diff --git a/src/ProcessBus.App.Wpf/MainWindow.xaml b/src/ProcessBus.App.Wpf/MainWindow.xaml
index 97dfe7b8..f51ea27b 100644
--- a/src/ProcessBus.App.Wpf/MainWindow.xaml
+++ b/src/ProcessBus.App.Wpf/MainWindow.xaml
@@ -2002,7 +2002,7 @@
                                                         <Style TargetType="Border">
                                                             <Setter Property="Visibility" Value="Collapsed"/>
                                                             <Style.Triggers>
-                                                                <DataTrigger Binding="{Binding IsRunning}" Value="True">
+                                                                <DataTrigger Binding="{Binding IsSelectedSvReceiving}" Value="True">
                                                                     <Setter Property="Visibility" Value="Visible"/>
                                                                 </DataTrigger>
                                                             </Style.Triggers>
@@ -2014,7 +2014,7 @@
                                                                 <Style TargetType="Ellipse">
                                                                     <Setter Property="Opacity" Value="0.55"/>
                                                                     <Style.Triggers>
-                                                                        <DataTrigger Binding="{Binding IsRunning}" Value="True">
+                                                                        <DataTrigger Binding="{Binding IsSelectedSvReceiving}" Value="True">
                                                                             <DataTrigger.EnterActions>
                                                                                 <BeginStoryboard x:Name="LiveDotPulseStoryboard">
                                                                                     <Storyboard AutoReverse="True" RepeatBehavior="Forever">
@@ -2205,6 +2205,7 @@
                                                             Command="{Binding CopyEvidenceCommand}"
                                                             ToolTip="Copy original engineering values and passive capture evidence"/>
                                                     <ComboBox Height="28" MinWidth="140"
+                                                              IsEnabled="{Binding CanSelectEngineeringScale}"
                                                               Style="{StaticResource CompactComboBoxStyle}"
                                                               ItemsSource="{Binding EngineeringScaleOptions}"
                                                               SelectedItem="{Binding SelectedEngineeringScale, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
diff --git a/src/ProcessBus.App.Wpf/ViewModels/MainWindowViewModel.cs b/src/ProcessBus.App.Wpf/ViewModels/MainWindowViewModel.cs
index 936b7ae1..dcd8a8ae 100644
--- a/src/ProcessBus.App.Wpf/ViewModels/MainWindowViewModel.cs
+++ b/src/ProcessBus.App.Wpf/ViewModels/MainWindowViewModel.cs
@@ -57,7 +57,7 @@ public sealed class MainWindowViewModel : INotifyPropertyChanged, IDisposable
     private string? _lastPhasorSourceStreamId;
     private readonly Dictionary<string, SvEngineeringScale> _explicitSvScales = new(StringComparer.OrdinalIgnoreCase);
     private const string RawScaleText = "Raw counts (uncalibrated)";
-    private const string NominalScaleText = "4I+4V nominal (1 mA / 10 mV per count)";
+    private const string NominalScaleText = "4I+4V nominal";
     private string _waveformShowMode = "Both";
     private string _waveformLayoutMode = "Overlay";
     private string _waveformTimebase = "2 cycles";
@@ -110,6 +110,7 @@ private set
             _isRunning = value;
             OnPropertyChanged();
             OnPropertyChanged(nameof(CanSwitchCaptureMode));
+            OnPropertyChanged(nameof(IsSelectedSvReceiving));
         }
     }
 
@@ -764,6 +765,8 @@ public SvStreamItem? SelectedStream
             ApplySelectedStreamScale(value?.StreamId);
             OnPropertyChanged(nameof(SelectedEngineeringScale));
             OnPropertyChanged(nameof(EngineeringScaleExplanation));
+            OnPropertyChanged(nameof(CanSelectEngineeringScale));
+            OnPropertyChanged(nameof(IsSelectedSvReceiving));
             RestoreSelectedStreamWorkspace(value);
             OnPropertyChanged();
             OnPropertyChanged(nameof(AnalogValues));
@@ -880,6 +883,11 @@ public int CurrentWorkspaceTabIndex
     public IReadOnlyList<string> WaveformMarkerOptions { get; } = ["On", "Off"];
     public IReadOnlyList<string> EngineeringScaleOptions { get; } = [NominalScaleText, RawScaleText];
 
