The generic Breakaway Cable is designed for Braked Trailers and many Caravans 1m long x 3mm diameter

This is designed to apply the trailer handbrake in the event of the trailer separating from the towing vehicle.  As the trailer stops the cable should brake away from the towing vehicle. 

Fitting: The split ring on one end of the cable can be opened slightly with a flat bladed screwdriver, allowing it to be attached to the trailer braking system, usually the lowest part of the handbrake lever mechanism by passing it through a hole or loop in the steelwork. .

Instructions for use:
When attaching the trailer to the towing vehicle, the break away cable should be uncoiled and passed throught the attachment point provided, alternatively passed around the towing bracket or some other firm attachment point.  The spring clip on the end of the cable should be fastened back to the cable to form a loop...