“Maintenance” as used in this chapter in relation to lighting systems, includes the replacement of any obsolete equipment with new modern equipment found by the board of supervisors or legislative body of a city to be necessary for the proper operation of the district. The replacement of any such obsolete equipment with any such new modern equipment, except for work done by a utility district or a private utility company under contract with the maintenance district, shall be subject to the requirements of Article 56 (commencing with Section 20840) of Chapter 1 of Part 3 of Division 2 of the Public Contract Code.