Important: Unsigned analogs (UDINT, UINT, USINT, 3D_BCD and 4D_BCD) cannot have rate of change alarms. If you try to configure a rate of change alarm you will see an Invalid point type for rate of change alarm error message.

Rate of Change alarming detects either a faster or slower than expected change in the value of a point.

1	Alarm type
2	Update value
3	Interval
4	Alarm limits

 

1	Alarm type

Select Rate of Change.

2	Update value

Check to update the point value in runtime applications, e.g. Alarm Viewer, each time the value changes.

3

Interval

Length of the interval between sampling the point for Rate of Change alarming.

The interval is a number of:

Seconds

Minutes or

Hours

Important: The minimum Rate of Change interval is 5 seconds.

If the difference between the current value of the point and the value at the last sample time exceeds alarm limits, an alarm message will be generated.

4	Alarm limits

Specify positive alarm limits to check for increasing values and negative alarm limits to check for decreasing values.

An alarm is generated when the:

Difference between the current value of the point and its value at the last sample interval exceeds an alarm limit.

The current point is not evaluated for an alarm condition each time the value changes.

An alarm is generated under these conditions:

Alarm	Generates alarms for:
Alarm High	Faster than expected changes
Warning High	Faster than expected changes
Alarm Low	Slower than expected changes
Warning Low	Slower than expected changes

In the Alarm Limits, you can specify:

Values that are	To check for:
Positive	Increasing values
Negative	Decreasing values.

Use the following table as a guide.

Alarm Limit	Limit Value	Rate of Change Value is ...	Point value is ....
Alarm High	Positive	³ Alarm High	Increasing too fast
Alarm High	Negative	£ Alarm High	Decreasing too fast
Warning High	Positive	³ Warning High	Increasing too fast
Warning High	Negative	£ Warning High	Decreasing too fast
Warning Low	Positive	£ Warning Low	Increasing too slowly
Warning Low	Negative	³ Warning Low	Decreasing too slowly
Alarm Low	Positive	£ Alarm Low	Increasing too slowly
Alarm Low	Negative	³ Alarm Low	Decreasing too slowly

More information

Step 3.2.2. Select alarm criteria.

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