For mobile bolters and explosion-protected loaders, what are the detector alarm and trip concentrations?

• A. Alarm at 1%, trip electricity supply at 2%
• B. Alarm at 0.25%, trip at 0.5%
• C. Alarm at 2%, trip at 3%
• D. No alarm or trip

Answer: (A)

The key idea here is how gas detection on mobile mining equipment protects against ignition in a methane-bearing environment. For mobile bolters and explosion-protected loaders, a methane detector provides a two-tier response: an alarm at a low concentration to warn people, and an automatic trip to shut down the electric power if gas rises further.

Setting the alarm at 1% methane gives early notice so workers can take action before conditions worsen. If methane reaches 2%, the system trips the electricity supply to the machine, removing the ignition source and preventing potential ignition of a flammable atmosphere. Methane’s explosive range in air is about 5% (lower explosive limit), so these thresholds are intentionally well below that level to create a safe margin and stop equipment before dangerous levels are reached.

The other options either trigger too readily or provide no automatic shutdown, which would reduce safety and increase risk. Hence, alarm at 1% and trip at 2% is the best and standard approach for these devices.