diff --git a/src/AR.Iec61850/FaultRecords/Iec61850FaultRecordService.cs b/src/AR.Iec61850/FaultRecords/Iec61850FaultRecordService.cs
index b38a88a6..d2e5c8ff 100644
--- a/src/AR.Iec61850/FaultRecords/Iec61850FaultRecordService.cs
+++ b/src/AR.Iec61850/FaultRecords/Iec61850FaultRecordService.cs
@@ -12,7 +12,9 @@ public enum Iec61850FaultRecordFileKind
     Information,
     Combined,
     Archive,
-    VendorPackage
+    VendorPackage,
+    /// <summary>Optional relay-settings sidecar, never a standalone COMTRADE record.</summary>
+    RioSidecar
 }
 
 public sealed class Iec61850FaultRecordFile
@@ -108,6 +110,7 @@ public static class Iec61850FaultRecordCatalogBuilder
             [".dat"] = Iec61850FaultRecordFileKind.Data,
             [".hdr"] = Iec61850FaultRecordFileKind.Header,
             [".inf"] = Iec61850FaultRecordFileKind.Information,
+            [".rio"] = Iec61850FaultRecordFileKind.RioSidecar,
             [".cff"] = Iec61850FaultRecordFileKind.Combined,
             [".zip"] = Iec61850FaultRecordFileKind.Archive
         };
@@ -140,6 +143,11 @@ public static Iec61850FaultRecordCatalog Build(
             .GroupBy(
                 file => BuildRecordId(file.RemoteDirectory, file.BaseName),
                 StringComparer.OrdinalIgnoreCase)
+            // A lone RIO is ordinary root inventory, not a downloadable
+            // COMTRADE record. Only exact directory + base-name matches join
+            // an already recognized primary CFG/DAT/package identity.
+            .Where(group => group.Any(file =>
+                file.Kind != Iec61850FaultRecordFileKind.RioSidecar))
             .Select(group => BuildRecord(group.Key, group))
             .OrderByDescending(record => record.LastModifiedUtc ?? DateTimeOffset.MinValue)
             .ThenBy(record => record.RecordId, StringComparer.OrdinalIgnoreCase)
diff --git a/tests/AR.Iec61850.Tests/ComtradeRioSidecarCatalogTests.cs b/tests/AR.Iec61850.Tests/ComtradeRioSidecarCatalogTests.cs
new file mode 100644
index 00000000..98772c65
--- /dev/null
+++ b/tests/AR.Iec61850.Tests/ComtradeRioSidecarCatalogTests.cs
@@ -0,0 +1,66 @@
+using AR.Iec61850.FaultRecords;
+using AR.Iec61850.Mms;
+
+namespace AR.Iec61850.Tests;
+
+public sealed class ComtradeRioSidecarCatalogTests
+{
+    private static MmsFileDirectoryEntry File(string path, uint size = 10) =>
+        new() { Path = path, Name = Path.GetFileName(path), SizeBytes = size };
+
+    [Fact]
+    public void RioSidecar_JoinsOnlySameDirectoryAndBasename_WithCaseInsensitiveExtension()
+    {
+        var catalog = Iec61850FaultRecordCatalogBuilder.Build(
+        [
+            File("dir/line_1.cfg", 100),
+            File("dir/line_1.dat", 200),
+            File("dir/line_1.RIO", 30),
+            File("dir/line_2.RIO", 40), // no matching primary
+            File("other/line_1.RIO", 50) // wrong folder
+        ]);
+        var record = Assert.Single(catalog.Records);
+        Assert.Equal("line_1", record.BaseName);
+        Assert.True(record.IsComplete);
+        Assert.Equal(3, record.Files.Count);
+        Assert.Contains(record.Files, f =>
+            f.Name == "line_1.RIO" &&
+            f.Kind == Iec61850FaultRecordFileKind.RioSidecar &&
+            f.RemotePath == "dir/line_1.RIO");
+        Assert.DoesNotContain(record.Files, f =>
+            f.RemotePath == "other/line_1.RIO");
+        Assert.Equal(330L, record.KnownSizeBytes);
+    }
+
+    [Fact]
+    public void OrphanRio_IsNotAuthorizedAsFaultRecord()
+    {
+        var catalog = Iec61850FaultRecordCatalogBuilder.Build([File("line_1.RIO")]);
+        Assert.Empty(catalog.Records);
+        Assert.True(Iec61850FaultRecordCatalogBuilder.IsSupportedFile("line_1.rio"));
+    }
+
+    [Fact]
+    public void RioDoesNotMakeIncompleteCfgOrDatComplete()
+    {
+        var catalog = Iec61850FaultRecordCatalogBuilder.Build(
+            [File("line_1.cfg"), File("line_1.rio")]);
+        var record = Assert.Single(catalog.Records);
+        Assert.False(record.IsComplete);
+        Assert.Equal(2, record.Files.Count);
+        Assert.False(record.CanDownload && record.IsComplete);
+    }
+
+    [Fact]
+    public void DuplicateOrDifferentlyCasedRemoteDirectoryListing_DoesNotDuplicateRio()
+    {
+        var catalog = Iec61850FaultRecordCatalogBuilder.Build(
+        [
+            File("dir/line_1.cfg"),
+            File("dir/line_1.dat"),
+            File("dir/line_1.RIO"),
+            File("dir/line_1.RIO")
+        ]);
+        Assert.Equal(3, Assert.Single(catalog.Records).Files.Count);
+    }
+}