@@ -85,11 +85,6 @@ namespace GridKit
       [[gnu::always_inline]] void internalConstraints(const ScalarT* x, const ScalarT* external, ScalarT* g) const override;
       [[gnu::always_inline]] void externalConstraints(const ScalarT* x, const ScalarT* external, ScalarT* g) const override;
 
-      bool inService() const
-      {
-        return in_service_;
-      }
-
     private:
       void initializeParameters(const ModelDataT& data);
       void setDerivedParameters();
 
@@ -79,12 +79,6 @@ namespace GridKit
 
       [[gnu::always_inline]] void internalConstraints(const ScalarT* x, const ScalarT* external, ScalarT* g) const override;
 
-      /// Angle reference, with zero imaginary voltage
-      bool reference() const
-      {
-        return reference_;
-      }
-
     private:
       void initializeParameters(const ModelDataT& data);
 
 
@@ -10,7 +10,7 @@ variable bounds $x_l \le x \le x_u$. The objective and each constraint sum
 component contributions. Each constraint and variable bound belongs to one
 component, which tags the constraint as an inequality or leaves it an equality
 with $g_l = g_u = 0$. An inequality without a finite limit has infinite bounds.
-Every island needs a reference bus.
+Every island needs a reference bus, which the system does not check.
 
 Cartesian voltages make a solution directly a starting point for a phasor
 dynamics simulation. Branch power is quadratic in these voltages; ZIP
@@ -57,13 +57,9 @@ are added to the owning constraints; they are not separate constraints. For
 example, a generator contributes power to a bus's balance without reading its
 voltage.
 
-Case bus numbers are resolved when constructing the system. Electrical islands
-follow in-service branches, independently of component dependencies.
-
-`SystemModel` also accepts a vector of owned components for direct composition.
-That constructor validates component parameters and connections. Electrical
-island and reference checks use the network metadata supplied by the case-data
-constructor.
+Case bus numbers are resolved when constructing the system. `SystemModel` also
+accepts a vector of owned components for direct composition. That constructor
+validates component parameters and connections.
 
 ## Derivatives
 
 
@@ -21,9 +21,6 @@ namespace GridKit
     template <typename scalar_type, typename index_type>
     class Bus;
 
-    template <typename scalar_type, typename index_type>
-    class Branch;
-
     /**
      * @brief AC optimal power flow in Cartesian coordinates
      *
@@ -46,7 +43,6 @@ namespace GridKit
       using IndexVectorT = LinearAlgebra::Vector<IdxT, IdxT>;
       using CsrMatrixT   = typename EvaluatorT::CsrMatrixT;
       using BusT         = Bus<ScalarT, IdxT>;
-      using BranchT      = Branch<ScalarT, IdxT>;
       using Block        = typename ComponentT::Block;
       using CooBufferT   = typename ComponentT::CooBuffer;
 
@@ -92,12 +88,11 @@ namespace GridKit
       void addBus(const BusData<RealT, IdxT>& data);
 
       template <typename ModelT, typename DataT>
-      ModelT& addDevice(const DataT& data);
+      void addDevice(const DataT& data);
 
-      int               compose();
-      int               bind(ComponentT& component, const Block& offset);
-      std::vector<IdxT> unreferencedIslands() const;
-      int               allocateDerivatives();
+      int compose();
+      int bind(ComponentT& component, const Block& offset);
+      int allocateDerivatives();
 
       std::unique_ptr<CsrMatrixT> assemble(IdxT rows, IdxT cols, Assembly& assembly) const;
       int                         refill(CsrMatrixT& matrix, const Assembly& assembly) const;
@@ -108,9 +103,6 @@ namespace GridKit
       /// Buses by number
       std::map<IdxT, BusT*> buses_;
 
-      /// Electrical connectivity, independent of component dependencies
-      std::vector<BranchT*> branches_;
-
       Assembly jacobian_assembly_;
       Assembly hessian_assembly_;
     };
 
@@ -6,7 +6,6 @@
 #pragma once
 
 #include <format>
-#include <map>
 #include <set>
 #include <stdexcept>
 #include <string>
@@ -43,7 +42,7 @@ namespace GridKit
 
       for (const auto& branch : data.branch)
       {
-        branches_.push_back(&addDevice<BranchT>(branch));
+        addDevice<Branch<ScalarT, IdxT>>(branch);
       }
 
       for (const auto& generator : data.generator)
@@ -71,8 +70,7 @@ namespace GridKit
     }
 
