permissible

A permissible is any tool, vehicle, or equipment that has been officially tested and approved for use in coal mines and other underground workings where explosive gas or dust may be present. The certification means the device will not create a spark, flame, or heat source capable of igniting methane, coal dust, or other volatile atmospheres. In coal mining, permissible status is not optional; it is a legal requirement enforced by mining authorities and insurance regulators.

The most familiar permissibles are mining locomotives and shuttle cars used to haul coal and supplies underground. A permissible locomotive uses a special electrical system with insulated windings and no exposed commutators that could arc. The brakes, wheels, and bearing surfaces are designed to minimize friction and spark generation even under hard use. Permissible vehicles must also have enclosed flame paths on engines, special battery terminals, and approved cable systems. Smaller tools, including hand drills, cap lamps, and even surveying equipment, can be manufactured to permissible standards.

Certification is conducted by independent laboratories accredited by mining safety authorities. In the United States, equipment is tested to standards set by the Mine Safety and Health Administration (MSHA) or equivalent bodies in other countries. Testing examines electrical leakage, surface temperatures under load, potential spark sources, and the path any flame would take if ignition occurred. Once approved, a permissible device carries a metal tag or plate stamped with the certification number and the date of approval. Re-certification may be required after repairs or significant modifications.

The cost of permissible certification is substantial; a permissible locomotive can cost 20 to 50 percent more than a conventional model because of the reinforced electrical systems, specialized materials, and testing protocols. This premium has driven long equipment lifecycles underground, as mines keep permissible vehicles in service for decades rather than replace them frequently. Worn-out permissibles are sometimes retrofitted rather than scrapped.

Non-compliance carries heavy penalties. Using non-permissible equipment in a gassy mine can result in fines, operational shutdowns, and personal liability for supervisors. The term 'permissible' originated in North American coal mining and is now standard in mining terminology worldwide, though certification standards and naming conventions vary by country and regulatory jurisdiction.