81. Exemption from arrest and personal appearance.—In a suit instituted against a public officer in respect of any act purporting to be done by him in his official capacity—
(a) the defendant shall not be liable to arrest nor his property to attachment otherwise than in execution of a decree, and,
(b) where the Court is satisfied that the defendant cannot absent himself from his duty without detriment to the public service, it shall exempt him from appearing in person.