Looking in my Flight Handbook it states
Autoland
During an autoland, with three autopilots selected,AC and DC buses 1,2, and 3 are automatically isolated. AC and DC bus 4 continue to power their respective tie bus. This provides an independent power source for each autopilot. There is no indication of this isolation. The electric synoptic is inhibited at this time.
During a 3 engine autoland approach, electrical connections are automatically made to ensure AC and DC busses 1 2,3, and 4 are powered and an independent source for each autopilot is maintained.
With a LAND 3 indication, the level of redundancy of the autopilot system is such that the occurence of any single fault would not prevent the autopilot system from making an automatic landing (fail operational).
Since LAND 3 comes on at 1500 feet RA it seems to me that the isolation takes place at this time.
No direct quote - just reading between the lines...
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I have a need for speed!