Can't remember the exact details, but the Canberra has a mix of crash switches and crash strips. The latter are under the belly in pairs, and upon ground contact they release a fire inhibitant into the belly fuel tank. The 4 crash switches only operate when subject to a longtitudinal deceleration (ie not when pulling 'g') and they operate engine fire bottles as well as isolate the aircraft battery and the generators. This should therefore isolate all of the electrical systems.