If the mediator is unable to effect settlement of the controversy within 15 days after his appointment and the mediator declares that factfinding is appropriate to the resolution of the impasse, either party may, by written notification to the other, request that their differences be submitted to a factfinding panel. Within five days after receipt of the written request, each party shall select a person to serve as its member of the factfinding panel. The board shall, within five days after such selection, select a chairman of the factfinding panel. The chairman designated by the board shall not, without the consent of both parties, be the same person who served as mediator pursuant to Section 3590.