The user of a positive displacement type air compressor which is not provided with automatic means for limiting the operating temperature to a safe level shall provide a fusible plug fitted close to the outlet valves or discharge ports of every stage of compression: Provided that the provisions of this regulation shall not apply to air compressors with a free air delivery of less than 8.5 m3 per minute or in which compression does not take place in the presence of lubricating oil.