Pub. Util. Code, § 28851

If there is a question whether an employee organization represents a majority of employees or whether the proposed unit is or is not appropriate, such matters shall be submitted to the California State Mediation and Conciliation Service for disposition. The California State Mediation and Conciliation Service shall promptly hold a public hearing and may, by decision, establish the boundaries of any collective bargaining unit and provide for an election to determine the question of representation. Provided, however, any certification of an employee organization to represent or act for the employees in any collective bargaining unit shall not be subject to challenge on the grounds that a new substantial question of representation within such collective bargaining unit exists until the lapse of one year from the date of certification or the expiration of any collective bargaining agreement, whichever is later.