At what general body concentration would you immediately shut off power to an explosion-protected electrically powered loader, vehicle or plant with a trailing cable?

• A. 1.0%
• B. 1.5%
• C. 1.25%
• D. 2.0%

Answer: (C)

The test is about when to cut power to explosion-protected equipment with a trailing cable in a gas-bearing atmosphere. The rule uses a specific gas concentration in the air as the trigger: once the general body concentration reaches 1.25%, you must immediately shut off the power to that equipment. This threshold serves as a clear safety margin to prevent ignition sources from operating in an environment where methane (the common hazard in coal mines) is present and could ignite if an electrical fault or arcing occurred.

Why this limit works: as gas concentration in the mine air increases, the likelihood that an ignition source will ignite a flammable mixture rises. By stopping power at 1.25% concentration, you remove the electrical ignition source while ventilation is used to bring the atmosphere back to a safer level, reducing the chance of an explosion. Trailing cables and electrical components are more vulnerable to sparks and faults, so establishing a firm action trigger helps ensure a rapid, consistent safety response.

The other numbers aren’t the defined trigger in this rule because they would either be too conservative or not conservative enough relative to the established safety standard. The 1.25% mark is the agreed threshold that prompts immediate shutdown to protect workers and equipment in the hazardous zone.