HMI - Alarms



Ladder Logic

RSLogix 5000

Structured Text








In general Alarms in control systems are set of predefined conditions. Those conditions signal an abnormal condition. The main purpose of an alarm is to signal that something has gone wrong, or that particular stage of processing has been reached. For example, an alarm might indicate that the oil level in hydraulic tank has reached a low level or very low level, or very simply an alarm can signal the end of a shift.

Alarm Levels

Every alarm is associated with a level of severity or priority that indicates how critical or how important or even any intervention is needed. In the case of the low oil level in the hydraulic system then the intervention of the operator is needed to refill the hydraulic tank with oil as soon as possible. In the case of very low oil level then the hydraulic system is in very critical condition and hence it should be shut down to prevent any damage to the machines and to the equipments. By contrast, in the case of the end of a shift, the severity would be minimal. The severity of an alarm usually depends upon the circumstances, the factory application, the nature of the equipment, safety, availability of backup systems, potential costs of damage or downtime and so on.

Alarm Purpose

One of the major purposes of an alarm is to alert the operator of any abnormal condition being monitored. The operator should then acknowledge the alarm, indicating that he/she has been seen it. This is separate from the issue of taking corrective action, if any, which might not happen right away. It is also separate from the issue of whether the alarm condition returns to normal, which it might do on its own, even without any external intervention. For example when an high temperature is reached, if the cooling fan start, it will cool down the system and hence the alarm condition will no longer exists. Acknowledgment merely indicates that someone has noticed the occurrence of the alarm. A high or medium priority alarm usually requires acknowledgment, while a very low priority alarm might not. Although the condition that generated the alarm may go away the alarm itself is not considered handled until it is acknowledged. This is very important and it could be useful to improve and optimize the system.

Alarm Groups

Alarms may be organized into groups this facilitates the tracking and the management. Groups might represent different areas of a system or even subsystems, and equipment, operator responsibility or a specific functionality.

Alarm Types

There exist two types of alarms

  • Real Time Alarms are the alarms that show the current alarms in the system.
  • Historical alarms alarms are simply the logging of real time alarms. This will give a historical of the alarms that will allow system engineers to prevent alarms.

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Ladder Logic

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Bottle Application