A deviation Alarmis commonly used to report an excess deviation between a desired set point level of a process value and an actual measurement of that value. The deviation Alarmbecomes active when the deviation exceeds or drops below a defined limit.

For example, if a set point had a value of 10, a high deviation Alarmlimit of 2 and a low deviation Alarmlimit of -1 then the low sub state is entered if the process value drops below 9; the high sub state is entered if the process value raises above 12. If the set point were changed to 11 then the new deviation values would be 10 and 13 respectively. The set point can be fixed by a configuration, adjusted by an Operatoror it can be adjusted by an algorithm, the actual functionality exposed by the set point is application specific. The deviation Alarmcan also be used to report a problem between a redundant data source where the difference between the primary source and the secondary source exceeds the included limit. In this case, the SetpointNodewould point to the secondary source.

The LowLimitand LowLowLimitshall be negative, indicating a number below the target value and the HighLimitand HighHighLimitshall be positive, indicating a number above the target value. If provided, the limits shall not be zero and shall follow these rules:

• HighHighLimit> HighLimit
• LowLimit > LowLowLimit.

For example, if the LowLimitis -2 then a LowLowLimitof -1 would not be allowed, but a LowLowLimitof -3 would be allowed.

The NonExclusiveDeviationAlarmTypeis a special level Alarmutilized with one or more non-exclusive states. If for example both the High and HighHigh states need to be maintained as active at the same time then an instance of NonExclusiveDeviationAlarmTypeshould be used.

The NonExclusiveDeviationAlarmTypeis based on the NonExclusiveLimitAlarmType. It is formally defined in Table 101.

Table 101– NonExclusiveDeviationAlarmType definition

Attribute	Value
BrowseName	NonExclusiveDeviationAlarmType
IsAbstract	False
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the NonExclusiveLimitAlarmType defined in clause 5.8.20.
HasProperty	Variable	SetpointNode	NodeId	PropertyType	Mandatory
HasProperty	Variable	BaseSetpointNode	NodeId	PropertyType	Optional
ConformanceUnits
A & C Non-Exclusive Deviation

The SetpointNode Propertyprovides the NodeIdof the set point used in the deviation calculation. In cases where the Alarmis generated by an underlying system and if the Variableis not in the AddressSpace, a NULL NodeIdshall be provided.

The BaseSetpointNode Propertyprovides the NodeIdof the original or base setpoint. The value of this node is the value of the setpoint to which an AdaptiveAlarmcan be reset if any algorithmic changes need to be discarded. The value of this node usually contains the originally configured set point.

The ExclusiveDeviationAlarmTypeis utilized with multiple mutually exclusive limits. It is formally defined in Table 102.

Table 102– ExclusiveDeviationAlarmType definition

Attribute	Value
BrowseName	ExclusiveDeviationAlarmType
IsAbstract	False
References	NodeClass	BrowseName	DataType	TypeDefinition	Modelling Rule
Inherits the Propertiesof the ExclusiveLimitAlarmTypedefined in clause 5.8.19.3.
HasProperty	Variable	SetpointNode	NodeId	PropertyType	Mandatory
HasProperty	Variable	BaseSetpointNode	NodeId	PropertyType	Optional
ConformanceUnits
A & C Exclusive Deviation

The SetpointNode Propertyprovides the NodeIdof the set point used in the Deviationcalculation. If this Variableis not in the AddressSpace, a NULL NodeIdshall be provided.

The BaseSetpointNode Propertyprovides the NodeIdof the original or base setpoint. The value of this node is the value of the set point to which an AdaptiveAlarmcan be reset if any algorithmic changes need to be discarded. The value of this node usually contains the originally configured set point.