Evid. Code, § 646

(a) As used in this section, “defendant” includes any party against whom the res ipsa loquitur presumption operates.

(b) The judicial doctrine of res ipsa loquitur is a presumption affecting the burden of producing evidence.

(c) If the evidence, or facts otherwise established, would support a res ipsa loquitur presumption and the defendant has introduced evidence which would support a finding that he was not negligent or that any negligence on his part was not a proximate cause of the occurrence, the court may, and upon request shall, instruct the jury to the effect that:

(1) If the facts which would give rise to res ipsa loquitur presumption are found or otherwise established, the jury may draw the inference from such facts that a proximate cause of the occurrence was some negligent conduct on the part of the defendant; and

(2) The jury shall not find that a proximate cause of the occurrence was some negligent conduct on the part of the defendant unless the jury believes, after weighing all the evidence in the case and drawing such inferences therefrom as the jury believes are warranted, that it is more probable than not that the occurrence was caused by some negligent conduct on the part of the defendant.