OPGW and ADSS: the fibre decides the drum.

OPGW replaces the shield wire at the top of a transmission line. The optical unit sits inside a stainless or aluminium tube, and that tube is surrounded by stranded aluminium-clad steel or alloy wires that carry fault current and lightning. The cable does two jobs at once.

ADSS carries no current. Aramid yarn takes the tensile load, the cable is jacketed in polyethylene, and track-resistant jackets are used where the cable will sit in a high electric field. It is lashed or suspended on its own fittings below the conductors, often on the same structures.

Fibre count is the first number on the drum tag: twelve, twenty-four, forty-eight, more. Single-mode fibre is the norm on utility routes, and the tag should say what standard the fibre meets. A drum with more fibres is a different proposition from one with a small count and the same metal weight.

Where the tag is missing, the count can sometimes be recovered from the splice closure that came with the drum, or from the cable maker's print on the jacket. If neither exists, the fibre is treated as unknown and the drum is reviewed on metal alone, with the tube counted as scrap.

• Fibre count and type, read from the tag and confirmed against the tube count.
• Single-mode or multimode, since utility routes are almost always single-mode.
• Attenuation and OTDR results where the drum was tested before storage.
• Jacket and tube condition, especially any crush or water ingress at an end.

Optical aerial work produces closures, deadend clamps, tension sets, suspension clamps and slack storage. These are often left in the same yard as the drums and usually change hands with them. A closure with trays inside is worth opening before it is written off as mixed scrap.

Fittings are matched to cable diameter, so an ADSS clamp set does not fit OPGW. Sorting hardware by cable type before a visit makes the review quicker, stops usable fittings being weighed as scrap, and shows which drums were installed as complete spans with their own tension sets.

A crushed tube, a kinked span end or water sitting in a loose tube can take a drum out of optical use entirely. The metal remains, but the lot is then a metal lot. An OTDR trace is the only reliable way to say whether the fibre survived, and without one the buyer has to assume the worst.

A drum that has been dropped on its flange or dragged across a yard may look sound and still fail a trace. The jacket hides the tube, and the tube hides the fibre. That is why the test record, the isolation clearance and the handling history matter as much as the metal on the outside, and why the photographs should include both drum ends.

Do not cut an OPGW drum to inspect the tube. A cut end admits moisture and the fibre cannot be tested afterwards without a splice.