I have 2 TIUs on my layout, using 6 channels controlling various areas. On TIU #1 both FIXED are powered by a Z4000 using the side receiver. These feed the two main lines. Both VARIABLEs are powered by separate handles on an ancient ZW. These control two large storage areas on the lowest level. On TIU#2, FIXED are not used and both VARIABLEs are powered by a singly PH-180; they had used a second ZW but when I bought, and didn't use, the PH-180 for a grandson's layout, I replaced the ZW with the PH-180 to save space. These two channels serve the main yard and a branch line, respectively.
There are external breakers on all transformer outputs.
All hot feeds go from the TIU outputs to a large control panel and feed my 70+ blocks, each of which has a toggle switch set in maps of the layout. DZ switch controllers are al,o on the maps.
Ground is via a 12-gauge common buss loop around the layout. Ground is not fed through the TIUs, but all black TIU outputs are connected to the buss.