Ataytis Assurance · Fault Tree Analysis
Fault tree validation
Which fault tree rules apply under each notation profile, the messages they produce and how to fix them, and the advisories.
Rules by profile
| Rule | IEC 61025:2006 | Traditional / NUREG-0492 | Custom |
|---|---|---|---|
| Exactly one top event | Error | Error | Error |
| No cycles | Error | Error | Error |
| Probability 0 to 1; failure rate, repair rate and mission time 0 or more | Error | Error | Error |
| Top event has no parent | Error | Error | Warning |
| Events and gates alternate | Error | Error | — |
| One event above each gate; one gate beneath each top, intermediate and transfer-out event | Error | Error | — |
| Nothing beneath basic, dormant, undeveloped, house, zero and conditional events | Error | Error | — |
| Gate input counts | Error | Error | Warning below 2 inputs |
| Voting, priority sequence, spare, house, zero and dormancy values | Error | Error | — |
| Every node reachable from the top event | Error | Error | — |
| Each transfer has one reference, to a diagram in the table | Error | Error | — |
| FDEP and Priority OR gates | Error | Error | Allowed, with their own input rules |
| Conditioning event on INHIBIT and Priority AND gates | — | Error on INHIBIT, warning on Priority AND | — |
| Tree with no gates | Warning | Warning | — |
| Disconnected parts of the diagram | — | — | Reported |
Messages and fixes
| Message | Fix |
|---|---|
| Top Event is missing. | Add one row with Node Type Top Event. A diagram that starts from a Transfer Out is exempt. |
| Top Event occurs 2 times. | Keep one top event per diagram. Put separate trees under separate Diagram IDs. |
| IEC notation requires the top event to be the root of the hierarchy. | Clear Parent ID on the top event's row. |
| Each gate needs exactly one event above it. Give this gate a parent event, or remove the extra ones. | Set the gate's Parent ID to the event it defines. If the gate appears on more than one row, keep one. |
| Intermediate Event needs exactly one gate beneath it saying how it happens. | Add a gate between the event and its causes, or change it to a basic event. Two gates under one event also produce this message. |
| AND Gate cannot feed or gate directly. A fault tree alternates events and gates. | Put an intermediate event between two gates, or a gate between two events. |
| Basic Event must not be developed further. | Remove the rows that name it as their parent, or change it to an intermediate event with a gate beneath it. |
| OR Gate requires at least 2 inputs. Add or remove inputs beneath it. | Add inputs, or change the gate type. The message gives the required count for each gate. |
| The voting threshold must be an integer from 1 to 3. | Set Vote Threshold on the voting gate's row. |
| Voting input count must match the 3 connected diagram inputs. | Correct Vote Input Count, or leave it blank. |
| Spare mode must be Cold, Warm or Hot. | Correct Spare Mode on the Spare gate's row. |
| Required spares must be an integer from 1 to 2. | Correct Spare Required Count on the Spare gate's row. |
| A house event — also written External Event — is either certain or impossible, so its probability must be 0 or 1, not 0.4. | Set the probability to 0 or 1, or change the event to an undeveloped event. |
| The house event state must be TRUE/FALSE or ON/OFF and agree with its probability. | Make Event State agree with Probability, or clear one of them. |
| A zero event must remain FALSE and can only have probability 0. | Set Probability to 0 and Event State to FALSE or blank. |
| Probability must be a finite value from 0 to 1. | Check for percentages entered as whole numbers, such as 5 for 0.05. |
| Each transfer symbol needs exactly one paired transfer relationship or linked diagram reference. | Give the transfer one Linked Diagram ID, or one Transfer relationship, not both and not neither. |
| This transfer points to diagram …, which is not present in the data. | Correct Linked Diagram ID, or add the linked diagram's rows to the table. |
| 3 fault-tree nodes cannot be reached from the top event. | Check Parent ID on those rows. Every node must lead up to the top event. |
| This diagram has no logic gates, so it is a cause tree rather than a fault tree. | Add gates between events, or switch the profile to Custom. |
| IEC 61025:2006 Annex A does not define the fdepGate. Switch the notation policy to Custom to use it. | Switch the profile to Custom, or use a gate the standard defines. |
| Traditional notation states a INHIBIT gate's enabling condition through an attached Conditioning event. Add one. | Attach a conditional event to the gate, or switch the profile. |
| No Node type column is mapped, so notation rules are not checked. | Map a Node Type column to check the tree against the profile. |
Numbers and names in these messages come from your data, so the counts and types shown will differ.
Advisories
Advisories never stop a tree drawing. Turn them off by setting Diagram → Dynamic gate conventions → Advisory diagnostics to Hide advisories.
| Advisory | Severity | Raised when |
|---|---|---|
| Repeated events | Information | An event feeds more than one gate. |
| Non-coherent gate | Information | A NOT, NAND or NOR gate is used under IEC 61025:2006 or Traditional / NUREG-0492. |
| Priority AND equal to Sequential | Information | A Priority AND has more than two inputs under the default convention. |
| Degenerate vote | Information | A voting threshold is 1, or equals the number of inputs. |
| Spare mode and dormancy conflict | Warning | A cold spare's input has a dormancy factor other than 0, or a hot spare's input other than 1. |
| Partly numbered spare inputs | Warning | Some, but not all, inputs of a Spare gate have a priority sequence. |
| Spare gate incomplete | Warning | A Spare gate has no spare mode or no required spares, under IEC 61025:2006 or Traditional / NUREG-0492. |
| Unused dormancy factor | Information | An event with a dormancy factor is not an input to a Spare gate. |
| Symbol substitution | Information | A node type is drawn with a borrowed symbol under the active profile. |