Cal. Public Utilities Code - PUC § 90680. Section 90680.
All bonds including refunding bonds issued by a district are legal investments for all trust funds and for the funds of all insurance companies, banks, both commercial and savings, and trust companies, and for the State School Fund and for all sinking funds under the control of the State Treasurer. Whenever any money or funds may by law be invested in or loaned upon the security of bonds of cities, cities and counties, counties, or school districts, in the state, such money or funds may be invested in or loaned upon the security of the bonds of the district; and whenever bonds of cities, cities and counties, counties, or school districts by law may be used as security for
the faithful performance or execution of any court or private trust or of any other act, bonds of the district may be so used.