Confined Space Assessment Calculator

Under the Confined Spaces Regulations 1997, a confined space is any largely enclosed place (chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or similar) where, by its enclosed nature, there is a reasonably foreseeable specified risk — fire/explosion, loss of consciousness from gas/fume/vapour or oxygen depletion, loss of consciousness from heat, drowning from rising liquid, or asphyxiation/entrapment from free-flowing solids. It is the specified risk, not a fixed depth, that makes a space "confined". A trench is therefore a confined space when such a risk is reasonably foreseeable, regardless of depth (HSE has withdrawn the old prescriptive 1.2m trench rule). Examples: manholes, tanks, sewers, silos, roof voids, and service ducts.

A permit to work (PTW) should be used for all high-risk confined space entry: where there is a risk of hazardous gas, where the space is not naturally ventilated, for entries over 1.5m deep, or where rescue would be difficult. The PTW documents safety precautions and emergency procedures.

At minimum, test for oxygen levels (19.5-23.5% normal). For sewers/drains: add methane (LEL), hydrogen sulphide (H2S), and carbon monoxide (CO). A 4-gas monitor covers these. Test before entry, at multiple heights, and monitor continuously during work.

Minimum team: one entrant and one standby person at the entry point at all times. For high-risk entries, add a supervisor and rescue team. The standby person must NEVER enter the space to attempt rescue without proper equipment - this is how most multiple fatalities occur.

Neither the Confined Spaces Regulations 1997 nor HSE guidance L101 sets a fixed air-changes figure — HSE requires forced/mechanical ventilation for hazardous spaces but leaves the actual turnover rate to your risk assessment. As an industry rule of thumb, suppliers commonly quote around 7 air changes to purge a space and roughly 7.5–20 air changes per hour for continuous ventilation (Allegro recommends 20, RamFan around 7.5). For a 5 m³ manhole, 10–20 air changes/hour works out at about 50–100 m³/hr airflow. The fan should blow clean air in from the surface, not extract from below. Always retest the atmosphere after ventilating and continuously during work.