Section 30. A person or a corporation, private or municipal, owning or operating a line of wires over or under streets or buildings shall use only strong and proper wires safely attached to strong and sufficient supports and insulated at all points of attachment; shall remove all wires the use of which is abandoned; shall properly insulate every wire where it enters a building, and, if such wire is other than a wire designed to carry an electric light, heat or power current, shall attach to it at a proper point in the circuit, near the place of entering the building, and so situated as to avoid danger from fire, an appliance adapted at all times to prevent a current of electricity of such intensity or volume as to be capable of injuring electrical instruments or of causing fire from entering the building by means of such wire beyond the point at which such appliance is attached; and shall properly insulate every wire within a building designed to carry an electric light, heat or power current.