Bend radius

01The fault that stays live

Fibre is more tolerant than it looks, until it isn't. Bend a strand of single-mode glass too tightly and the light no longer stays trapped inside the core by total internal reflection — it leaks out through the cladding, raising attenuation without breaking anything. The link stays up. Traffic flows. The loss just sits there, quiet, accumulating.

Standard single-mode fibre — ITU-T G.652 — has a minimum bend radius of around 30 mm for permanent installations and 50 mm when handled during installation. Those figures matter most not in the splice case or the cabinet, but in the two or three centimetres of slack loop that an installer coils carelessly behind an enclosure lid, or the cable that gets pinched under a duct fitting. A radius of 10 mm, barely bigger than a thumb, can add several tenths of a decibel. On a link with a tight power budget — common at high split ratios — that fraction is the difference between a marginal pass and a fault.

The frustrating part is how the fault presents. An OTDR trace will often show the bend as a small step loss at a plausible distance, or miss it entirely if the bend is close to a connector. Signal degrades slightly. Speeds drop or error counts rise. The customer calls. A technician arrives, checks connectors, measures optical power, scratches their head. The bend sits undetected until someone physically rehandles the fibre and the loss disappears.

Bend-insensitive fibre — G.657, in two grades — was developed precisely to widen the margin. G.657.A2, common in drop cables, tolerates a 7.5 mm radius with acceptable loss. G.657.B3 manages 5 mm, which is why it dominates indoor and blown-fibre work where tight corners are unavoidable. The trade-off is marginal: slightly higher splice loss at the factory, completely acceptable in practice.

Knowing which fibre class is in the ground matters when reading a trace and the numbers don't quite add up.