Calculate Equipment State Duration per Shift

here a solution "complete" (corresponding to the status of the information received 😁) avoiding the use of the function List.Accumulate.

starting table:

output Table:

the code for the output table:

let

    ct = Table.TransformColumnTypes(Table,{{"StateDateTime", type datetime}, {"EquipmentState", type text}, {"ShiftIndex", Int64.Type}}),
    idx=List.RemoveLastN(List.Distinct(ct[ShiftIndex]),1),
    lstR=List.Generate(
        ()=>[r=1,pos=List.PositionOf(ct[ShiftIndex],idx{0},Occurrence.Last), lr=sfht(ct{pos},ct{pos+1})], 
        each [r]<=List.Count(idx),
        each [r=[r]+1, pos=List.PositionOf(ct[ShiftIndex],idx{[r]},Occurrence.Last),lr=sfht(ct{pos},ct{pos+1}) ], 
        each [lr]
    ),
   tc= Table.Combine({Table.FromRecords(List.Combine(lstR)),Table}),
   grp = Table.Group(tc, {"ShiftIndex"}, {{"shIdx", each duration(_)}}),
    #"Expanded shIdx" = Table.ExpandTableColumn(grp, "shIdx", {"StateDateTime", "EquipmentState", "Duration"}, {"StateDateTime", "EquipmentState", "Duration"})
in
    #"Expanded shIdx"

the two functions used:

sfht

let

    listRows=(rL,rF)=>
    let
    shift=#duration(0,0,720,0),
    dL=rL[StateDateTime],
    dF=rF[StateDateTime],
    lr=List.Generate(
        ()=>[r=rL&[StateDateTime=#datetime(Date.Year(dL),Date.Month(dL),Date.Day(dL)+sh{1},sh{0},0,0)],idx=1],
        each [r][StateDateTime]<=dF,
        each [r=if Number.Mod(idx,2)=1 then [r]& [StateDateTime=[r][StateDateTime]+shift] else [r]&[ShiftIndex=[r][ShiftIndex]+1],idx=[idx]+1],
     each [r]   
    ), 
    sh=if Time.Hour(dL) < 6 then {6,0} else if Time.Hour(dL)<18 then {18,0} else {6,1}

    in lr

in listRows

and finally duration:

let
count=(tab)=>
let
    ts=Table.Sort(tab,{"StateDateTime"}),
    ai = Table.AddIndexColumn(ts, "i", 0, 1)
   in
   Table.RemoveColumns(Table.AddColumn(ai, "Duration", each try if [ShiftIndex]=ai[ShiftIndex]{[i]+1} then Duration.TotalMinutes(ai[StateDateTime]{[i]+1}-[StateDateTime]) else "" otherwise""),{"i"})
in count