As the passengers were boarding, a sudden power surge was indicated by way of relay lights and instrument flags flashing on and off. A flight attendant indicated there was smoke coming from the rear galley area. The crew immediately turned off the galley power switch and all aircraft power, and advised all passengers to disembark in an orderly fashion. The first officer then proceeded to the rear of the aircraft with a fire extinguisher, however it was not required. Later examination found that both the rear galley circuit breakers had popped. The cause for the smoke was traced to a fault in the main three phase coupling plug. A section of the female to male coupling plug had short circuited and burnt. The coupling was replaced and the aircraft returned to service.