Hot battery bus. As has been said, is connected directly to the battery. It is not a tie bus. The battery bus is connected to the HBB by a contactor.
The a/c is fitted with a Common Return Network. Big bonding leads and metal busses that run along the fuselage and wings. I understand it weighs about 400 lb.
Each electrical device is connected to it for earth return.
The big loads such as Air con compressors are fed from variable frequency electronic controllers. +/-270vdc.
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