In the case of a gas line severed by excavation, which row shows readings necessitating evacuation?

The correct choice indicates that evacuation is necessary based on the presence of gas concentrations that pose significant safety risks. Specifically, Row 4 at 12% of the Lower Explosive Limit (LEL) and Row 2 at 2.7% of the LEL signal a need for immediate action due to the potential for an explosive atmosphere; once concentrations reach even a small fraction of the LEL, the risk escalates. Row 3 at 40% of the LEL indicates a critical and immediate danger, making evacuation imperative.

Monitoring for gas presence in an excavation scenario is crucial since gas lines can leak quickly and at lower concentrations may not be detectable without proper equipment, thus leading to a possible hazardous situation. In any case where readings reflect LEL percentages that are moderate to high, the safety protocols dictate evacuation to protect personnel from the risk of explosion or health hazards associated with toxic gases.

Other choices lack the necessary combination of readings that indicate a clear and present danger to warrant evacuation. For example, Row 1 at 1% of LEL indicates a minimal risk that typically wouldn't necessitate evacuation unless other factors were in play. Similarly, Row 3 at 5% of the LEL does not reach the threshold where evacuation