§ 24.4 - Requirements for approval.

Single-shot blasting units approved under this part shall be practical in construction and operation as well as suitable for service conditions underground. They shall offer no probable ignition hazard when used in gassy mine atmospheres. They shall meet, under laboratory test conditions, the following requirements of minimum performance:

(a) The unit shall fire consistently single electric blasting caps through a total resistance of 41/2 ohms.

(b) The unit shall not produce sparks capable of igniting explosive mixtures of methane and air. 1

(c) Units of the battery type shall be designed to prevent accidental connection to the firing circuit. Batteries having a short-circuit current greater than 10 amperes, shall be enclosed in an effectively locked or sealed housing.

(d) Storage batteries supplying current for both blasting and illumination at the same time shall have ample capacity for the combined service or shall be equipped with an automatic device that cuts off the lamp during shot firing. The lamp part of the unit shall meet the requirements of the current lamp schedules.

(e) Magneto units shall be operable only by a special key or handle.

(f) A unit that depends on some special modification such as a shunt resistance, to prevent igniting sparks, shall have its housing sealed or locked to prevent removal or disturbance of this safety feature.