What is a confined space?
A confined space is a wholly or largely enclosed space with restricted access, insufficient natural ventilation and no intended use for continuous occupancy, in which workers can be seriously harmed by the atmosphere, released substances or stored energy. Examples include tanks, silos, reactor vessels, sewer manholes, crawl spaces and pipe tunnels. Work in such a space mostly takes place during inspection, cleaning, maintenance and repair, and always calls for a work permit, a pre-entry gas test and an attendant outside the space.
A confined space often looks harmless. The danger lies in what you cannot see or smell: too little oxygen, toxic gases or an explosive mixture.
🧠 When does a space count as a confined space?
The Dutch Working Conditions Decree (Arbobesluit) has no separate definition of a “confined space”. The term comes from practice, sector health and safety catalogues and company procedures. A space is generally treated as confined when several of these characteristics come together:
- Restricted entry and exit - a narrow manhole, hatch or ladder, which makes getting in and out, and above all rescue, difficult.
- No or insufficient ventilation - gases and vapours accumulate; heavy gases such as CO2 and H2S sink to the lowest point.
- Not designed for continuous occupancy - the space is only entered for temporary work.
- Hazardous substances or energy - product residues, sludge, ignition sources, agitators, steam or electrical voltage.
Small or seemingly harmless spaces also count, such as a meter pit, an open pit in which nitrogen lingers or a cable basement. The British Confined Spaces Regulations 1997 take a similar approach: a space that is wholly or largely enclosed and where there is a reasonably foreseeable specified risk of fire, explosion, loss of consciousness, asphyxiation or drowning. The US OSHA standard 29 CFR 1910.146 distinguishes between an ordinary confined space and a permit-required confined space, which may only be entered under a permit.
🏭 Where do you find confined spaces?
| Sector | Typical confined spaces | Main hazard |
|---|---|---|
| Chemicals and petrochemicals | Storage tanks, reactor vessels, columns, heat exchangers | Product residues, inert gas (nitrogen), explosive mixture |
| Water and sewerage | Sewer manholes, pumping stations, treatment plant basements | H2S, methane, oxygen deficiency, drowning |
| Food and agriculture | Silos, mixing vessels, fermentation tanks, manure pits | CO2 from fermentation, dust explosion, engulfment in bulk material |
| Energy | Boilers, flue gas ducts, wind turbine hubs, transformer vaults | CO, heat, limited escape route |
| Buildings and utilities | Crawl spaces, shafts, cable basements | Oxygen deficiency, slip, trip and fall hazards |
| Shipping | Cargo tanks, double bottoms, ballast tanks | Gases from cargo, rust that consumes oxygen |
Notably, the biggest hazards often do not come from the space itself but from the process around it: a valve that was not isolated, an agitator that starts via the control system, or nitrogen flowing in through a forgotten connection.
⚠️ What hazards do workers face in a confined space?
- Asphyxiation - oxygen is displaced by nitrogen, argon or CO2, or consumed by rusting and decomposition. At a very low oxygen level a worker can lose consciousness after a few breaths, without any prior warning.
- Poisoning and intoxication - H2S from sewers and sludge, CO from combustion, solvent vapours from product residues. At high concentrations H2S can no longer be smelled, because the sense of smell shuts down.
- Fire and explosion - flammable vapours or dust above the lower explosive limit, or an oxygen level that is too high. The rules for ATEX and hazardous area zones then apply.
- Energy and mechanical hazards - agitators, screw conveyors, steam, hydraulic pressure and electrical voltage. Isolation following Lock-out Tag-out is therefore essential.
- Engulfment and drowning - collapsing bulk material in a silo, or water and sludge that suddenly flows in.
- Heat, slips, trips and falls - and an escape route that is barely usable in case of panic or injury.
A well-known and recurring pattern in accidents is that colleagues who rush to help a victim are overcome themselves. That is why the most important rule for the attendant and colleagues is: never go in without independent respiratory protection and a rescue plan.
🔍 Which gas limits apply to pre-entry testing?
Article 3.5g of the Dutch Arbobesluit states that a worker may only enter a space where there may be a risk of asphyxiation, intoxication, poisoning, fire or explosion if an investigation shows that the hazard is not present. The article itself sets concrete values:
| Parameter | Arbobesluit art. 3.5g (NL) | OSHA 29 CFR 1910.146 (US) | Common company practice |
|---|---|---|---|
| Oxygen, lower limit | Risk of asphyxiation below 18 vol% | Hazardous below 19.5 vol% | Often 19 or 19.5 vol% |
| Oxygen, upper limit | Risk of fire or explosion above 21 vol% | Hazardous above 23.5 vol% | Normal air contains 20.9 vol% |
| Flammable gases and vapours | Hazard above 10% of the lower explosive limit (LEL) | Hazardous above 10% LFL | Many sites: entry only at close to 0% LEL |
| Toxic substances | Hazard above the occupational exposure limit (art. 4.3), for example CO 23 mg/m³ (about 20 ppm, 8 hours) | Above the permissible exposure limit | Gas detector alarm thresholds at or below the limit |
As an illustration: the lower explosive limit of methane is around 4.4 vol%, so 10% LEL corresponds to about 0.44 vol% (4,400 ppm). Always test in a fixed order: first oxygen, then flammable gases, then toxic substances. A catalytic LEL sensor needs oxygen to measure correctly. Also test at several heights, because heavy gases collect at the bottom and light gases such as methane at the top.
