SMMT Guide on Trailers > Trailer Electrics

Trailer Electrics - Lighting Systems

All lights must be in working order.

Lenses and reflectors must he in good condition.Bulbs must be of correct wattage for the application.


Electrical Connection
Trailers are fitted with a 7-pin plug (I2N) for connection to the towing vehicle lighting system.

These are wired to an international standard to ensure correct operation on any towing vehicle. Towbars are fitted with the corresponding socket. Although not a legal requirement in the UK, it is advisable to use a system incorporating a rear fog light bypass to disable the vehicle fog light when the trailer is connected, preventing glare in the rear view mirror.Where additional electrical circuits are required, a supplementary 7-pin plug (12S) and socket is available. This is commonly used for reversing lights, battery charging and powering refrigerators and interior lights in caravans.

Some European vehicles are now fitted with a 13-pin electrical socket incorporating the functions of the 12N & 12S sockets. Special connecting lead assemblies are available to allow connection between this socket and the 12N & 12S system on the trailer.