(1) All employees whose names appear on the list certified by the board as being a complete list of the employees within the defined appropriate unit shall be eligible to vote.

(2) Any prospective voter may be challenged for cause.

(3) All employees whose names do not appear upon the list certified by the board as being a complete list of the employees within the defined appropriate unit shall be challenged by the agent of the board.

(4) A challenged voter shall be permitted to vote but his ballot shall not be cast. It shall instead be sealed in a separate, unmarked envelope under the supervision of the agent of the board and then inserted in a special identifiable form envelope provided by the board for that purpose and returned to the board.