+    // Before SV discovery this is a display preset only. No incoming samples,
+    // physical units or fabricated measurements are implied.
+    public bool CanSelectEngineeringScale => SelectedStream is not null;
+    public bool IsSelectedSvReceiving => IsRunning && SelectedStream is { IsActive: true };
+
     private SvEngineeringScale ResolveSelectedStreamScale(string streamId)
     {
         if (_explicitSvScales.TryGetValue(streamId, out var explicitMode))
@@ -899,8 +907,9 @@ details is not null &&
 
     public string SelectedEngineeringScale
     {
-        get => SelectedStream is not null &&
-               ResolveSelectedStreamScale(SelectedStream.StreamId) == SvEngineeringScale.Nominal4I4V
+        get => (SelectedStream is null
+                ? SvEngineeringScaleFactors.IdlePresentationDefault
+                : ResolveSelectedStreamScale(SelectedStream.StreamId)) == SvEngineeringScale.Nominal4I4V
             ? NominalScaleText : RawScaleText;
         set
         {
@@ -925,7 +934,7 @@ public string SelectedEngineeringScale
     }
 
     public string EngineeringScaleExplanation => SelectedStream is null
-        ? "Select an SV stream to see its measurement units."
+        ? "Waiting for publisher. Nominal 4I+4V is a display preset, not a measurement; no values until recognized SV data arrives."
         : SelectedEngineeringScale == NominalScaleText
             ? "Assumed 4I+4V nominal: 1 mA / 10 mV per count. Display estimate only; not calibrated or SCL-verified. Choose Raw counts anytime."
             : "Raw SV counts: no electrical units inferred. Use nominal 4I+4V only when its layout is confirmed.";
@@ -2182,6 +2191,8 @@ private void SynchronizeSelectedStreamRow()
             _selectedStream = refreshed;
             RestoreSelectedStreamWorkspace(refreshed);
             OnPropertyChanged(nameof(SelectedStream));
+            OnPropertyChanged(nameof(IsSelectedSvReceiving));
+            OnPropertyChanged(nameof(CanSelectEngineeringScale));
             OnPropertyChanged(nameof(AnalogValues));
             OnPropertyChanged(nameof(Phasors));
             OnPropertyChanged(nameof(Waveform));
@@ -2218,6 +2229,8 @@ private void SyncUiSelectionToRuntimeStream(string? streamId)
         OnPropertyChanged(nameof(SelectedStream));
         OnPropertyChanged(nameof(SelectedEngineeringScale));
         OnPropertyChanged(nameof(EngineeringScaleExplanation));
+        OnPropertyChanged(nameof(CanSelectEngineeringScale));
+        OnPropertyChanged(nameof(IsSelectedSvReceiving));
     }
 
     private void RestoreSelectedStreamWorkspace(SvStreamItem? stream)
@@ -2546,6 +2559,7 @@ private void RaiseAll(DateTime now, bool force = false)
         {
             _lastGlobalRaiseUtc = now;
             OnPropertyChanged(nameof(StreamStatusText));
+            OnPropertyChanged(nameof(IsSelectedSvReceiving));
             OnPropertyChanged(nameof(OperatorCaptureSummaryText));
             OnPropertyChanged(nameof(DataSourceName));
             OnPropertyChanged(nameof(IsLiveMode));
diff --git a/src/ProcessBus.Core/Models/SvEngineeringScale.cs b/src/ProcessBus.Core/Models/SvEngineeringScale.cs
index 9f9bf89c..4ba7dfbb 100644
--- a/src/ProcessBus.Core/Models/SvEngineeringScale.cs
+++ b/src/ProcessBus.Core/Models/SvEngineeringScale.cs
@@ -12,6 +12,11 @@ public enum SvEngineeringScale
 
 public static class SvEngineeringScaleFactors
 {
+    // UI preset shown before any publisher is detected. It is not applied to
+    // an unknown stream until its decoded profile is verified or the operator
+    // explicitly selects nominal. No electrical values exist during idle.
+    public const SvEngineeringScale IdlePresentationDefault = SvEngineeringScale.Nominal4I4V;
+
     // This explicitly opted-in 4I+4V lab/nominal profile is 1 mA per
     // current count and 10 mV per voltage count. It is NOT universal
     // IEC 61850 SV scaling and must never be auto-selected by svID.
diff --git a/tests/ProcessBus.Tests/SvNominalProfilePolicyTests.cs b/tests/ProcessBus.Tests/SvNominalProfilePolicyTests.cs
index 9c73ff32..f5fcd859 100644
--- a/tests/ProcessBus.Tests/SvNominalProfilePolicyTests.cs
+++ b/tests/ProcessBus.Tests/SvNominalProfilePolicyTests.cs
@@ -5,6 +5,16 @@ namespace ProcessBus.Tests;
 
 public sealed class SvNominalProfilePolicyTests
 {
+    [Fact]
+    public void IdleUiPresetIsNominalButCannotInventMeasurementEvidence()
+    {
+        Assert.Equal(SvEngineeringScale.Nominal4I4V, SvEngineeringScaleFactors.IdlePresentationDefault);
+        var noSamples = new AnalogValuesSnapshot();
+        Assert.Null(noSamples.Ua.RmsValue);
+        Assert.Null(noSamples.Ia.RmsValue);
+        Assert.Null(noSamples.Ic.AngleDegrees);
+    }
+
     [Theory]
     [InlineData("Raw 4I4V instMag/q candidate profile", true)]
     [InlineData("OMICRON raw 4I4V instMag/q profile / OMICRON_CMC_SV1", true)]