The following will cause the power isolation logic to be set:
• Triple channel arm - all three channels either engaged or armed.
• None of the three channels generating NO LAND 3 logic.
• Approach mode armed.
• Airplane above alert height (200 feet RA)
I borrowed this from Spanner Turner. Its important to remember that the bus isolation request is given only when the 3 A/Ps agree that they are good to go for autoland. I highlighted this yesterday when I mentioned CMDENG. What is written above makes it a little more clear.
The approach logic requires that all 3 A/Ps capture LOC and GS and are below 1500 feet but above 200 feet RA. No bus isolation request occurs before this point.
As far as the VHF glitch goes, I suspect that its the L VHF that has the power interuption when the main battery transfer relay drops out and the main battery relay kicks in. Even the slightest power interuption could cause a problem for the electronics. Thats why the modern aircraft have the no break power transfer.