Alarm types (high, low, deviation, rate) in SCADA systems - Full Explanation

Introduction

Imagine you are monitoring a machine that must stay within safe limits. If something goes wrong, you need to know quickly. Different alarm types help you spot problems by alerting you when values go too high, too low, change unexpectedly, or change too fast.

Explanation

High Alarm

A high alarm triggers when a measurement goes above a set upper limit. This warns operators that a value is too high and may cause damage or unsafe conditions. The alarm stays active until the value returns below the limit.

High alarms alert when values exceed a safe upper threshold.

Low Alarm

A low alarm activates when a measurement falls below a set lower limit. It signals that a value is too low, which might indicate a problem like loss of pressure or temperature. The alarm clears when the value rises above the limit again.

Low alarms warn when values drop below a safe lower threshold.

Deviation Alarm

A deviation alarm compares a measurement to a reference value, such as a setpoint or another variable. It triggers if the difference between them becomes too large, indicating something is off balance or out of sync.

Deviation alarms detect when a value strays too far from a reference.

Rate Alarm

A rate alarm watches how fast a value changes over time. If the change is too rapid, it signals a possible sudden fault or unsafe condition. This helps catch problems that might not be obvious from just the value itself.

Rate alarms alert when values change faster than a safe speed.

Real World Analogy

Think of driving a car with a dashboard. The high alarm is like a warning light for overheating engine temperature. The low alarm is like a fuel gauge showing empty. The deviation alarm is like noticing your speed is very different from the flow of traffic. The rate alarm is like feeling the car suddenly jerk or accelerate too fast.

High Alarm → Engine temperature warning light when too hot

Low Alarm → Fuel gauge warning when fuel is too low

Deviation Alarm → Speed being very different from other cars on the road

Rate Alarm → Feeling sudden jerks or rapid acceleration in the car

Diagram

Diagram

┌───────────────┐
│   Measurement │
└──────┬────────┘
       │
       ▼
┌───────────────┐      ┌───────────────┐      ┌───────────────┐      ┌───────────────┐
│   High Alarm  │      │   Low Alarm   │      │Deviation Alarm│      │   Rate Alarm  │
│ Triggers if   │      │ Triggers if   │      │Triggers if    │      │Triggers if    │
│ value > high  │      │ value < low   │      │difference >   │      │change too     │
│ limit        │      │ limit        │      │threshold     │      │fast          │
└───────────────┘      └───────────────┘      └───────────────┘      └───────────────┘

Diagram showing a measurement feeding into four alarm types, each triggering on different conditions.

Key Facts

High Alarm → Activates when a value exceeds a predefined upper limit.

Low Alarm → Activates when a value falls below a predefined lower limit.

Deviation Alarm → Activates when the difference between a value and its reference exceeds a set threshold.

Rate Alarm → Activates when the speed of change of a value exceeds a safe rate.

Common Confusions

Believing a high alarm triggers only on dangerous conditions.

Believing a high alarm triggers only on dangerous conditions. High alarms trigger whenever a value goes above the set limit, which may be for safety or operational reasons, not always danger.

Thinking deviation alarms compare values to fixed limits instead of references.

Thinking deviation alarms compare values to fixed limits instead of references. Deviation alarms compare a value to a dynamic reference, like a setpoint or another variable, not just fixed high or low limits.

Assuming rate alarms trigger based on value magnitude rather than speed of change.

Assuming rate alarms trigger based on value magnitude rather than speed of change. Rate alarms focus on how fast a value changes over time, not the value itself.

Summary

High and low alarms alert when values cross set upper or lower limits to ensure safe operation.

Deviation alarms detect when a value strays too far from a reference point, signaling imbalance.

Rate alarms warn about rapid changes in values that could indicate sudden problems.