Belt barriers are benign objects with exactly one way of hurting somebody, and it is not the one people expect.

What happens without a brake

A retractable cassette is a spring under tension. Unclip the belt on an unbraked unit and several metres of webbing and a metal clip travel back at whatever speed the spring chooses.

It catches fingers, and at head height it is worse. Anybody who has worked in a venue has seen it happen at least once.

What a brake does

It governs the return speed so the belt retracts steadily rather than snapping. Good cassettes retract at a walking pace regardless of how much webbing is out.

It is a small mechanical component and it is the first thing removed when a barrier is built down to a price.

Long spans need more of it

More webbing out means more stored energy. A standard cassette fitted with a nine-metre belt returns considerably faster than the same cassette with three metres.

Long-span belts should carry a heavier brake as standard. If a supplier quotes the same cassette across every length, ask why.

Cold and grit change it

Brake grease stiffens in cold and thins in heat, and grit in an unsealed cassette either jams the mechanism or wears it until it stops braking at all.

Outdoors, specify a sealed cassette and tell the supplier the temperature range. A brake tuned for a warm foyer is not the same product.

Test it on arrival

Extend a belt fully, unclip it, and watch it return. If it snaps rather than travels, do not put it into service where the public can reach it.

That is a ten-second check and it is worth doing on the first unit of every delivery.