diff --git a/src/Ybus.jl b/src/Ybus.jl
index 7a8ceb527..84374ec4f 100644
--- a/src/Ybus.jl
+++ b/src/Ybus.jl
@@ -820,6 +820,114 @@ function _get_filtered_components(
     return components
 end
 
+"""
+Apply reductions that always run during Ybus construction, before any user-specified
+`network_reductions`. Currently this performs two steps:
+
+  - Merge closed zero-impedance `DiscreteControlledACBranch` components by collapsing
+    their endpoint buses (mutates `nr.bus_reduction_map` and `nr.reverse_bus_search_map`).
+  - Identify any other two-terminal branches whose endpoints both resolve to the same
+    surviving bus after the merge, and exclude them so they cannot produce self-loop
+    rows in the incidence matrix downstream.
+
+Returns `(breaker_switches, collapsed_branch_names)`:
+
+  - `breaker_switches`: closed breakers with non-trivial impedance that still need to
+    be modeled as Y-bus branches.
+  - `collapsed_branch_names`: names of non-breaker two-terminal branches to be excluded
+    from `_get_ybus_two_terminal_ac_branches`.
+"""
+function _apply_implicit_reductions!(sys::PSY.System, nr::NetworkReductionData)
+    breaker_switches = _merge_zero_impedance_breakers!(sys, nr)
+    collapsed_branch_names = _eliminate_collapsed_branches!(sys, breaker_switches, nr)
+    return breaker_switches, collapsed_branch_names
+end
+
+"""
+Merge closed `DiscreteControlledACBranch` components with near-zero impedance by
+collapsing their endpoints in the network reduction maps. Returns the closed breakers
+with non-trivial impedance that must still be modeled as Y-bus branches.
+"""
+function _merge_zero_impedance_breakers!(sys::PSY.System, nr::NetworkReductionData)
+    breaker_switches = Vector{PSY.DiscreteControlledACBranch}()
+    reverse_bus_search_map = get_reverse_bus_search_map(nr)
+    bus_reduction_map = get_bus_reduction_map(nr)
+    for br in PSY.get_components(PSY.DiscreteControlledACBranch, sys)
+        PSY.get_available(br) || continue
+        PSY.get_branch_status(br) == PSY.DiscreteControlledBranchStatus.CLOSED ||
+            continue
+        r = PSY.get_r(br)
+        x = PSY.get_x(br)
+        if r == 0.0 && x < ZERO_IMPEDANCE_LINE_REACTANCE_THRESHOLD
+            from_bus_number = PSY.get_number(PSY.get_from(PSY.get_arc(br)))
+            to_bus_number = PSY.get_number(PSY.get_to(PSY.get_arc(br)))
+            _update_bus_maps!(
+                reverse_bus_search_map,
+                bus_reduction_map,
+                to_bus_number,
+                from_bus_number,
+            )
+            push!(nr.removed_arcs, (from_bus_number, to_bus_number))
+        else
+            push!(breaker_switches, br)
+        end
+    end
+    return breaker_switches
+end
+
+"""
+Identify two-terminal branches whose endpoints both resolve to the same surviving
+bus after the zero-impedance breaker merge. Such branches would produce self-loop
+rows in the incidence matrix and corrupt downstream reduction algorithms.
+
+Filters collapsed entries out of `breaker_switches` in place. Returns a set of names
+for non-breaker branches that the caller must exclude from
+`_get_ybus_two_terminal_ac_branches`.
+"""
+function _eliminate_collapsed_branches!(
+    sys::PSY.System,
+    breaker_switches::Vector{PSY.DiscreteControlledACBranch},
+    nr::NetworkReductionData,
+)
+    collapsed = Set{String}()
+    reverse_bus_search_map = get_reverse_bus_search_map(nr)
+    isempty(reverse_bus_search_map) && return collapsed
+    for br in PSY.get_components(PSY.ACTransmission, sys)
+        PSY.get_available(br) || continue
+        br isa PSY.ThreeWindingTransformer && continue
+        br isa PSY.DiscreteControlledACBranch && continue
+        if _record_if_collapsed!(br, nr, reverse_bus_search_map)
+            push!(collapsed, PSY.get_name(br))
+        end
+    end
+    filter!(
+        br -> !_record_if_collapsed!(br, nr, reverse_bus_search_map),
+        breaker_switches,
+    )
+    return collapsed
+end
+
+# Returns `true` if both endpoints of `br` resolve to the same surviving bus, in
+# which case the arc is added to `nr.removed_arcs`. Used by
+# `_eliminate_collapsed_branches!` to flag and audit branches that the implicit
+# breaker merge has turned into self-loops.
+function _record_if_collapsed!(
+    br::PSY.ACTransmission,
+    nr::NetworkReductionData,
+    reverse_bus_search_map::Dict{Int, Int},
+)
+    arc = PSY.get_arc(br)
+    fr_no = PSY.get_number(PSY.get_from(arc))
+    to_no = PSY.get_number(PSY.get_to(arc))
+    fr_resolved = get(reverse_bus_search_map, fr_no, fr_no)
+    to_resolved = get(reverse_bus_search_map, to_no, to_no)
+    fr_resolved == to_resolved || return false
+    push!(nr.removed_arcs, (fr_no, to_no))
+    @debug "Implicit reduction: branch $(PSY.get_name(br)) excluded; endpoints " *
+           "$fr_no and $to_no both collapse to bus $fr_resolved."
+    return true
+end
+
 """
     Ybus(
         sys::PSY.System;
@@ -907,29 +1015,7 @@ function Ybus(
         end
     end
 
