Section 2(7) in The Chennai Corporation Officers Service Rules, 1975
(7)A person is said to be "on duty" as member of a service-(a)when he is performing the duties of a post borne on a cadre of such service or in undergoing the probation, instruction or training prescribed for such service;(b)when he is on joining time; or(c)when he is absent from duty during vacation or authorised holidays or on casual leave issued by the State Government having been on duty immediately before and immediately after such absence;
