire or explosion, loss of consciousness from gas, fume, vapour or lack of oxygen, drowning from rising liquid, asphyxiation from a free-flowing solid, or loss of consciousness from heat. A large grain silo is a confined space. An open-topped pit can be one. A locked cupboard usually is not.
What the regulations actually forbid
Regulation 4 of the Confined Spaces Regulations 1997 says nobody shall enter a confined space to carry out work unless it is not reasonably practicable to achieve that purpose without entry. Entry is the last resort, and the first question is always whether the job can be done from outside: cameras, remote sampling, robotic cleaning, and designing the plant so routine work does not need entry.
The safe system of work
Where entry cannot be avoided, regulation 4(2) requires a system of work that renders the work safe as regards the specified risks. In practice that is isolation of incoming services and materials, cleaning or purging, testing the atmosphere before and continuously during entry, ventilation, controlling ignition sources, a top man in communication with the entrant, defined entry duration, and equipment suitable for the atmosphere.
The permit, and why it exists
A confined space entry permit is the document that proves every one of those steps was checked by a named person immediately before entry, and that the space is handed back at the end. It exists because the failures are ones that a checklist catches: the isolation that was assumed, the gas test taken an hour earlier, the second entrant nobody logged.
Rescue is the part that kills people
Regulation 5 requires suitable and sufficient arrangements for rescue to have been made before anyone enters. Would-be rescuers who go in without breathing apparatus make up a large share of confined space deaths, and they are usually colleagues who went in on instinct. A rescue arrangement means trained rescuers, the right equipment on site, a tripod and winch where the access allows it, and a plan that does not depend on somebody entering unprotected.
Why methane in a confined space is so dangerous
It is dangerous in two separate ways at once. It is flammable across a wide range in air, so a spark can ignite an accumulation. And because it displaces air, it reduces the oxygen concentration, so a space can asphyxiate an entrant well before it reaches a concentration that would explode. That is why atmospheric testing checks oxygen, flammability and toxicity rather than any one of them.
Medium risk, and what the tiering means
Industry classifies confined space work as low, medium or high risk according to the atmosphere, the access and the rescue difficulty, and training is provided against those tiers. The tiers are an industry convention rather than a legal classification, and a space does not become safe because somebody labelled it medium risk.
Questions people ask about confined space
What is classed as a confined space?
A place that is substantially enclosed and where one or more specified risks are reasonably foreseeable: fire or explosion, loss of consciousness from gas, fume, vapour or lack of oxygen, drowning, asphyxiation from a free-flowing solid, or loss of consciousness from heat.
Is a permit required to enter a confined space?
The regulations require a safe system of work rather than a permit by name, but a written permit is the normal and expected way of showing that system was applied before entry.
Do rescue arrangements have to be in place before entry?
Yes. Regulation 5 requires suitable and sufficient arrangements for rescue to have been made before any person enters.
Is a manhole a confined space?
Usually yes, because a sewer or drainage chamber carries foreseeable risks of gas, oxygen deficiency and rising liquid. Each space still has to be assessed on its own facts.