Before entering a trench more than 4 feet deep where a hazardous atmosphere could exist (e.g., near landfill or sewer), the employer must:

解析

Section 1541(g)(1)(A) requires the atmosphere to be tested before employees enter an excavation deeper than 4 feet where an oxygen deficiency or other hazardous atmosphere could reasonably be expected - near landfill, near sewer, near stored hazardous substances - and (g)(1)(B) then requires precautions against atmospheres below 19.5 percent oxygen. Testing comes first because it is what identifies the control. Ventilating blind (a) or issuing respirators blind (d) both replace the measurement with a guess, and a cartridge chosen without knowing the contaminant is useless against an oxygen deficiency. A trench is not automatically a permit-required confined space (c); section 5157 turns on limited entry and egress plus a listed hazard, and the excavation orders cover most trenches.