Light trailers up to 750kg maximum laden weight are classed as cat O1 vehicles.
There is no requirement for O1 trailers to have a braking system but, they must be fitted with a ‘secondary coupling’ so that in the event of a breakaway or a primary coupling failure, the trailer is prevented from running away. The secondary coupling has to be strong enough to restrain the fully laden trailer, and short enough to prevent the drawbar striking the ground, thereby retaining some residual steering.
If a cat O1 trailer is fitted with brakes, they must be operational on all wheels and it will be treated as a Cat O2.
Above 750kgs up to 3,500kgs, maximum laden weight, a trailer is a cat O2 vehicle. All cat O2 trailers are required to have an operational braking system on all wheels and, it needs to be auto-reversible. In general, this is achieved via an over-run (inertia) primary coupling, allied with auto-reverse mechanisms in each brake assembly, which detects when the wheels are operating in reverse mode. This self releases when the trailer moves forward.
All O2 trailers must be fitted with a breakaway cable that, in the event of a detachment or primary coupling failure, will activate emergency braking, bringing the trailer to a standstill. ALTERNATIVELY, (but only up to 1500kgs maximum laden weight), an O2 trailer can be fitted with a secondary coupling as per O1 trailers.
A secondary coupling restrains the trailer and needs to be securely attached to the towing vehicles tow frame.
A breakaway cable allows the trailer to separate from the towing vehicle, but with the brakes fully applied, at which point the cable snaps and will need replacing.