What is the burden of proof for testimony on public policy grounds?

testimony on public policy grounds bears the burden of proof on any foundational issues of fact. (See, e.g., People v. Badgett (1995) 10 Cal.4th 330, 364 [41 Cal.Rptr.2d 635, 895 P.2d 877] [The claimant of the marital privilege for confidential communications has the burden of proving, by a preponderance of the evidence, the facts necessary to sustain the claim.].) Likewise, under section 500, [e]xcept as otherwise provided by law, a party has the burden of proof as to each fact the existence or nonexistence of which is essential to the claim for relief or defense that he is asserting.