Trailers and caravans towed by light vehicles must have a braking system if the trailer gross mass exceeds a certain value. An electric braking system is commonly used to activate the drum-type friction brakes on the trailer.
Braking effect can be increased or reduced by the driver, adjusting a control unit to suit the load on the trailer. This manual override can be used in certain driving conditions to dampen a trailer's tendency to sway.
When the brakes in the towing vehicle are applied, the brake-light circuit sends the signal to the control unit. The control unit then sends an appropriate current to the trailer brake actuators, to operate the trailer brakes, at the level selected.
In some places, trailers above a specified weight must also have an auxiliary battery-powered braking system that automatically applies the brakes and keeps them applied if the trailer ever breaks away from its towing vehicle.
