Breakaway cables are a  legal requirement for all trailers with brakes.An emergency breakaway cable must be fitted to the parking brake linkage and the other end clipped or fixed round some fixture on the towing vehicle so that, in the event of the trailer becoming detached from the towing vehicle, the cable will apply the parking brake automatically, before snapping itself. It is not recommended to loop the cable round the towball. (But do so if there is no alternative attachment point.) It is a separate offence not to use the breakaway cable provided.