RSA 266:63 Trailer Breakaway Safety Chains.

Every trailer or semi-trailer including farm and agricultural vehicles shall have, in addition to the tow-bar or coupling device, a safety chain or cable to prevent breakaway from the towing vehicle. Each chain or cable shall have an ultimate strength at least equal to the gross weight of the trailer and load being towed. Chains or cables shall be connected to the towed and towing vehicle to prevent the tow-bar from dropping to the ground in the event the tow-bar fails. This provision shall not apply to:
I. Truck-tractor and semi-trailer units equipped with fifth wheel mechanisms;
II. Full trailers being driven as part of a truck-tractor, semi-trailer, or full trailer unit when the full trailer is attached to the semi-trailer by means of a converter dolly or fifth wheel