Alarm suppression and shelving in SCADA systems - Step-by-Step Execution

Process Flow - Alarm suppression and shelving

Alarm Triggered

↓

Check Suppression Rules

↓

Suppress Alarm

↓

Shelve Alarm Temporarily

↓

Shelving Time Expires

↓

Alarm Active Again

↓

Operator Response

↓

Alarm Cleared

When an alarm triggers, the system checks if it should be suppressed or shelved temporarily before alerting the operator.

Execution Sample

SCADA systems

alarm_triggered = True
if suppression_active:
    suppress_alarm()
elif shelving_active:
    shelve_alarm()
else:
    alert_operator()

This code checks if an alarm should be suppressed or shelved before alerting the operator.

Process Table

Step	Alarm Triggered	Suppression Active	Shelving Active	Action Taken	Alarm State
1	True	True	False	Suppress Alarm	Alarm Suppressed
2	True	False	True	Shelve Alarm Temporarily	Alarm Shelved
3	True	False	False	Alert Operator	Alarm Active
4	False	False	False	No Action	No Alarm

💡 Alarm state changes based on suppression and shelving flags; stops when alarm is cleared or no alarm present.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
alarm_triggered	False	True	True	True	False
suppression_active	False	True	False	False	False
shelving_active	False	False	True	False	False
alarm_state	No Alarm	Alarm Suppressed	Alarm Shelved	Alarm Active	No Alarm

Key Moments - 3 Insights

Why does the alarm not alert the operator when suppression is active?

Because in step 1 of the execution table, suppression_active is True, so the system suppresses the alarm instead of alerting.

What happens when shelving is active but suppression is not?

At step 2, shelving_active is True and suppression_active is False, so the alarm is shelved temporarily, delaying the alert.

Why does the alarm become active again after shelving expires?

After shelving time expires, shelving_active becomes False, so the alarm moves to active state as shown in step 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the alarm state at step 2?

A Alarm Shelved

B Alarm Suppressed

C Alarm Active

D No Alarm

💡 Hint

Check the 'Alarm State' column in row for step 2.

At which step does the alarm get suppressed?

A Step 4

B Step 3

C Step 1

D Step 2

💡 Hint

Look at the 'Action Taken' column for 'Suppress Alarm'.

If shelving_active was True at step 3, what would happen?

A Alarm would be suppressed

B Alarm would be shelved

C Alarm would alert operator

D No alarm would trigger

💡 Hint

Refer to how shelving_active True affects action in steps 1 and 2.

Concept Snapshot

Alarm suppression and shelving:
- Suppression stops alarm alerts temporarily.
- Shelving delays alarm alerts for a set time.
- System checks suppression first, then shelving.
- Alarm alerts operator only if neither active.
- Shelving expires returns alarm to active state.

Full Transcript

When an alarm triggers in a SCADA system, it first checks if suppression is active. If yes, the alarm is suppressed and no alert is sent. If suppression is not active, it checks if shelving is active. Shelving temporarily delays the alarm alert. If neither suppression nor shelving is active, the alarm alerts the operator. After shelving time expires, the alarm becomes active again. This flow ensures operators are not overwhelmed by alarms and can manage them effectively.