Cal. Pub. Util. Code § 12104

Every appointee to a permanent position shall be on probation for six months, except that the general manager may establish a probationary period for up to 12 months for professional, scientific, technical, administrative, management, or executive positions, or for positions requiring certification by the state pursuant to Chapter 9 (commencing with Section 13625) of Division 7 of the Water Code. The general manager may terminate the appointment during the probationary period. If the appointment is not then terminated it shall be permanent, subject to this division and the rules and regulations adopted pursuant to this division.