Most failure modes are defined relative to a failure mode control variable (FMCV). This is the variable that is referenced by the failure mode to determine when failure occurs. (For those failure modes that are defined as distributions, the control variable represents the x-axis of a failure distribution plot.)
For FMCV-based failure modes, the failure mode calculates the FMCV “age” that will result in its next failure at the start of the realization, after each time the mode is repaired, and after each time the component is replaced. When the FMCV exceeds this value, the failure occurs.
Each (FMCV-based) failure mode has its own FMCV, which is defined in terms of a specified “base variable”. Two built-in based variables are available to both the Function and the Action element. These are “Total time” which accrues time regardless of whether or not the component is operating and “Operating time”, which accrues time when the component is operating. The latter is the default base variable for all failure modes in GoldSim. A third base variable, "Number of actions completed", is available only to the Action element.
Base variables can be changed by selecting a Failure Mode on the Failure Modes tab of a reliability element and clicking the Settings… button. The failure mode base variable is selected from a drop-down list at the top of the dialog.
The two built-in base variables that reference time are quite different in the way they accrue time. The "Operating time" base variable accrues time only when the component is operating, whereas the "Total time" base variable accrues time even if the component or its parent has been turned off. Both can be reset when the failure mode is repaired or the component is replaced.
The model file ControlVariables.gsm, found in the Reliability Examples folder in your GoldSim directory directory (accessed by selecting File | Open Example... from the main menu), illustrates the difference between these three base variables. The model contains three elements, each of which has a single Specified Value Exceeded failure mode that fails at a value of 100 days (100 completed actions for the element using the Number of actions completed failure mode). All three elements are turned on at 75 days and turned off at 250 days. The Action element’s action is triggered once per day, and the component that uses the Operating time control variable undergoes a Preventive maintenance event at 125 days that lasts for exactly 5 days.
In the plot below, we see the behavior of the element whose failure mode uses a FMCV based on Total time:
You can see that the Total time control variable accrues time from the start of the simulation, so even though the element is only turned on at 75 days, it fails at 100 days.
The plot below shows the behavior of the element whose failure mode uses a FMCV based on Operating time:
In this case, you see that the control variable accrues time from 75 to 125 days, when a PM:Preventive Maintenance mode occurs. The control variable value stops accruing time at 125 days by the PM event, and then is reset and starts to accrue time again when the element is placed back in service at 130 days. At 230 days, the element fails, as it has been operating without a PM for 100 days.
The following plot shows the behavior of the Action element whose failure mode uses a FMCV based on Number of completed actions:
From the plot, you can see that this element fails at 175 days, after its action has been triggered 100 times. The Number of Actions Completed control variable is similar to the Total time control variable in that it makes no distinction between the component being operating or off.
Learn more about:
Failure Mode Control Variables
Understanding Failure Mode Base Variables