-    #Building map for removed Breaker/Switches
-    breaker_switches = Vector{PSY.DiscreteControlledACBranch}()
-    for br in PSY.get_components(PSY.DiscreteControlledACBranch, sys)
-        !PSY.get_available(br) && continue
-        r = PSY.get_r(br)
-        x = PSY.get_x(br)
-        status = PSY.get_branch_status(br)
-        if status == PSY.DiscreteControlledBranchStatus.CLOSED
-            if r == 0.0 && x < ZERO_IMPEDANCE_LINE_REACTANCE_THRESHOLD
-                from_bus_number = PSY.get_number(PSY.get_from(PSY.get_arc(br)))
-                to_bus_number = PSY.get_number(PSY.get_to(PSY.get_arc(br)))
-                _update_bus_maps!(
-                    reverse_bus_search_map,
-                    bus_reduction_map,
-                    to_bus_number,
-                    from_bus_number,
-                )
-                push!(nr.removed_arcs, (from_bus_number, to_bus_number))
-            else
-                push!(breaker_switches, br)
-            end
-        end
-    end
+    breaker_switches, collapsed_branch_names = _apply_implicit_reductions!(sys, nr)
 
     bus_ax = sort!(collect(keys(bus_reduction_map)))
     axes = (bus_ax, bus_ax)
@@ -940,7 +1026,8 @@ function Ybus(
         bus_lookup[b] = ix
     end
     adj = SparseArrays.spdiagm(ones(Int8, busnumber))
-    branches = _get_ybus_two_terminal_ac_branches(sys)
+    branches =
+        _get_ybus_two_terminal_ac_branches(sys; skip_names = collapsed_branch_names)
     append!(branches.breaker_switches, breaker_switches)
     transformer_3W =
         _get_filtered_components(PSY.ThreeWindingTransformer, sys, PSY.get_available)
diff --git a/src/YbusACBranches.jl b/src/YbusACBranches.jl
index 473ffd357..5f490e8d2 100644
--- a/src/YbusACBranches.jl
+++ b/src/YbusACBranches.jl
@@ -22,17 +22,23 @@ end
 
 function _populate_ybus_branch_vector!(
     vec::Vector{T},
-    sys::PSY.System,
+    sys::PSY.System;
+    skip_names::Set{String} = Set{String}(),
 ) where {T <: PSY.ACTransmission}
     iter = PSY.get_components(T, sys)
     sizehint!(vec, length(iter))
     for br in iter
-        PSY.get_available(br) && push!(vec, br)
+        PSY.get_available(br) || continue
+        PSY.get_name(br) in skip_names && continue
+        push!(vec, br)
     end
     return
 end
 
