Section 56D. No contract for the purchase of equipment, supplies or materials, the actual or estimated cost of which amounts to $25,000 or more, and no contract for the purchase of generation, transmission or distribution equipment, the actual cost or estimated cost of which amounts to $25,000 or more, except in cases of special emergencies involving the health, safety or welfare of the people or their property, shall be awarded unless proposals for the same have been invited by advertisement in at least one newspaper published in the city or town in which the lighting plant is located, or, if there is no such newspaper, in a newspaper published in the same county, such publication to be at least one week before the time specified for the opening of said proposals. Such advertisement shall state the time and place for opening the proposals in answer to said advertisement, and shall reserve to the municipal light commission the right to reject any or all such proposals. All such proposals shall be opened in public. No bill or contract shall be split or divided for the purpose of evading any provision of this section. This section shall not apply to contracts for the supply of electricity to a municipal lighting plant.