Confined space assessment

What counts as a confined space

Under the Confined Spaces Regulations 1997 a confined space is defined by two features at once — not just by being small or tight:

• It is substantially enclosed — a chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or similar space. It need not be fully sealed.
• Because of that enclosure, a “specified risk” is reasonably foreseeable. The Regulations list five: serious injury from fire or explosion; loss of consciousness from a rise in body temperature (heat); loss of consciousness or asphyxiation from gas, fume, vapour or lack of oxygen; drowning from a rising liquid; and asphyxiation from, or entrapment by, a free-flowing solid such as grain, sand or flour.

Both parts must be present. A loft or a service riser is usually not a confined space — but it can become one if, for example, a leak builds up gas or oxygen is displaced. Manholes, storage tanks, sewers and unventilated chambers commonly qualify, and deep excavations (broadly trenches over about 1.2 m, where heavier-than-air gases can collect) can qualify too. A space's status can change with the contents, the weather and the work being done, so the test is reassessed, not decided once.

If only one part of the test is met, the 1997 Regulations do not apply — but a normal risk assessment under the Management of Health and Safety at Work Regulations 1999 still does.

Why rescue is the part that kills people

HSE is blunt about this: a large share of people who die in confined spaces are not the original entrants but would-be rescuers who go in without protection when something goes wrong. An oxygen-deficient or toxic atmosphere can incapacitate a rescuer in seconds. That is why emergency and rescue arrangements (reg 5) are a legal duty that must be sorted before entry — non-entry rescue (a retrieval line on a tripod and winch) wherever possible, trained rescuers and the right equipment on site, and a plan that never relies on dialling 999 alone.

The order the law works in

1. Avoid entry (reg 4(1)) — do the work from outside if it is reasonably practicable, by remote inspection, long-reach tools, or draining and cleaning from outside.
2. Safe system of work (reg 4(2)) — if entry is unavoidable: test and monitor the atmosphere, ventilate, isolate and lock off inflows and energy, sort access and egress, communications, lighting, the right PPE/RPE and a posted attendant.
3. Adequate emergency arrangements (reg 5) — in place before work starts.

These are concurrent duties, not a menu. Confined-space entry is normally controlled by an entry permit, and the decision to enter is made on site by a competent person — never from a web tool. This assessment helps you structure that thinking; it does not authorise entry.

Sources: the Confined Spaces Regulations 1997 and HSE's Approved Code of Practice L101 “Safe work in confined spaces” (see also hse.gov.uk/confinedspace).