Hier die Logik der 737NG: [1]

The overwing and mid-exit emergency exits lock when:

• three of the four Entry/Service doors are closed and
• either engine is running and
• the airplane air/ground logic indicates that the airplane is in the air or both thrust levers are advanced.

They unlock when any one of the above conditions is not met or DC power is lost.

Quelle

[1]

Emergency Exits Sicherung? - Quax37: www.flightforum.ch
http://www.flightforum.ch/forum/showthread.php?t=55260