Safety
Safety
This domain tests hazard recognition and control in treatment plants, pump stations, sewers, and construction sites. Expect permit-space entry, chemical exposure, electrical safety, traffic control, and emergency response.
Core rules and concepts
- Confined spaces: OSHA 29 CFR 1910.146. Test the atmosphere before entry and continuously; acceptable oxygen is 19.5–23.5%, flammable gas below 10% LEL, and H2S below 10 ppm.
- Test order matters: oxygen first, then flammables, then toxics. Ventilate and retest; never enter to rescue without supplied air and a harness/retrieval system.
- Lockout/tagout under 1910.147 controls stored energy; verify zero energy, not just the tag.
- Chlorine gas is heavier than air and has an IDLH of 10 ppm; ammonia is lighter and also highly toxic. Use SCBA and notify others.
- Trenching: protective systems are required at 5 ft depth or deeper; spoil piles and equipment stay back from the edge.
Worked example
A wet-well entry meter reads 18.5% O2, 4 ppm H2S, and 8% LEL. Although H2S and LEL are below action levels, oxygen is below 19.5%, so the space is oxygen-deficient and entry is prohibited. Ventilate, find the cause, retest, and use a permit with an attendant and retrieval system. If the meter had read 19.6% O2, the other readings would still require evaluation, but the atmosphere would not be immediately dangerous on oxygen alone.
Traps
Treating one safe reading as clearance; entering after ventilation without retesting; relying on a dust mask for chlorine or H2S; and assigning a lone worker as both entrant and attendant.