Right then, this might help:
Crash Protection System
(a) In order to minimise fire risk and make the aircraft electrically dead (except for emergency services) in the event of a crash landing, 20 crash trip elements and 8 inertia switches are provided. The crash trip elements are beneath the forward fuselage and undercarriage fairings; the inertia switches are in the port and starboard nose equipment bays.
(b) The elements and switches are divided into four groups, each group operates a pair of crash service relays associated with an engine and is known as an engine channel. Each engine channel is sub-divided into two halves. The complete system, consisting of all four engine channels, is divided into independent halves, port and starboard. The port system consists of Nos 1 and 4 engine channels and the starboard system Nos 2 and 3 engine channels.
(c) Operation of the system.
(i) Operation of a single complete engine channel shuts down two alternators, discharges the associated fire extinguishers (first shot) and illuminates the amber warning light on panel 10P.
(ii) Operation of all engine channels disconnects the batteries from their normal busbars and connects then to the emergency service busbars only, shuts down all alternators, discharges all engine fire extinguishers (first shot) and illuminates four amber warning lights on panel 10P.
This is from the Pilots Notes of the Short Belfast.
Hopefully it will help answer most of your questions.