BACKGROUND
The change described in this service bulletin will allow the flight crew to remove power from the non-essential cabin systems if smoke or flames occur.
We have received numerous reports of smoke or flames in the passenger cabin related to the wiring for the passenger cabin in-flight entertainment system, cabin lighting and passenger seats.
It is not easy to remove the electrical power to these systems to stop the smoke or flames.
New relays can be installed to allow the flight crew to remove the electrical power from the non-essential passenger cabin systems using the left and right utility bus switches.
Looking through the SB and bearing in mind I don't work 747's, it seems to add a couple of relays in so that the cabin lighting bus is now switchable from the utility bus switches (was not before) and the IFE system power is depowered when the utility bus switches are off (was not before - used seperate switch).
Again, I don't know about the 747 but on the 75 and 76 the utility bus switches don't knock off the lights or entertainment systems.
We have seperate IFE power switches in the cabin on our fleet but I know we have a mod program coming up to install a switch in the flight deck for all our aircraft that have IFE. Our new aircraft (Airbus and Boeing) in the the last 12 months have come with this as standard fit. It's labelled Commercial power on the Bus but does the same thing.