     /**
-     * @brief Every problem with the data, including an island without a
-     * reference bus
+     * @brief Every problem with the data
      */
     template <typename scalar_type, typename index_type>
     Model::ConfigurationChecks SystemModel<scalar_type, index_type>::verify() const
@@ -91,11 +89,6 @@ namespace GridKit
                      std::format("{}: invalid bus number", bus->id()));
       }
 
-      for (const IdxT number : unreferencedIslands())
-      {
-        checks.fail(std::format("OptimalPowerFlow::SystemModel: the island of bus {} has no reference bus", number));
-      }
-
       std::set<std::string> ids;
       for (const auto& component : components_)
       {
@@ -274,7 +267,7 @@ namespace GridKit
      */
     template <typename scalar_type, typename index_type>
     template <typename ModelT, typename DataT>
-    ModelT& SystemModel<scalar_type, index_type>::addDevice(const DataT& data)
+    void SystemModel<scalar_type, index_type>::addDevice(const DataT& data)
     {
       using Ports = typename DataT::Ports;
       static_assert(ModelT::NUM_PORT_VARIABLES.size() == Utilities::enum_size<Ports>());
@@ -302,9 +295,7 @@ namespace GridKit
         }
       }
 
-      ModelT& added = *device;
       components_.push_back(std::move(device));
-      return added;
     }
 
     /**
@@ -371,61 +362,5 @@ namespace GridKit
       component.bound_ = true;
       return 0;
     }
-
-    /**
-     * @brief Lowest bus number of each island without a reference bus
-     *
-     * In-service branches connect the buses of their ports.
-     */
-    template <typename scalar_type, typename index_type>
-    std::vector<index_type> SystemModel<scalar_type, index_type>::unreferencedIslands() const
-    {
-      // Each bus points toward the root bus of its island
-      std::map<const ComponentT*, const ComponentT*> parent;
-      for (const auto& [number, bus] : buses_)
-      {
-        parent[bus] = bus;
-      }
-
-      const auto root = [&parent](const ComponentT* bus)
-      {
-        while (parent.at(bus) != bus)
-        {
-          bus = parent.at(bus);
-        }
-        return bus;
-      };
-
-      for (const auto* branch : branches_)
-      {
-        const auto& ports = branch->ports();
-        if (!branch->inService() || ports[0].component == nullptr || ports[1].component == nullptr)
-        {
-          continue;
-        }
-        parent[root(ports[1].component)] = root(ports[0].component);
-      }
-
-      std::set<const ComponentT*> referenced;
-      for (const auto& [number, bus] : buses_)
-      {
-        if (bus->reference())
-        {
-          referenced.insert(root(bus));
-        }
-      }
-
-      std::vector<IdxT>           unreferenced;
-      std::set<const ComponentT*> reported;
-      for (const auto& [number, bus] : buses_)
-      {
-        const auto* island = root(bus);
-        if (!referenced.contains(island) && reported.insert(island).second)
-        {
-          unreferenced.push_back(number);
-        }
-      }
-      return unreferenced;
-    }
   } // namespace OptimalPowerFlow
 } // namespace GridKit
@@ -273,25 +273,11 @@ namespace GridKit
         return success.report(__func__);
       }
 
-      /// Invalid bus numbers, connections, and islands are rejected
+      /// Invalid bus numbers and connections are rejected
       TestOutcome configuration()
       {
         TestStatus success = true;
 
-        auto no_reference                                             = SystemTestData::data();
-        no_reference.bus[0].parameters[OPF::BusParameters::reference] = false;
-
-        success *= configurationRejected([&]()
-                                         { return SystemT(no_reference).verify(); });
-
-        // Out-of-service branches 2-3 and 1-3 leave bus 3 an island without a reference bus
-        auto island                                                    = SystemTestData::data();
-        island.branch[1].parameters[OPF::BranchParameters::in_service] = false;
-        island.branch[2].parameters[OPF::BranchParameters::in_service] = false;
-
-        success *= configurationRejected([&]()
-                                         { return SystemT(island).verify(); });
-
         auto repeated          = SystemTestData::data();
         repeated.bus[1].number = 1;