Corrosion resistance, far more than mechanical rating - because the mechanical loads are modest and constant while the environment attacks continuously.

A suspension clamp on a telecoms cable carries a load well within its rating for its whole life. What actually ends its service is the environment: rain, ultraviolet, freeze-thaw, industrial atmosphere, and salt near the coast.

So the specification that matters is material and coating. Hot dip galvanised steel is the general standard for brackets and bands; stainless is specified for coastal and aggressive environments; and polymer components must be ultraviolet-stabilised or they become brittle and fail within a few years.

The failure mode is worth understanding: a corroding fitting rarely announces itself. It holds until it does not, and the result is a cable down across a road. Because aerial routes are inspected rarely and are hard to reach, over-specifying corrosion protection is one of the cheapest decisions available on the whole route.

Because the clamp grips by compressing the cable, and cables differ enormously in what they will tolerate.

A clamp is designed for a diameter range and for a particular construction. Fitted to a cable outside that range it either slips - because there is not enough grip - or crushes, because the clamping force intended for a larger cable is concentrated on a smaller one.

Fibre optic cable is the sensitive case. Optical fibre is degraded by micro-bending and by pressure on the buffer tubes, and the damage does not show as a broken cable. It shows as increased attenuation - a route that works but performs below specification, which is diagnosed as an optical problem long before anyone suspects a clamp.

So confirm the cable's outside diameter and its construction against the clamp's stated range, and use clamps specified for fibre where fibre is involved. ADSS cable in particular has its own hardware designed around it.

All-Dielectric Self-Supporting cable - it contains no metal at all and carries its own weight over a span without a separate messenger wire.

Conventional aerial cable is either lashed to a steel messenger strand or built as figure-of-eight with an integral messenger. ADSS dispenses with the messenger entirely: aramid strength members inside the cable take the tension, and the whole construction is non-conductive.

That matters most where the cable shares poles with electrical distribution. A metallic messenger on a power pole is an induction and safety problem; an all-dielectric cable is not.

The consequence for hardware is that ADSS needs clamps designed for it. The cable carries substantial tension in its own jacket, so dead-ends must grip over a long length and distribute the load rather than pinching. Suspension clamps must avoid point loading, and on high-voltage routes they must also manage the electrical field at the cable surface, which can otherwise cause dry-band arcing that erodes the jacket.

By the sag the design permits, the tension the cable and the poles can take, and the worst-case loading the route will see.

A cable strung between two poles hangs in a curve. Increasing tension reduces sag but loads the cable and the poles more; reducing tension increases sag, which must stay above the required ground clearance at the lowest point.

The governing case is not a still summer day. It is the cable loaded with ice and pushed by wind, at low temperature where the cable has contracted and tension is highest - and separately the hottest day, where the cable has expanded and sag is greatest.

So the design has to satisfy clearance at maximum sag and strength at maximum tension, which is why span lengths on the same cable differ between climates.

Manufacturers publish sag and tension tables for their cables against span and loading condition. Those, and the local loading standard, are what set the span - not the pole spacing that happens to exist.

It becomes vulnerable, which is why downlead protection is a distinct product rather than an afterthought.

A cable running horizontally between poles is out of reach. The moment it turns and runs down the pole it is within reach of vehicles, ladders, vandalism, strimmers and animals - and it is also at a bend, which is a mechanical weak point.

U-guards or downlead guards are channel sections fixed over the cable along the vulnerable lower section of the pole, protecting it from impact and from casual interference while keeping it tidy.

The bend itself needs attention too: the cable's minimum bend radius must be respected where it transitions from the span to the downlead, and a bracket or coil holder is used to control it rather than letting the cable find its own radius.

On fibre routes this is a common source of unexplained loss - the cable is intact, the connectors are clean, and the attenuation comes from a bend at the pole that nobody looks at.

Frequently yes, under a joint-use arrangement - but the separation, the clearances and the working practices are governed by the electricity network's rules rather than the telecoms operator's.

Joint use is economically attractive because it avoids a second pole line, and it is common in many networks. The telecoms cable is installed in a defined communications zone below the electrical conductors, with a specified vertical separation that must be maintained under all loading conditions - including maximum sag on a hot day, which is when the gap is smallest.

ADSS cable is normally required, because a metallic messenger in the induced field of a power line is both a safety hazard and an operational problem.

Working practices change completely: anyone climbing a shared pole is working near live conductors, which brings competence, permit and clearance requirements that do not apply on a telecoms-only route.

Agree the arrangement with the network operator before designing, since their standard dictates the hardware as much as the cable does.

Periodic visual survey, with attention to the fittings rather than the cable - because the cable rarely fails first.

Walk or drive the route and look for corrosion at brackets and bands, clamps that have slipped or rotated, cable that has moved in a suspension clamp, damaged or missing downlead guards, and poles that have moved, leaned or been struck.

Check sag against the design at a known temperature - a span that has sagged beyond its design has either lost tension at a dead-end or the cable has crept, and both need attention before clearance is lost.

Vegetation is the routine finding: growth against a cable abrades the jacket over time and adds load in wind.

On fibre routes, trend the optical loss. A slow rise in attenuation on a span with no visible damage usually means a clamp is compressing the cable or a bend has tightened - and finding it from the measurement is far cheaper than finding it from an outage.