Unspecified flight crewmember directed passengers to evacuate the aircraft at the gate when electrical power was removed for maintenance and a light smoke began to enter the cabin.

Narrative

At the gate; Maintenance was attempting to fix a low oil pressure advisory light. I stepped off the flight deck so they would have room to work. Aircraft was on APU power with external disconnected. Aircraft also had 139 passengers on board. A replacement electronic display part was brought to the aircraft at which time I came back on board to find maintenance had disconnected all power to the aircraft with the APU running. I advised maintenance personnel they needed to restore power to the airplane or we may need to deplane the passengers. They said they were unable to restore power due to the fact that a display had been removed and there were open cannon plugs so connecting electrical power was not an option. I immediately went back into the main cabin to assess the situation and proceeded to the rear of the aircraft to talk with the flight attendants. I observed light smoke entering into the cabin from the air ducting systems. I immediately directed the passengers to deplane the aircraft; without bags; via the jetbridge exit at door 1L. Passengers deplaned without incident. Maintenance suspected the light smoke was of hydraulic fluid origin. Aircraft was taken out of service; and a spare aircraft was obtained. The light smoke appears to have been caused by a slight differential pressure that allowed tail cone gases to enter into the main cabin. A potential hazardous condition was avoided by the immediate deplaning of passengers. Maintenance may want to reconsider procedures allowing passengers to remain onboard with all electrical power disconnected. In this situation; all cabin interphone and PA systems were unavailable to use. Use of emergency power to activate the PA system may inadvertently power an electrical bus maintenance need to have deactivated for safety. In this case; a loud verbal announcement was made to the passengers to deplane the aircraft.