Section 21F. The provisions of sections twenty-one A to twenty-one E, inclusive, shall not apply to the construction, reconstruction, operation and maintenance of overhead electrical conductors and their supporting structures and associated equipment by authorized electrical workers; nor to any authorized person engaged in the construction, reconstruction, operation and maintenance of overhead electrical or communications circuits or conductors and their supporting structures and associated equipment of rail transportation systems, electrical generating, transmission or distribution systems, or communications systems.

As used in sections twenty-one A to twenty-one F, inclusive, the words ''high voltage'' shall mean a voltage in excess of four hundred and forty volts, measured between conductors, or measured between the conductor and the ground; the words ''mechanical barrier'' shall mean, temporary devices for separating and preventing contact between material or equipment and overhead electrical conductors, such as:—

(a) Series of poles or the equivalent;

(b) Nonconductive enclosures around conductors.

''De-energizing'' shall mean removing the voltage from electrical conductors.

''Temporary relocation'' shall mean:

(a) Removing electrical conductors from poles;

(b) Elevating electrical conductors from poles;

(c) Rerouting electrical conductors.

''Authorized person'' shall mean:

(a) Employees of a light and power company with respect to the electrical system of such a company, and the employees of a transportation system with respect to the electrical circuits of such system;

(b) Employees of communication utilities, state, county or municipal agencies having authorized circuit construction on the poles or structures of an electric power company or transportation system or communication system;

(c) Employees of an industrial plant with respect to the electrical system of such plant;

(d) Employees of any electrical or communications contractor with respect to work under his supervision.

''Warning sign'' shall mean a weather resistant sign of not less than five inches by seven inches with a yellow background and black lettering reading as follows: ''WARNING—Unlawful to operate this equipment within SIX FEET of high voltage lines.''