12 Causal Connections

If a model item has a failure diagram, the user can edit the causal connections between failure concepts using the Properties viewer. Properties such as probability, progression rate, polarity and causal strength are used in different types of FMECA. The properties of various causal connections that exist within a failure diagram are displayed in the table below. Each causal connection type is defined in terms of its source and target nodes within the diagram.

Table 8: Causal Connection Properties

Source	Target	Property	Definition
Cause	Mechanism	-	N/A
Mechanism	Fault	Causal probability	The probability that a failure mechanism will generate a fault.
Mechanism	Fault	Progression rate	The speed with which a failure mechanism will develop a fault. This is a relative scale from instantaneous (low) to multiple operating cycles (high).
Fault	Functional Failure Mode	Probability	The probability that a fault will result in the activation of an FFM.
Fault	Functional Failure Mode	Causal Strength	The degree to which the target functional output flow will be perturbed up or down by the source fault.

To edit Causal Connections:

• Open the Failure Diagram editor of an item
• Select the Criticality tab in the Properties viewer
  • Causal Strength and Polarity are in the General tab
• Select a Causal Connection between two Failure Concepts
• Edit the causal connection using the slider, checkbox or numerical field

Example (Air Filter failure diagram): Causes "Damaged surface protection", "Untimely maintenance actions", "Solid particle contaminants" feed Mechanism "Corrosive attack", feeding Faults "Perforated", "Pitted", "Corroded", each connected via a weighted OR-gate ("−") to the FFM "Refine Gas Mass flow rate (Air Filter)".

A "Failure Paths Metrics" panel below the diagram lists, per Failure Path: Baseline NFMR, Baseline Failure Rate, Baseline MTTF, Baseline Reliability, Baseline P(f), Effectiveness Factor, Adjusted P(f), Revised Reliability, Revised NFMR, Revised MTTF, Revised Failure Rate. Example rows for Air Filter's three faults:

Failure Path	Baseline NFMR	Baseline Failure Rate	Baseline MTTF	Baseline Reliability	Baseline P(f)	Effectiveness Factor	Adjusted P(f)	Revised Reliability	Revised NFMR	Revised MTTF	Revised Failure Rate
Air Filter	1.00		1000000.00	0.9999089	0.000091		0.000091	0.9999089	1.00	1000000.00
Corroded (Air Filter)	0.3333333	0.3333	3000000.00	0.9999696	0.0000303	0.00	0.0000303	0.9999696	0.3333333	3000000.00	0.3333
Perforated (Air Filter)	0.3333333	0.3333	3000000.00	0.9999696	0.0000303	0.00	0.0000303	0.9999696	0.3333333	3000000.00	0.3333
Pitted (Air Filter)	0.3333333	0.3333	3000000.00	0.9999696	0.0000303	0.00	0.0000303	0.9999696	0.3333333	3000000.00	0.3333

Selecting the "Corroded -> Gas Mass flow rate" connection shows in the Properties viewer, Criticality tab: Causal Probability slider from 1 to 10, shown at maximum (10.00, labeled "Max").