An employer shall ensure that, wherever any transformer or switchgear in which higher voltage is used is installed otherwise than in a building, the transformer or switchgear is adequately protected either by -
(a) suitable fencing not less than 2.4 m high, or
(b) some other effective means for preventing any unauthorised person gaining access to the equipment or to anything connected thereto which is used as a conductor
unless it is completely enclosed by -
(i) a metal casing which is connected to earth, or
(ii) some other equally suitable non-metal casing.