Confined space entry is one of the highest risk activities carried out in the UK industry. The hazards are usually invisible, the atmosphere can change within minutes, and incidents follow a recurring pattern: one worker is overcome, colleagues rush in without protection, and the casualty count rises.

A confined space is not defined by its size. It is defined by two things: whether the space is substantially enclosed, and whether a specified risk is reasonably foreseeable inside it. A grain silo of several hundred cubic metres qualifies. A well ventilated store cupboard containing nothing hazardous does not.

This application note explains what a confined space is under UK law, the gas hazards you are likely to meet, how to test the atmosphere correctly before and during entry, and the detection, escape and rescue equipment needed to work safely.

What Is a Confined Space?

Those thresholds only matter once you have established that you are dealing with a confined space, and that is a narrower legal test than most people assume.

A confined space is not defined by its size. Under the Confined Spaces Regulations 1997 it is defined by two conditions, and both must be met:

1. The space is substantially enclosed, though not necessarily entirely.
2. There is a reasonably foreseeable specified risk arising from that enclosed nature.

A grain silo holding several hundred cubic metres qualifies. A well ventilated store cupboard containing nothing hazardous does not.

The Regulations name chambers, tanks, vats, silos, pits, trenches, pipes, sewers, flues and wells as examples, but the list is far longer in practice. The phrase doing the real work is "reasonably foreseeable". It is not enough to say that a space has never caused a problem. The question is whether a competent person, looking at the space and at the work being carried out inside it, would foresee the risk.

The Five Specified Risks

The second half of that test rests on the specified risks, which the Regulations set out precisely. If a space is substantially enclosed and any one of these is reasonably foreseeable, the Regulations apply:

• Serious injury to any person at work arising from fire or explosion
• Loss of consciousness arising from an increase in body temperature
• Loss of consciousness or asphyxiation arising from gas, fume, vapour or the lack of oxygen
• Drowning arising from an increase in the level of a liquid
• Asphyxiation arising from a free flowing solid, or the inability to reach a respirable environment because of entrapment by a free flowing solid

Free flowing solids include flour, grain, sugar and sand.

Three of those five risks are atmospheric. That is why gas detection sits at the centre of almost every confined space safe system of work.

The Confined Spaces Regulations 1997 impose three duties in strict order of priority. A lower duty only becomes relevant once the one above it cannot be met.

Avoid entry. Regulation 4(1) states that no one may enter a confined space to carry out work unless it is not reasonably practicable to achieve that purpose without entry. This is a real hurdle, not a formality. Remote inspection, CCTV survey, robotic cleaning, external sampling points and design changes should all be considered, and the reasoning recorded, before entry is authorised.

Work to a safe system of work. Regulation 4(2) requires that where entry is unavoidable, it is carried out under a system of work that renders it safe in relation to the specified risks.

Prepare emergency arrangements. Regulation 5 prohibits entry until suitable and sufficient rescue arrangements have been prepared.

These sit alongside Approved Code of Practice L101, Safe work in confined spaces, third edition, published 2014. An ACOP carries special legal status: if you are prosecuted and it is shown that you did not follow it, you must demonstrate that you complied with the law some other way, or the court will find you at fault.

The Regulations do not apply to work below ground in a mine, to diving projects covered by the Diving at Work Regulations 1997, or to the master and crew of a sea-going ship carrying out normal shipboard activities. Separate legislation covers those cases.

The second of those duties is where most of the practical effort goes.

Pre-entry gas testing is the single most important control in a confined space safe system of work. Getting it wrong is what turns a routine job into a fatality.

Test from outside, before anyone goes in. Never enter a space to test it. Use a pumped gas detector with a sampling line fed in from outside so the atmosphere is proven before anyone is exposed to it. A diffusion instrument clipped to a worker tells you what is happening at the entrant's chest once they are already inside. It does not tell you whether it is safe to go in.

Test in the right order: oxygen, then flammable, then toxic. The order is not arbitrary. Catalytic bead LEL sensors need oxygen to function, so an LEL reading taken in an oxygen deficient atmosphere is unreliable. Establishing the oxygen level first tells you whether the rest of the readings can be trusted.

Test at every level. Atmospheres in confined spaces stratify. Carbon dioxide, hydrogen sulphide and most solvent vapours are heavier than air and collect at the bottom. Methane and hydrogen are lighter and collect at the top. A single reading at the entry point can easily miss a lethal pocket two metres below. Sample at the top, middle and bottom, and record each reading on the permit.

Allow for transit and response time. A pumped instrument does not read instantly. The sample has to travel the full length of the tubing before it reaches the sensors, and the sensors then need time to stabilise. Long sampling lines, bends, moisture and dirty filters all increase the delay, and a reading taken too early reads clean simply because the sample has not arrived yet. Use a water trap or float probe where liquid is present, and check filter and tubing condition before every use.

Keep monitoring throughout. Pre-entry testing proves the atmosphere at one moment. It does not stay proven. Disturbing sludge or scale releases trapped gas, hot work consumes oxygen and generates carbon monoxide, ventilation can fail, and process upsets can push gas in from connected pipework. Every entrant should wear a personal multi-gas detector for the duration of the work, and an area monitor at the entry point gives the top person independent visibility of conditions.

Prove the instrument. A detector that has not been bump tested is an unproven instrument. Bump test before every use to confirm that the sensors and alarms respond, and calibrate to the manufacturer's schedule. Docking stations automate both and keep the records that an enforcing officer will ask for.

How much of this applies to a given job depends on how the entry is graded.