Challenges for general disqualification may be taken on one or both of the following grounds, and for no other:
(a) A want of any of the qualifications prescribed by this code to render a person competent as a juror.
(b) The existence of any incapacity which satisfies the court that the challenged person is incapable of performing the duties of a juror in the particular action without prejudice to the substantial rights of the challenging party.