Evaluating WinCC Horn Audible Alarm Triggering with Hidden and 'To Be Hidden' Messages in SIMATIC PCS 7

In SIMATIC PCS 7 and WinCC, whether an incoming process alarm triggers the physical or sound-card horn depends strictly on the message state and the WinCC Horn configuration. An alarm that is already actively suppressed and categorized in the 'Hidden List' (Hide state = 1) will NOT trigger the audible horn. However, if an alarm enters the system and is routed to the 'To Be Hidden' list (due to dynamic hide timers or pending acknowledgment criteria), the alarm event triggers the Horn at the exact moment of arrival before being transferred to the hidden archive. Suppressing horn actuation for hidden alarms requires parameterizing the WinCC Horn editor to filter out messages bearing the 'Hidden' message attribute.

WinCC Smart Alarm Hiding and Horn Trigger Mechanics

In modern process control rooms, Alarm Flooding during plant upsets impairs operator responsiveness. SIMATIC PCS 7 implements Smart Alarm Hiding conforming to ISA-18.2 to suppress nuisance, redundant, or secondary consequence alarms:
- The WinCC Horn Subsystem (CCAlert.exe):
* Evaluates incoming message telegrams from the alarm logging event buffer in real time.
* When an alarm matches configured criteria (Message Class = Alarm/Warning, Priority > 8, or specific Area), the Horn triggers external signal sounders, sirens, or PC sound card WAV files.
- The Paradox of the Hidden Alarm:
* Operators frequently report hearing the audible horn activate while the primary WinCC Alarm Banner displays zero incoming messages.
* Inspecting the To Be Hidden List reveals the offending alarm with an arrival timestamp coinciding exactly with the horn trigger.
* This occurs because the alarm was captured and processed by the Horn subsystem before the dynamic hiding engine completed its suppression evaluation.

The Lifecycle of Hidden Messages - Incoming vs To Be Hidden vs Hidden

Understanding how an alarm transitions through hiding states is critical for aligning audible behavior:
1. Manual Alarm Hiding (Static Suppression):
- An operator manually hides an alarm from the faceplate or message list.
- The message is placed in the Hidden List with attribute Hidden = TRUE.
- When subsequent occurrences of this alarm arrive from the controller, the Alarm Logging engine immediately passivates the event: The Horn is NOT triggered.
2. Automatic Alarm Hiding (Dynamic State Suppression):
- Driven by controller logic (e.g. unit shutdown mode) or WinCC Alarm Hiding rules.
- When an alarm fires, it first enters the To Be Hidden List:
* In this intermediate state, the alarm is evaluated against acknowledgment rules. If an alarm requires acknowledgment before hiding, or if a delay timer is configured, the event is treated as an active incoming alarm.
* The WinCC Horn detects an active incoming message in the Alarm Logging queue and sounds the horn immediately.
* Fractions of a second later, the hiding logic completes, and the message vanishes from the main banner, leaving operators bewildered.

Configuring WinCC Horn Message Filters for Hidden Alarms

To ensure that hidden or to-be-hidden alarms never trigger audible sounders:

Open the WinCC Horn Editor:
- In WinCC Explorer, double-click Horn in the project navigation tree.
Configure Message Assignment Filter Rules:
- Navigate to the Message Assignment tab.
- Select the target signal tag or sound output row.
- Click the Filter or Selection button.
- In the filter dialog, add the condition:
text State <> 'Hidden' AND State <> 'To Be Hidden'
- Alternatively, filter out the specific Message Class assigned exclusively to suppressed or maintenance alarms.
Align Alarm Hiding Configuration in Alarm Logging:
- Open Alarm Logging in WinCC Explorer.
- Navigate to Alarm Hiding > Settings.
- Ensure the option Acknowledge before hiding is disabled if alarms should be suppressed silently without operator intervention.
- Save the configuration and restart WinCC Runtime.

Alarm State vs Visual Display and Audible Horn Trigger Table

The following table details the operational and visual behavior of alarms across hiding states:

Alarm State / ClassificationDisplayed in Active Message List?Displayed in 'To Be Hidden' List?Displayed in 'Hidden' Archive List?Triggers Audible Horn by Default?Standard Active AlarmYES (Red flashing)NONOYESIncoming (Pre-Hiding Transition)Momentary / NOYES (Pending evaluation)NOYES (Unless filtered in Horn)Actively Hidden (Manual Hide)NONOYESNOActively Hidden (Automatic Hide)NONOYESNOLocked Alarm (MsgLock = 1)NO (Suppressed in AS)NONONO (Never leaves controller)

Verification

To confirm that hidden alarms no longer actuate the audible horn:
1. In WinCC Runtime, open the Alarm Logging overview window and navigate to the To Be Hidden List and Hidden List tabs.
2. Manually hide a non-critical test alarm (e.g. tank level high warning).
3. In CFC test mode, force the alarm condition active.
4. Verify that:
- The active message list remains clean with zero new rows.
- The horn output tag (@Horn_Trigger) remains 0 and no sound is emitted.
- The alarm appears silently in the Hidden List.
5. Unhide the alarm, re-trigger the condition, and verify that the horn sounds normally for standard unsuppressed alarms.

Limitations and Operational Precautions

When configuring alarm hiding and audible signaling:
1. Safety-Critical Alarms Must Never Be Hidden: ISA-18.2 mandates that safety-instrumented system (SIS) alarms, trip interlocks, and toxic gas alarms must be locked out from both manual and automated hiding.
2. Client-Specific Horn Silence: Ensure the horn silence pushbutton (@Horn_Silence) is accessible on all operator screens to allow operators to mute horns instantly during plant disturbances.
3. Redundant Server Horn Synchronization: In redundant OS server pairs, verify that the horn trigger tag is synchronized so that acknowledging an alarm on one client mutes sounders across the entire control room.

Engineer FAQ

Why did my horn trigger when there were no alarms in the active message list?

Because an alarm entered the 'To Be Hidden List'. The WinCC Horn triggered upon the message's physical arrival before the alarm hiding engine completed its suppression transition.

Can I configure different horn sounds for different plant areas?

Yes. In the WinCC Horn Editor under Message Assignment, you can map distinct WAV audio files or hardware digital outputs to specific Plant Areas or Message Classes.

What is the difference between hiding an alarm and locking an alarm?

Hiding an alarm occurs in WinCC (OS layer); the alarm is logged in the database but hidden from operator view. Locking an alarm (MsgLock) occurs in the AS controller; the alarm telegram is never generated at all.

How does an operator unhide an alarm in WinCC?

The operator opens the 'Hidden List' view in the WinCC Alarm Control, selects the hidden message, and clicks the 'Unhide' button, returning it to active monitoring.