Feature Description
The Severity Rating Integer Attribute is a end item Failure Effects Checklist Attribute with assigned integers indicating level of Severity as shown in in Attribute Value window below.
For a given failure, the Risk Priority Number (RPN) severity rating is derived from the severity class that has been assigned to end item effects that include that failure among their causes. If more than one end item effect includes a given failure among their causes, the effect with the highest severity is used in the calculation.
|
|
Most eXpress models override the built-in severity classes using a checklist attribute to define the different severity levels that can be assigned to a failure effect.
The example at left shows the checklist “Severity” attribute that would be used to override the default severity classes with those defined within MIL-STD-1629A.
|
To override severity classes in eXpress, the Attribute must be a checklist Attribute named “Severity” that is associated only with failure effects. The different classes are entered as entries in the checklist. The “Mutually Exclusive” and “Assign Values” checkboxes must be enabled and a unique value must be assigned to each entry, the maximum value being assigned to the most severe class. In the example above, the highest value (4) has been assigned to the most severe class (“Category I – Catastrophic”).
|
There are two ways that the RPN severity rating can be used within an eXpress FMECA chart. First of all, it can be included in its own column. To do this, select the “Severity Rating (for RPN)” entry from the list on the right- hand side of the FMECA Plus Options dialog and move it over to the list on the left. Once you have added this field, you can rename the column header using the “Name” box at the bottom of the dialog. Use the “Move Up” and “Move Down” buttons to modify the order in which columns will appear in the FMECA.
|
|
|
|
Of course, the severity rating is also used when calculating the value that appears in the Risk Priority Number (RPN) column. In the
example at left, the column “RPN (Risk Priority Number)” has been added to the FMECA, along with the columns for the individual severity, occurrence and detection ratings. In this example, all four RPN-related columns have been included in a column group labeled “Risk Priority Number” and the “RPN (Risk Priority Number” sub-column has been renamed “RPN” (as shown at left).
|
The following excerpt show how the final columns from the preceding example would appear in a calculated FMECA chart:
In this example, the values 1, 2, 3 & 4 assigned to the MIL-STD-1629A severity rankings have been mapped to 1, 4, 7 & 10 in the 10-point severity rating. If you wish to change how the values map, you can do so by creating a user-defined severity map Attribute. This Attribute must be created as follows:
|
|
-
It must be named “SeverityMap”
-
It must be of type Checklist
-
It must be associated with the Design
-
It must be “Mutually Exclusive”
-
It must have values assigned
-
It must have one entry for each Severity rating
(on a ten-point scale) to which Severity class values will be mapped (in the example below: 1, 2, 5 & 10)
-
For each entry, assign the minimum Severity
Class value that is to be mapped to that entry
(in the example below, the Severity Class 4 and
above will be mapped to severity rating 10)
The following excerpt shows how the Severity
rankings would be calculated when the severity
map Attribute shown above is defined
in the model where the FMECA is run:
|
The following excerpt shows how the Severity rankings would be calculated when the severity map Attribute shown above is defined in the model where the FMECA is run:
As you can see, the 4-point MIL-STD-1629A severity rankings have now been mapped to 1, 2, 5 & 10 in the 10-point severity rating. The calculated RPN takes this change into account as well.
One final way in which you can define RPN severity values in eXpress is to create a user-defined severity rating Attribute. This allows you to manually define (or import) the severity rating for each failure effect…and have this value used when calculating the final Risk Priority Number.
|
|
Create an integer Attribute named “SeverityRating”. Associate the Attribute with Failure Effects.
Use this Attribute to separately define the value to be used for the RPN severity rating for each failure in the FMECA.
Notice that these Attribute values must be assigned to the object failure effect that are represented by each row in the FMECA, not to the design effects that appear as local, next higher or end item effects.
This gives you greater freedom in assigning severity ratings. You are not restricted to values between 1 and 10. You are also not required to enter the same value for failures with the same Severity Class.
|
|
When the severity rating Attribute is included as a column in the FMECA, the Risk Priority Number will be calculated using the values from this Attribute, rather than the built-in severity rating calculation.
The FMECA setup pictured at left is nearly identical to the one shown previously—the only difference is that the “SeverityRating” Attribute is used for the severity rating.
Because the name of the Attribute (with no space) would look odd as a column header, you can insert a carriage return (a double back-slash) into the column name. Inserting a space would also do the trick.
|
|
In this final FMECA excerpt, the RPN is calculated using a user-defined severity rating Attribute, rather than the built-in severity rating calculation.
Notice that, for some failures, the top severity class (“Category I – Catastrophic”) has been assigned a rating of 10, and for others a rating of 9. Although this approach gives you the flexibility to do this, it also requires you to enter or import individual severity rating values for all object failure effects that constitute a line item in the FMECA. The two approaches discussed earlier both derive the severity rating from the assigned severity class.
As expected, the RPN column uses the value from the Attribute, rather than the built-in calculation
