Except as provided in this section, municipal corporations or public trusts thereof, the Grand River Dam Authority, rural electric cooperatives or investor-owned electric utilities shall not furnish retail electric service to an electric consuming facility which is currently being served, or which was being served and the electric facilities are in place to render such a service, by a municipal corporation or public trust thereof, the Grand River Dam Authority, a rural electric cooperative or an investor-owned electric utility unless the entities involved have agreed by mutual consent, in writing, to such transaction. For purposes of this section, the term “electric consuming facility” means everything that utilizes electric energy from a central station source.