diff --git a/Project.toml b/Project.toml
index d8b15d7..3d2ad78 100644
--- a/Project.toml
+++ b/Project.toml
@@ -11,15 +11,21 @@ JSON = "682c06a0-de6a-54ab-a142-c8b1cf79cde6"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 OpenAPI = "d5e62ea6-ddf3-4d43-8e4c-ad5e6c8bfd7d"
 PowerCoreOpenAPIModels = "b7b40286-e793-417d-a9a0-b1583e4da1cb"
+PowerDynamicsOpenAPIModels = "044a0b22-31f8-4ef6-8282-c9a61b3013f6"
+PowerInvestmentsOpenAPIModels = "33cb4396-f4d9-4f59-8585-787ebb56cb1b"
 PowerOpenAPIModels = "0730f07c-cff6-4c3b-a9df-c546153be50a"
 PowerOperationsOpenAPIModels = "a372b6d7-45a2-44c2-8199-6a724b72e8ff"
+PowerTimeSeriesOpenAPIModels = "8c2f6a0d-6b0e-4d0a-9d8f-2b5e6a4f9c31"
 Unicode = "4ec0a83e-493e-50e2-b9ac-8f72acf5a8f5"
-
 [sources]
 # These packages are generated and not yet released, so they resolve from the branch
 # rather than the registry. Switch to a tagged rev once one exists.
 PowerCoreOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerCoreOpenAPIModels.jl"}
 PowerOperationsOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerOperationsOpenAPIModels.jl"}
+PowerOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerOpenAPIModels.jl"}
+PowerDynamicsOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerDynamicsOpenAPIModels.jl"}
+PowerInvestmentsOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerInvestmentsOpenAPIModels.jl"}
+PowerTimeSeriesOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerTimeSeriesOpenAPIModels.jl"}
 InfrastructureSystems = {rev = "IS4", url = "https://github.com/Sienna-Platform/InfrastructureSystems.jl"}
 
 [compat]
@@ -30,7 +36,10 @@ JSON = "1"
 LinearAlgebra = "1"
 OpenAPI = "0.2"
 PowerCoreOpenAPIModels = "0.1"
+PowerDynamicsOpenAPIModels = "0.1"
+PowerInvestmentsOpenAPIModels = "0.1"
 PowerOpenAPIModels = "0.1"
 PowerOperationsOpenAPIModels = "0.1"
+PowerTimeSeriesOpenAPIModels = "0.1"
 Unicode = "1"
 julia = "^1.10"
diff --git a/src/pm_io/psse.jl b/src/pm_io/psse.jl
index e64a591..1d121b6 100644
--- a/src/pm_io/psse.jl
+++ b/src/pm_io/psse.jl
@@ -951,17 +951,15 @@ function _psse2pm_shunt!(pm_data::Dict, pti_data::Dict, import_all::Bool, nb)
 
                 sub_data["ext"]["NREG"] = pop!(switched_shunt, "NREG")
             elseif pm_data["source_version"] ∈ ("30", "32", "33")
-                sub_data["initial_status"] = ones(Int, length(sub_data["y_increment"]))
+                # Pre-v35 SWITCHED SHUNT records carry no per-block status field. BINIT
+                # already holds the total in-service admittance, which `bs` above passes
+                # on, so every block must start out of service whatever MODSW says —
+                # counting any of them in would double-count that same admittance.
+                sub_data["initial_status"] = zeros(Int, length(sub_data["y_increment"]))
             else
                 error("Unsupported PSS(R)E source version: $(pm_data["source_version"])")
             end
 
-            if switched_shunt["MODSW"] ∈ (0, 1, 2)
-                # BINIT is treated as the total shunt admittance.
-                # Keep Y_increase but zero all initial states to avoid double counting.
-                sub_data["initial_status"] = zeros(Int, length(sub_data["y_increment"]))
-            end
-
             sub_data["index"] = length(pm_data["switched_shunt"]) + 1
             sub_data["source_id"] = ["switched shunt", bus_number, sub_data["index"]]
 
