It shall be a defence in any legal proceedings to recover damages and in any prosecution, in so far as the proceedings or prosecution are or is based on an allegation of a contravention, in relation to a mine or quarry, of—
(a)a provision of this Act, of an order made thereunder or of regulations (not being a provision which expressly provides that a person is to be guilty of an offence); or
(b)a direction, prohibition, restriction, or requirement given or imposed by a notice served under or by virtue of this Act by an inspector; or
(c)a condition attached to an exemption, consent, approval or authority granted or given under or by virtue of this Act by the Minister or an inspector;
to prove that it was impracticable to avoid or prevent the contravention.