California Code, Public Utilities Code - PUC § 30730

The board may, by resolution, order that any of the moneys in the funds under its control which are not necessary for current operating expenses be invested in any obligations, bonds or securities in which a county could invest such funds;  provided, however, that (1) any such investments shall be made in such a manner that the moneys in such funds will be available at the times and in the amounts necessary to accomplish the purpose for which said funds were established, and (2) no such investment shall be made in contravention of any provision or covenant in any proceedings for the authorization and issuance of bonds, notes, contracts or other evidences of indebtedness.