No electrical permit shall be required for making repairs or maintenance replacements to any wiring system. No electrical permit shall be required for the installation or alteration of wires to operate an electrical bell, gas lighting apparatus, call bell, burglar alarm, telephone, telegraph, direct messenger, fire alarm or other similar installation or alteration when a transformer is not required and when the voltage used is not over twenty-four volts. No permit shall be required for the installation or alteration of any electrical system used in connection with electric light, heat, or power in any industrial plant, central station, substation, or public utility building if an expert electrician is regularly employed and has charge of electrical work in such plant, station, or building.