For a longwall face near the face, at what concentration should equipment be tripped?

• A. 2%
• B. 1%
• C. 1.25%
• D. 3%

Answer: (A)

The key idea is that electrical equipment near the longwall face should automatically shut down when methane levels rise enough to pose a real ignition risk. The face area has intense activity, limited fresh-air dilution, and potential gas pockets, so having a lower, preemptive trip point is a crucial safety measure.

Setting the trip threshold at two percent methane near the face provides a sensible margin: it’s high enough to avoid excessive nuisance trips during normal operation, but low enough to stop equipment before gas concentrations can escalate toward dangerous levels or before ventilation can fully compensate. If the threshold were much lower, trips would be too frequent and disrupt production; if it were higher, more methane could accumulate, increasing the chance of ignition once ignition sources are present.

So, two percent is chosen as the appropriate trip concentration near the face to balance safety and practicality, ensuring equipment is de-energized before the risk becomes unacceptable.