what safety device must be fitted to a trailer braking

A trailer that has its own braking system must be fitted with a breakaway cable (or breakaway system).

What the required device is

• The additional safety device is a breakaway cable/system that links the trailer to the towing vehicle.

• If the trailer becomes uncoupled, the cable pulls a pin or switch that immediately applies the trailer’s brakes and brings it to a stop.

Why a breakaway cable is required

• It ensures the trailer does not run away uncontrolled if the coupling fails, greatly reducing the risk of a serious collision.

• In many licensing/theory-test style questions (such as UK-style towing questions), “breakaway cable” is the officially expected answer when asked what safety device must be fitted to a trailer braking system.

TL;DR: For the question “what safety device must be fitted to a trailer braking system?”, the correct answer is: a breakaway cable (breakaway braking system).

Information gathered from public forums or data available on the internet and portrayed here.