diff --git a/test/Project.toml b/test/Project.toml
index 073a6a9..7aa0341 100644
--- a/test/Project.toml
+++ b/test/Project.toml
@@ -1,5 +1,4 @@
 [deps]
-PowerOpenAPIModels = "0730f07c-cff6-4c3b-a9df-c546153be50a"
 Aqua = "4c88cf16-eb10-579e-8560-4a9242c79595"
 InfrastructureSystems = "2cd47ed4-ca9b-11e9-27f2-ab636a7671f1"
 JSON = "682c06a0-de6a-54ab-a142-c8b1cf79cde6"
@@ -7,15 +6,21 @@ LazyArtifacts = "4af54fe1-eca0-43a8-85a7-787d91b784e3"
 Logging = "56ddb016-857b-54e1-b83d-db4d58db5568"
 OpenAPI = "d5e62ea6-ddf3-4d43-8e4c-ad5e6c8bfd7d"
 PowerCoreOpenAPIModels = "b7b40286-e793-417d-a9a0-b1583e4da1cb"
+PowerDynamicsOpenAPIModels = "044a0b22-31f8-4ef6-8282-c9a61b3013f6"
 PowerFlowFileParser = "bed98974-b02e-5e2f-9ee0-a103f5c450dd"
+PowerInvestmentsOpenAPIModels = "33cb4396-f4d9-4f59-8585-787ebb56cb1b"
+PowerOpenAPIModels = "0730f07c-cff6-4c3b-a9df-c546153be50a"
 PowerOperationsOpenAPIModels = "a372b6d7-45a2-44c2-8199-6a724b72e8ff"
+PowerTimeSeriesOpenAPIModels = "8c2f6a0d-6b0e-4d0a-9d8f-2b5e6a4f9c31"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
-
 [sources]
 PowerFlowFileParser = {path = ".."}
 PowerCoreOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerCoreOpenAPIModels.jl"}
 PowerOperationsOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerOperationsOpenAPIModels.jl"}
 PowerOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerOpenAPIModels.jl"}
+PowerDynamicsOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerDynamicsOpenAPIModels.jl"}
+PowerInvestmentsOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerInvestmentsOpenAPIModels.jl"}
+PowerTimeSeriesOpenAPIModels = {url = "https://github.com/Sienna-Platform/PowerOpenAPIModels.git", rev = "main", subdir = "PowerTimeSeriesOpenAPIModels.jl"}
 
 [compat]
 julia = "^1.6"
diff --git a/test/test_parse_psse.jl b/test/test_parse_psse.jl
index 8c2c52f..9a127cd 100644
--- a/test/test_parse_psse.jl
+++ b/test/test_parse_psse.jl
@@ -126,3 +126,31 @@ end
     @test pm_v33["bus"][1]["area_slack"] === true
     @test !haskey(pm_v33["bus"][3], "area_slack")
 end
+
+@testset "PSSE pre-v35 switched shunt blocks start out of service" begin
+    # Pre-v35 SWITCHED SHUNT records have no per-block status field, so the parser has to
+    # fabricate one. BINIT already carries the total in-service admittance into `bs`, so
+    # every block must start at zero whatever MODSW says; an in-service block would
+    # double-count the admittance BINIT has already contributed.
+    raw = read_fixture(FOURTEEN_BUS_FIXTURE)
+    pm_data = parse_file(IOBuffer(raw); filetype = "raw")
+    @test pm_data["source_version"] == "33"
+
+    shunts = collect(values(pm_data["switched_shunt"]))
+    @test !isempty(shunts)
+    for shunt in shunts
+        @test shunt["initial_status"] == zeros(Int, length(shunt["y_increment"]))
+    end
+
+    # Bus 101's record is MODSW=1, which an earlier mode-specific patch already zeroed.
+    # MODSW=3 took the fabricated all-ones path and is the case this fixes.
+    modsw3 = replace(raw, "   101,1,0,1," => "   101,3,0,1,"; count = 1)
+    @test modsw3 != raw
+    pm_modsw3 = parse_file(IOBuffer(modsw3); filetype = "raw")
+    shunt_101 =
+        only(v for v in values(pm_modsw3["switched_shunt"]) if v["shunt_bus"] == 101)
+    @test shunt_101["control_mode"] == 3
+    @test shunt_101["step_number"] == [5]
+    @test length(shunt_101["y_increment"]) == 1
+    @test shunt_101["initial_status"] == [0]
+end