If working in a hazardous atmosphere is unavoidable, paragraph 4 of article 3.5g requires that the worker is permanently observed and that effective help can be given immediately in case of direct danger. For designated work on tank vessels, such as cleaning, maintenance and demolition, article 3.5h applies instead of article 3.5g: a gas expert holding a certificate of competence assesses the hazards beforehand and issues a statement.
🔧 How do you enter a confined space safely in seven steps?
- Avoid entry where possible - inspect with cameras, drones or robots and clean from outside. This is also the first rule of the British regulations.
- Assess the risks - include confined spaces in the RI&E and prepare a task risk analysis (TRA) for each job. For complex installations a risk assessment or HAZOP helps.
- Isolate and empty - drain, flush and ventilate the space. Fit blind flanges or remove spool pieces, and lock out agitators and pumps following Lock-out Tag-out. A software block in the control system is not isolation.
- Test and release - an authorised gas tester measures from outside and records the readings on the work permit. During longer work a personal gas detector keeps measuring continuously.
- Equip - use PPE such as respiratory protection, a rescue harness with lifeline and a personal gas detector. In potentially explosive spaces, tools and lighting are explosion-protected, for example of intrinsically safe design. For electric hand tools in conductive locations with restricted movement, NEN 1010 sets additional requirements, and the work itself falls under NEN 3140.
- Post an attendant - the attendant (in Dutch practice the mangatwacht) stays at the opening, keeps in contact with the workers inside, records who is inside and raises the alarm. The attendant never goes in.
- Practise the rescue - a rescue plan with a tripod, winch or stretcher, and arrangements with the in-house emergency response team. Do not rely solely on the fire brigade: it often arrives too late for someone without oxygen.
Before starting, every worker also runs through an LMRA. Companies with a VCA certificate usually have these steps built into their work instructions already.
🖥️ How does a confined space relate to IT and OT?
Confined-space work looks like a pure occupational safety topic, but automation plays a role in three ways:
- Process isolation - modern plants are operated remotely via SCADA or access for remote maintenance. An operator or supplier can start a pump or agitator without knowing that someone is inside the tank. Physical lock-out and a clear status on the operator screen are therefore both needed, and an interlock must never be the only measure.
- Connected gas detection - portable gas detectors with a sensor for oxygen, LEL, CO and H2S send their readings and man-down alarms wirelessly to a central screen, for example via mobile networks or LoRaWAN. The attendant or control room then sees immediately when a reading or a worker deviates. Good alarm management keeps such alerts from drowning among other alarms.
- Digital permits and registration - digital permit-to-work systems link the permit to isolation points, gas tests and entry registration, with a complete log for audits and incident investigation.
This brings new dependencies. A wireless gas monitoring system is an IoT application with its own network and security requirements, and must never be the only safety net: the local alarm on the detector and the attendant remain leading. In the water sector, sewage pumping stations are both confined spaces and critical OT installations, so occupational safety and cybersecurity literally meet in the same pit.
❓ Frequently asked questions
Is a gas test mandatory before entering a confined space?
Yes, in the Netherlands article 3.5g of the Arbobesluit requires an investigation before anyone enters a confined space where there may be a risk of asphyxiation, poisoning, fire or explosion. In practice this is a gas test for oxygen, flammable gases and toxic substances. During longer work a gas detector keeps measuring continuously.
What oxygen level is safe in a confined space?
Normal outdoor air contains 20.9 percent oxygen by volume. The Dutch Arbobesluit defines a risk of asphyxiation below 18 vol% and a risk of fire or explosion above 21 vol%, while OSHA in the US treats below 19.5 vol% or above 23.5 vol% as hazardous. Many companies apply stricter internal limits for a confined space, such as a minimum of 19 or 19.5 vol%.
What does a confined-space attendant do?
A confined-space attendant stays at the entrance, keeps continuous contact with the workers inside and records who is inside. In case of an alarm or deviation the attendant calls everyone out and starts the rescue procedure. The attendant does not go in; a rescue from inside is carried out only by trained rescuers with independent respiratory protection.
Is a work permit required for confined-space work?
Dutch law does not literally prescribe a work permit in every sector, but it does require investigation, control measures and permanent observation in hazardous atmospheres. In the process industry and under VCA, a confined-space entry permit is therefore standard practice. The permit records isolation, gas tests, equipment, attendant and rescue plan.
Can you ventilate a confined space with pure oxygen?
No, you never ventilate a confined space with pure oxygen. An oxygen level above 21 vol% sharply increases the fire risk, which is why the Dutch Arbobesluit explicitly names it as a hazard. Use forced ventilation with clean outside air and test again afterwards.
What is the difference between a confined space and an ATEX zone?
A confined space is about the space itself: restricted access, poor ventilation and possible hazardous atmospheres. An ATEX zone indicates how often an explosive atmosphere can occur. A confined space can also be an ATEX zone, in which case both sets of rules apply.
📌 In summary
A confined space is an enclosed workplace with restricted access and poor ventilation, where oxygen deficiency, toxic gases, explosion hazards and released energy can become life-threatening within seconds.
In the Netherlands, article 3.5g of the Arbobesluit under the Working Conditions Act requires you to demonstrate before entry that there is no hazard, with concrete limits such as 18 vol% oxygen and 10% of the lower explosive limit. In practice this means a work permit, isolation following Lock-out Tag-out, gas testing, PPE, an attendant and a practised rescue plan. Automation helps with connected gas detection and digital permits, but also calls for physical isolation against remote starts. Only trained and authorised personnel enter a confined space, and only after release.