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Calculating a Failure-Finding Interval for a SIF Action Plan

On a SIF analysis’ action plan, you can recommend a failure-finding maintenance action for a hidden failure and provide the failure-finding interval. A failure-finding interval is the length of time that it is considered safe to wait before performing failure-finding maintenance. The interval is calculated or estimated based on the desired availability and the frequency of failure of the protective device or system. Use the Calculate Failure-Finding Interval dialog to create an FFI.

Note: The failure-finding interval calculator is not available on secondary action plans.

Tip: After calculating the FFI, you can view the calculation inputs in the Facilitation view, Failure Data tab. Click the Failure Finding Information button to open the Failure Finding Info window.

To Calculate a Failure-Finding Interval

1.	On the analysis’ Facilitation view, double-click the failure mode to open the Maintenance Action Plan window.

2.	Make sure that editing is enabled.

3.	Select the Implementation view and the Primary Action Plan tab. For example:

4.	Click the calculator icon next to the Failure finding interval boxes.

The Calculate Failure-Finding Interval window appears:

5.	Select the formula that you wish to use. When you select a formula, it is displayed below the list. The formula’s calculation inputs appear in the Calculation details area.

If you select Optimal cost, see To Calculate a Failure-Finding Interval Based on Optimal Cost.

The following table provides a brief description of each formula and its inputs.

Formula	Inputs
Availability-based formula	UTIVE = Unavailability of the protective device MTIVE = Mean time between failure for the protective device
Single independent protected function	MTED = Mean time between failure for the protected function MTIVE = Mean time between failure for the protective device MMF = Allowed mean time between multiple failures
Multiple failure modes on a single protective device	MTED = Mean time between failure for the protected function m1 = Mean time between failure for protective device 1 m2 = Mean time between failure for protective device 2 m3 = Mean time between failure for protective device 3 m4 = Mean time between failure for protective device 4 m5 = Mean time between failure for protective device 5 MMF = Allowed mean time between multiple failures
Multiple redundant protective devices Protected system with multiple independent, fully redundant protective devices	MTIVE = Mean time between failure for the protective device MTED = Mean time between failure for the protected function MMF = Allowed mean time between multiple failures n = the number of redundant protective devices
K of n voting system Protected system with a set of parallel protective devices	MTIVE = Mean time between failure for the protective device MTED = Mean time between failure for the protected function MMF = Allowed mean time between multiple failures n = the number of parallel protective devices k = is the number of protective devices required to activate the system r = the number of units which have to be in a failed state in order to cause the whole system to fail: r = n - k + 1
Failure finding task causes a hidden failure The failure-finding task could cause the very failure which it is suppose to check and so leave the protective device in a (hidden) failed state from the moment the test is completed	MOTHER = Mean time between failure caused by phenomena other than the test MTED = Mean time between failure for the protected function MMF = Allowed mean time between multiple failures p = if P is the probability that the failure finding test will leave the device in a failed state, p (as a decimal) is the unavailability caused by the testing process
Economic	CM = cost of the multiple failure (cost per event) CFF = cost of doing the failure finding task (per task) MTIVE = MTBF of the protective device MTED = MTBF of the protected function
Economic voting system	CM = cost of the multiple failure (cost per event) CFF = cost of doing the failure finding task (per task) MTIVE = MTBF of the protective device MTED = MTBF of the protected function n = the number of redundant protective devices

6.	Provide the calculation inputs for the formula. When the inputs are supplied, the results of the calculation are shown in the Results area. The frequency closest to the failure-finding interval, without being longer than the failure-finding interval, is displayed.

7.	Click OK. The failure-finding interval is added to the action plan and the frequency is calculated and displayed.

To Calculate a Failure-Finding Interval Based on Optimal Cost

When you select Optimal cost, the Calculate Failure-Finding Interval dialog changes, as follows:

You can select predefined FM interval parameters, such as “Months and days”, to pre-fill the units of measure. Or you can override the parameter defaults to select other unit of measure categories. Then enter the inputs for your scenario, such as the cost of a functional failure, the failure finding task cost, the MTBF of the protected equipment and protective device. Then click Execute.

Here is an example using the parameter defaults of months and days:

When the calculation is executed, the chart lines display the total costs, failure-finding task costs, and functional failure costs. You can use the cursor pointer to view the data:

You can also click View as Table or View as Form to see the figures.

Note: The Optimal cost calculation is a “generally available” product feature. You must first enable feature 141 to use the functionality in APM. In the Enterprise window, select the Features view and the Enabled Features tab. Click Browse, select “Optimal cost calculation for Failure Finding Interval” and click OK. If APM is running as a smart client, click Refresh Enabled Features on the server. Then restart the client to use the functionality.

In the following example, the parameter defaults were overridden to use the length unit of measure.

The optimal period is the period where the total cost is the lowest, resulting in a failure-finding interval of 57 days.