-function _get_ybus_two_terminal_ac_branches(sys::PSY.System)::YbusACBranches
+function _get_ybus_two_terminal_ac_branches(
+    sys::PSY.System;
+    skip_names::Set{String} = Set{String}(),
+)::YbusACBranches
     branches = YbusACBranches(
         Vector{PSY.Line}(),
         Vector{PSY.MonitoredLine}(),
@@ -43,13 +49,25 @@ function _get_ybus_two_terminal_ac_branches(sys::PSY.System)::YbusACBranches
         Vector{PSY.DynamicBranch}(),
         Vector{PSY.DiscreteControlledACBranch}(),
     )
-    _populate_ybus_branch_vector!(branches.lines, sys)
-    _populate_ybus_branch_vector!(branches.monitored_lines, sys)
-    _populate_ybus_branch_vector!(branches.generic_arc_impedances, sys)
-    _populate_ybus_branch_vector!(branches.tap_transformers, sys)
-    _populate_ybus_branch_vector!(branches.phase_shifting_transformers, sys)
-    _populate_ybus_branch_vector!(branches.transformer2w, sys)
-    _populate_ybus_branch_vector!(branches.dynamic_branches, sys)
+    _populate_ybus_branch_vector!(branches.lines, sys; skip_names = skip_names)
+    _populate_ybus_branch_vector!(branches.monitored_lines, sys; skip_names = skip_names)
+    _populate_ybus_branch_vector!(
+        branches.generic_arc_impedances,
+        sys;
+        skip_names = skip_names,
+    )
+    _populate_ybus_branch_vector!(branches.tap_transformers, sys; skip_names = skip_names)
+    _populate_ybus_branch_vector!(
+        branches.phase_shifting_transformers,
+        sys;
+        skip_names = skip_names,
+    )
+    _populate_ybus_branch_vector!(branches.transformer2w, sys; skip_names = skip_names)
+    _populate_ybus_branch_vector!(
+        branches.dynamic_branches,
+        sys;
+        skip_names = skip_names,
+    )
     return branches
 end
 
diff --git a/src/radial_reduction.jl b/src/radial_reduction.jl
index 01c2820a0..142bfea58 100644
--- a/src/radial_reduction.jl
+++ b/src/radial_reduction.jl
@@ -36,7 +36,10 @@ by that branch.
 function _build_row_to_cols(A::SparseArrays.SparseMatrixCSC{Int8, Int}, buscount::Int)
     n_rows = size(A, 1)
     row_first_col = zeros(Int, n_rows)
-    row_to_cols = Vector{Tuple{Int, Int}}(undef, n_rows)
+    # Initialize with a (0, 0) sentinel so any row with fewer than two stored
+    # entries can be detected after the loop. Without this, such rows leave
+    # `undef` slots that silently propagate garbage memory downstream.
+    row_to_cols = fill((0, 0), n_rows)
     Arowval = SparseArrays.rowvals(A)
     for col in 1:buscount
         for k in SparseArrays.nzrange(A, col)
@@ -48,6 +51,17 @@ function _build_row_to_cols(A::SparseArrays.SparseMatrixCSC{Int8, Int}, buscount
             end
         end
     end
+    for r in 1:n_rows
+        if row_to_cols[r] == (0, 0)
+            n_found = row_first_col[r] == 0 ? 0 : 1
+            error(
+                "Incidence-matrix row $r has $n_found stored entr$(n_found == 1 ? "y" : "ies") " *
+                "but exactly 2 are required (one per branch endpoint). This usually " *
+                "indicates a self-loop arc or a network reduction that left a " *
+                "collapsed arc in the arc axis.",
+            )
+        end
+    end
     return row_to_cols
 end