Exceptions from liability under section 3
4.  No liability shall be incurred by the owner of a ship under section 3 by reason of any discharge or escape of oil from the ship, or by reason of any relevant threat of contamination, if he proves that the discharge or escape, or the relevant threat of contamination, as the case may be —
(a)resulted from an act of war, hostilities, civil war, insurrection or an exceptional, inevitable and irresistible natural phenomenon;
(b)was due wholly to anything done or left undone by another person, not being an employee or agent of the owner, with intent to do damage; or
(c)was due wholly to the negligence or wrongful act of a government or other authority in exercising its functions of maintaining lights or other navigational aids for the maintenance of which it was responsible.