You should get out of the manhole as soon as

• A. Flow increases slightly
• B. H2S exceeds 5 ppm
• C. Lower explosive level alarm activates
• D. None of the above

Answer: (C)

In sewer manhole work, the immediate danger is a flammable atmosphere. The lower explosive limit alarm signals that enough combustible gas is present to ignite if a spark or heat source is introduced. That makes exiting the space right away the safest action, because you could be exposed to an ignition risk even without any other warning. After evacuating, you would ventilate and re-check the air before re-entering under proper permit procedures. Flow changes or hydrogen sulfide readings—while important safety concerns—do not carry the same immediate ignition risk as a LEL alarm, so they are not the trigger to leave in this scenario.