4. This act does not apply:
(a) to agricultural operations ordinarily developed by the farmer himself or by fewer than three full-time employees;
(b) to maintenance and repair work in buildings or on civil engineering works carried out by employees ordinarily employed in maintenance work in buildings or on civil engineering works or who are production workers in an establishment, who are directly employed by an employer other than a contractor;
(c) to construction work on water supply systems, sewers, paving and sidewalks and to other work of the same nature carried out by the employees of urban or regional communities and municipalities;
(d) to construction work directly related to mining exploration or operations carried out by the employees of a mining firm;
(e) to construction work directly related to forest operations, carried out by the employees of forest operations firms;
(f) to construction work on power transmission lines, carried out by the employees of electric supply firms.