NR-33 Confined Exposure Time

NR-33 confined spaces: atmospheric limits & entry permit

Brazilian regulation NR-33 covers work in confined spaces — tanks, silos, sewers, vessels, vaults. Before anyone goes in, the atmosphere has to be monitored without interruption and stay inside 19.5% ≤ O₂ ≤ 23%, with toxic gases under their TLV and flammability under 10% LEL. Step outside those ranges and entry is off the table until ventilation or purging brings the values back to safe.

No entry happens without a written Permit to Work (PET — Permissão de Entrada e Trabalho), good for 12 hours at most and signed by the Supervisor of Entry. A dedicated external Watcher (Vigia) stays outside the entire time, with rescue equipment on hand and a direct line to the people inside. Training isn't optional either: 16 h for authorized Workers, 40 h for Supervisors and Watchers, and an 8 h refresher each year.

Applications

Oil and gas tanks, pulp digesters, sewage manholes, ship holds, grain silos, underground vaults, reactors, HVAC ducts. Use the tool to lay out ventilation cycles, gas-detector calibration intervals, and how long someone can stay before the air quality has to be checked again.

FAQ

Why is 19.5% the O₂ lower limit? Once you drop below 19.5%, thinking and coordination fall off fast. Below 16%, a person can lose consciousness in a matter of minutes.

Can the same PET cover several shifts? No. You issue a fresh permit for each shift, and also any time the conditions change — different people, different tools, a different atmosphere.

Does the Watcher count as rescue team? No. The Watcher sounds the alarm and helps people get out, but a separate, properly trained rescue team has to be on standby with the right PPE and retrieval gear.