Nothing herein shall be deemed to apply to plumbing systems constructed or maintained by a person doing his own work on his own property, personally or through regularly employed maintenance plumbers; provided, that all such persons shall comply with all applicable ordinances, rules and regulations in regard to plumbing and plumbing systems; nor to piping systems in industrial processing plants located outside the incorporated limits of the city unless such systems are connected to the city water or sewer system. (1960 Code, Sec. 2-503)