What Is Confined Space Entry?

A confined space is an enclosed or partially enclosed area that is large enough for a worker to enter, has limited or restricted entry/exit points, and is not designed for continuous occupancy. Confined space entry (CSE) is one of the highest-risk activities in oil and gas, refining, and EPC construction, responsible for multiple fatalities each year across the industry.

OSHA 29 CFR 1910.146 (Permit-Required Confined Spaces) establishes the regulatory framework in the United States. Internationally, similar requirements exist under UK CDM Regulations, EU Directive 92/57/EEC, and national safety codes.

Confined Space Examples

Industry	Confined Space Examples
Oil & Gas / Refining	Pressure vessels, columns, drums, heat exchangers, storage tanks
Pipelines	Large-diameter pipes (>24”), pipeline pig traps, valve vaults
Construction	Manholes, excavations >1.2 m deep, crawl spaces, ductwork
Marine	Ballast tanks, cargo holds, chain lockers
Water/Wastewater	Pump stations, wet wells, digesters, sewer lines

Atmospheric Hazards

Atmospheric testing is mandatory before and during confined space entry. The gas tester must confirm safe levels of:

Parameter	Safe Entry Level	Danger Level
Oxygen (O2)	19.5% - 23.5%	< 19.5% (oxygen deficient) or > 23.5% (oxygen enriched)
LEL (flammable gas)	< 10% of LEL	> 10% LEL (entry prohibited without special measures)
H2S (hydrogen sulfide)	< 10 ppm (8-hour TWA)	> 10 ppm TWA; > 15 ppm STEL; immediately dangerous at 100 ppm
CO (carbon monoxide)	< 25 ppm (8-hour TWA)	> 25 ppm TWA
SO2 (sulfur dioxide)	< 2 ppm (8-hour TWA)	> 2 ppm TWA
Other toxic gases	Below applicable PEL/TLV	Above PEL/TLV for specific substance

Common Mistake

Testing atmosphere at the entry point only. Heavier-than-air gases (H2S, CO2) accumulate at the bottom; lighter gases (methane) rise to the top. Test at multiple levels: top, middle, and bottom of the space before and continuously during entry.

Entry Permit Requirements

A confined space entry permit must document:

Permit Element	Details
Space identification	Tag number, description, location
Purpose of entry	Inspection, cleaning, repair, welding
Hazard identification	Atmospheric, physical (engulfment, entrapment), chemical, thermal
Isolation	LOTO verification, blinds installed, drains open, purge complete
Atmospheric test results	O2, LEL, H2S, CO readings with time, date, and tester signature
Ventilation	Forced mechanical ventilation type, flow rate, duct placement
PPE	Respiratory protection (SCBA or air-line if required), harness, hard hat
Communication	Method between entrant and attendant (voice, radio, signal line)
Rescue plan	Rescue team on standby, retrieval equipment in place, hospital route identified
Duration	Permit validity (typically 8-12 hours; re-test atmosphere each shift)

Roles

Role	Responsibility
Entrant	Enters the space; wears required PPE; exits immediately if conditions change
Attendant (Hole Watch)	Stationed at entry point; monitors entrant; initiates rescue if needed; never enters the space
Entry Supervisor	Authorizes entry; verifies all permit conditions are met; cancels permit if hazards arise
Rescue Team	Trained and equipped for confined space rescue; positioned within response time

Pro Tip

The most common fatal error in confined space incidents is untrained rescuers entering the space to help a downed worker, only to become victims themselves. OSHA data shows that over 60% of confined space fatalities are would-be rescuers. Always use trained rescue teams with proper equipment—never enter without a plan.

Confined space entry is managed under the site Permit to Work (PTW) system and requires coordination with isolation procedures (LOTO) and atmospheric monitoring.

Read the full guide to LEL/UEL and flammable gas safety