Air Carrier Captain reported while arriving at the gate and needing ground power; ramp personnel attempted to open the MCD before the cabin crew was ready resulting in a near inadvertent escape slide deployment.

Narrative

I recently had a line check and it was pointed out to me that the Manual says the seatbelt sign stays on until both engines have been turned off. I was previously under the impression that as long as the parking brake is set and the captain has determined that it is safe for the passengers to get up that it is Captains discretion whether or not the seatbelt sign is on. I have been trying to comply with this today with an INOP APU and it is causing issues. There is a delay in shutting down the right engine because we end up waiting for power. I can hear the passengers standing up and the FAs making PAs to tell everyone to remain seated. In ZZZ we were waiting for the jetway to come up to the aircraft so the rampers could plug in the external power. The right engine was running so the seatbelt sign was on. When the jetway pulled up the gate agent went to open the main cabin door from the outside. The door had not been disarmed because the seatbelt sign was still on so the FAs had not got up to do their door disarming. The FA saw the gate agent going to open the door from the outside and the FA gave her a 'X' hand signal through the window and thankfully the gate agent stopped before blowing the slide. The red flag was still in the window. My cabin crew members were shaken up over this and told me they thought the slide was going to deploy into the jetway. The first flight attendant asked me why it was taking so long to turn off the seatbelt sign? And she told me that every time the jetway pulls up with the door armed there is this fear that the slide is gonna get blown. I explained the need to wait for external power to shut down the engine. I think it would be a safer SOP for the Captain to turn off the seatbelt sign when the parking brake is set and the wheels are chocked before the jetway approaches the aircraft. This allows the FAs to have time to disarm the doors.