(a) when an employee is on leave of absence from his position, a temporary appointment to such position may be made for a period not exceeding the authorized duration of such leave of absence as prescribed by statute or rule;
(b) a temporary appointment may be made for a period not exceeding six months when it is found by the state civil service department or appropriate municipal civil service commission, upon due inquiry, that the position to which such appointment is proposed will not continue in existence for a longer period; provided, however, that where a temporary appointment is made to a position originally expected to exist for no longer than six months and it subsequently develops that such position will remain in existence beyond such six-month period, such temporary appointment may be extended, with the approval of the state civil service department or municipal civil service commission having jurisdiction, for a further period not to exceed an additional six months;
(c) when the department of civil service or appropriate municipal civil service commission of any city containing more than one county finds that a reduction or abolition of positions in the state service or such city service is planned or imminent and that such reduction or abolition of positions will probably result in the suspension or demotion of permanent employees, such department or commission may authorize temporary instead of permanent appointments to be made for a period not exceeding one year in positions in state service or such city service to which permanent employees to be affected by such abolition or reduction of positions will be eligible for transfer or reassignment. Successive temporary appointments shall not be made to the same position after the expiration of the authorized period of the original temporary appointment to such position. 2. Temporary appointments from eligible lists. A temporary appointment for a period not exceeding three months may be made without regard to existing eligible lists. A temporary appointment for a period exceeding three months but not exceeding six months may be by the selection of a person from an appropriate eligible list, if available, without regard to the relative standing of such person on such list. Any further temporary appointment beyond such six month period or any temporary appointment originally made for a period exceeding six months shall be made by the selection of an appointee from among those graded highest on an appropriate eligible list, if available. 3. Temporary appointments without examination in exceptional cases. Notwithstanding the provisions of subdivisions one and two of this section, the civil service department or municipal commission having jurisdiction may authorize a temporary appointment, without examination, when the person appointed will render professional, scientific, technical or other expert services (1) on an occasional basis or (2) on a full-time or regular part-time basis in a temporary position established to conduct a special study or project for a period not exceeding eighteen months. Such appointment may be authorized only in a case where, because of the nature of the services to be rendered and the temporary or occasional character of such services, it would not be practicable to hold an examination of any kind. 4. The state and municipal civil service commissions may, by rule, provide for the extension of some or all of the rights and benefits of permanent status to an employee who is appointed or promoted, after having qualified therefor in the same manner as required for permanent appointment or promotion, to a position left temporarily vacant by the leave of absence of the permanent incumbent thereof. Such rights and benefits shall be subject to such conditions and limitations as may be prescribed in the rules.