Abstract: A method of continuously monitoring the condition of a power transmission line insulator while it is in service, in which part of the insulator is bypassed by conductive means and in which, when the leakage current passed by the insulator and flowing through the conductive means exceeds a predetermined value, the conductive means is severed by a detonator or other explosive charge and actuating means is actuated which is operative to give a remote indication. Also a sensing and actuating device for use in the method, having conductive means and being adapted to be attached to an insulator so as to bypass part of it, which includes a detonator or other explosive charge adapted to sever the conductive means when leakage current through it exceeds a predetermined value and actuating means in the form of a mercury switch which is operable to give a remote indication when this happens.