Contents
What makes a space “permit-required” The atmosphere: numbers, order, and why the order matters Who does what Making the space safe before the first reading What the permit must record Country instruments The failures that recur Frequently asked questionsA vessel, a sump, a sewer, a silo, a tank car, the space under a floor slab. What makes it dangerous is not the size of the opening — it is that the air inside is not the air outside, and it can change while a person is in there.
What makes a space “permit-required”
Four tests, applied in order. The first three define a confined space; the fourth turns it into one that needs a permit.
| Test | Meaning |
|---|---|
| Large enough to enter | a person can get their body inside and do work |
| Limited entry or exit | a manway, a hatch, a shaft — not a doorway you can run out of |
| Not designed for continuous occupancy | it exists to hold product, not people |
| Holds a serious hazard | hazardous atmosphere, engulfment risk, converging walls, or any other recognised serious hazard |
That fourth test is where judgement lives. A clean, ventilated pit with no stored energy may be entered under a lighter regime in some jurisdictions. A grain silo, where the material itself can swallow a person in seconds, is permit-required no matter how good the air is.
The atmosphere: numbers, order, and why the order matters
| What is measured | Acceptable range | If it fails |
|---|---|---|
| Oxygen | 19.5% – 23.5% | do not enter; ventilate and re-test — enrichment above 23.5% is as dangerous as deficiency, because everything burns faster |
| Flammable gas or vapour | below 10% of the lower explosive limit | do not enter; find the source. For entry combined with hot work, many operators demand 0% |
| Toxic contaminants | below the permissible exposure limit for each substance | do not enter without the right respiratory protection and a documented reason |
Test oxygen first, flammables second, toxics third. The reason is physical, not procedural: the catalytic sensor that reads flammable gas burns the sample, and in an oxygen-poor atmosphere it reads low — a lethally rich space can look safe.
Two more rules that get skipped. Test at every level — top, middle and bottom — because heavy vapours sit low and light ones collect at the crown. And test from outside before the first entry: putting a head in to take a reading is entry.
Who does what
- Authorised entrant — goes in, wears what the permit specifies, keeps in contact, comes out on the alarm without arguing.
- Attendant — stays outside for the whole entry, keeps count of who is inside, watches the gas monitor and the plant around the space, orders evacuation, calls rescue. Never enters.
- Entry supervisor — verifies the tests, checks isolations, signs the permit, and cancels it when conditions change or the work ends.
- Rescue team — trained, drilled and available in the time the plan assumes. “Call the fire brigade” is a plan only if someone has checked how long they take and whether they can reach the space.
Rescuers who went in unprepared make up a large share of confined space deaths — a pattern NIOSH, the US research agency for occupational safety, has documented for decades. That is why the attendant’s discipline of staying outside is not caution; it is the control.
Making the space safe before the first reading
The gas test proves the preparation worked; it does not replace it. Before the tester arrives, the space has to be cut off from everything that could refill it:
- Isolate by blinding, not by valves. A closed valve is a decision someone can reverse; a spade or a physical disconnect with an air gap is a fact. Every line in and out of the space is blinded or disconnected, and the permit lists each one by number — “all lines isolated” is not a record, it is a hope.
- Drain, flush and purge whatever the space held. Product left in a dead leg or soaked into scale keeps giving off vapour long after the space looks empty, which is why a space that tested clean in the morning can fail at noon.
- Ventilate with a purpose. Blowing air at the manway moves the reading, not the hazard. Effective ventilation puts the delivery point deep in the space, pulls from the far end, and keeps running for the whole entry — and if it stops, everyone comes out, because the ventilation was a condition of the permit, not a comfort.
- Isolate mechanical and electrical energy too. Agitators, screw conveyors, and anything that can move is locked out before entry; atmosphere is the most famous confined space killer, but it is not the only one.
During the entry, monitoring does not stop. A pre-entry test is a snapshot of a space that breathes — disturbed sludge, warming walls and the work itself all change the air. Continuous monitors with alarms, worn in the breathing zone or placed at the work level, are standard practice at serious operators; where readings are taken periodically instead, the permit states the interval and someone actually keeps it.
What the permit must record
The space and the exact task. The isolations and how they were proved — blanks and blinds listed, not just “isolated”. Every gas reading with the time it was taken and the name of the person who took it. Ventilation arrangements, and whether they run continuously. Access and lighting, both rated for the atmosphere. Communication method. The rescue plan and who is standing by. The names of entrant, attendant and supervisor. And the expiry — the moment the authorisation dies.
If a confined space entry involves welding or cutting, a hot work permit is raised in addition, and the two are cross-referenced. Neither replaces the other.
Country instruments
| Jurisdiction | Instrument |
|---|---|
| United States | OSHA 29 CFR 1910.146 (general industry); 1926 Subpart AA (construction) |
| Mexico | NOM-033-STPS-2015, which requires written authorisation for entry, and NOM-027-STPS-2008 where welding or cutting is involved |
| China | GB 30871-2022, mandatory since 1 October 2022, which covers confined space entry (受限空间作业) among eight permitted special operations |
| Gulf operators | company standards such as Saudi Aramco GI 2.100, which lists confined space entry as its own permit type with a job safety analysis attached |
| International practice | HSE UK guidance HSG250 and IOGP guidance underpin most operator systems |
The failures that recur
The reading taken at the manway instead of inside. The second person who follows the first without being on the permit. Ventilation switched off because it was noisy. An attendant pulled away for ten minutes to do something else. A permit written for one shift and quietly used for the next. Each one is small, and each one has killed people.
Frequently asked questions
In what order should a confined space be tested?
Oxygen first, then flammable gases and vapours, then toxic contaminants. The order is not bureaucratic: a combustible gas sensor needs enough oxygen to read correctly, so an oxygen-deficient atmosphere can make the flammable reading meaningless.
What are the acceptable atmospheric limits?
Under OSHA 1910.146: oxygen between 19.5% and 23.5%, flammable gas below 10% of the lower explosive limit, and each toxic contaminant below its permissible exposure limit. Many operators set stricter internal limits, and for entry with hot work several require 0% LEL.
Can the attendant enter the space to help?
As a rule, no — the attendant stays outside, keeps count, watches conditions and raises the alarm. Some programmes allow a trained, equipped attendant to enter for rescue only after another qualified attendant has relieved them; treat that as the exception, never the plan. Entering unrelieved to help is the most common way one fatality becomes several.
How long is a confined space permit valid?
For the stated window only — typically one shift, and less if conditions change. Re-test after any break in the work, after any change in the plant, and before re-entry. The permit is cancelled when the work ends, when the atmosphere moves outside limits, or when anything on the permit stops being true.
Does a confined space need a permit if the atmosphere is clean?
It depends on the hazard, not on one reading. Where the only hazard was atmospheric and it has been eliminated and cannot return, some jurisdictions allow entry under an alternative procedure. Where engulfment, converging walls, moving parts or stored energy are present, the permit stays.