An apparatus comprising: a first elongate substantially electrically insulating structure arranged to contact, when the apparatus is in use, each of an electrically conductive supporting structure and an energized DC transmission line, the electrically conductive supporting structure supporting the energized DC transmission line which is located between a surface of Earth and the electrically conductive supporting structure; the apparatus further comprising a DC current measuring device electrically wired in series between the first elongate substantially electrically insulating structure and an electrical ground; and a second elongate substantially electrically insulating structure arranged to contact, when the apparatus is in use, each of the electrically conductive supporting structure and the energized DC transmission line, wherein the first elongate substantially electrically insulating structure and the second elongate substantially electrically insulating structure are substantially parallel to each other and, when the apparatus is in use, are positionable on opposite sides of an insulator that is about to be replaced and are in tension due to a weight of the energized DC transmission line, and wherein, during replacement of the insulator, the DC current measuring device is configured to measure a momentary DC leakage current of a momentary current spike from the energized DC transmission line passing through at least the first elongate substantially electrically insulating structure in millisecond time intervals, to store in a digital memory, a plurality of the momentary DC leakage currents measured by the DC current measuring device and to deliver a warning if the momentary DC leakage current reaches a threshold DC current value, wherein the threshold DC current value is indicative of a DC current flashover current value for at least the first elongate substantially electrically insulating structure. a first elongate substantially electrically insulating structure arranged to contact, when the apparatus is in use, each of an electrically conductive supporting structure and an energized DC transmission line, the electrically conductive supporting structure supporting the energized DC transmission line which is located between a surface of Earth and the electrically conductive supporting structure; the apparatus further comprising a DC current measuring device electrically wired in series between the first elongate substantially electrically insulating structure and an electrical ground; and a second elongate substantially electrically insulating structure arranged to contact, when the apparatus is in use, each of the electrically conductive supporting structure and the energized DC transmission line, wherein the first elongate substantially electrically insulating structure and the second elongate substantially electrically insulating structure are substantially parallel to each other and, when the apparatus is in use, are positionable on opposite sides of an insulator that is about to be replaced and are in tension due to a weight of the energized DC transmission line, and wherein, during replacement of the insulator, the DC current measuring device is configured to measure a momentary DC leakage current of a momentary current spike from the energized DC transmission line passing through at least the first elongate substantially electrically insulating structure in millisecond time intervals, to store in a digital memory, a plurality of the momentary DC leakage currents measured by the DC current measuring device and to deliver a warning if the momentary DC leakage current reaches a threshold DC current value, wherein the threshold DC current value is indicative of a DC current flashover current value for at least the first elongate substantially electrically insulating structure.