On a longwall shearer, what are the alarm and trip settings for the detector that monitors the general body methane around the machine?

• A. Must have an alarm at 1%, trip the cutters at 1.25%, and trip the machine at 2%.
• B. Alarm at 2%, trip at 3%, machine shut down at 4%.
• C. Alarm at 0.5%, no trip required.
• D. Alarm at 1.5%, trip the cutters at 2%, trip the machine at 2.5%.

Answer: (A)

These settings reflect how methane around the longwall machine is controlled with a stepped safety response: an early warning to give time to ventilate or adjust, a partial shutdown to reduce sources of methane near moving parts, and a full machine shutdown if the concentration keeps rising.

Why this combination is best: a 1% alarm provides a prompt alert so you can stop and ventilate or investigate before methane accumulates further. If the concentration reaches 1.25% around the cutters, tripping the cutters stops the active methane generation and removes the ignition risk from the moving part of the machine. If methane continues to rise to 2%, shutting down the entire machine stops all operations, giving the safest possible pause to address the hazard. This progression balances timely warning with graded responses to prevent ignition and control exposure near the machinery.

Other options would either delay warning (for example, alarm set higher than 1%), omit a necessary shutdown step, or use inconsistent thresholds that don’t align with the standard protective sequence for general body methane around the machine.