Abstract: The unit comprises a casing in the form of a portable box in which there are contained a pneumatic compressor and an electric motor for driving the compressor. The motor is a low voltage d.c. motor and in the casing there is also contained a transformer - rectifier combination for supplying both the motor and low voltage external utilizers. The shaft of the electric motor extends perpendicular to a wall of the casing and has, at an end accessible through an opening in this wall, a power take-off connectable to a mechanical utilizer. The shaft of the compressor is coaxial to the shaft of the motor and is coupled to the latter by means of a clutch. There are provided means for disengaging the clutch to prevent the shaft of the compressor being turned by the motor shaft when a mechanical utilizer is coupled to the power take off.