A system for operating alarm features of a trailer towed by a tractor in the event of unintended decoupling of the trailer from the tractor. The system may include a breakaway switch or its functional equivalent, a flasher, and a source of emergency power such as a battery. The output of the system may operate only lights though the flasher, or optionally, also at least one electric brake of the trailer. Where braking is provided, the supply of electrical power is uninterrupted as by the flasher. Importantly, an anti-feedback feature prevents unintended operation of the lights and brake due to back-feeding from the electrical system of the tractor under ordinary operating conditions.