Functional Safety Glossary

Reading time: 5 minutes - Difficulty: advanced
In this articole you can find out all the definitions about Functional Safety. From the meaning of Functional Safety to the mandatory documents prior to the SIL certificate issue explanation, you can check out all the terminology related to the safety equipment standards.

Definitions

Here are explained all the definitions that the standard IEC 61508 Parts 1-7 presents.

 

Functional Safety:

Part of the overall safety relating to the EUC and the EUC control system that depends on the correct functioning of the E/E/PE safety-related systems and other risk reduction measures.

 

Safe State:

State of the EUC when safety is achieved.

 

Electrical/Electronic/Programmable Electronic Control System:

System for control, protection or monitoring based on one or more electrical/electronic programmable (E/E/PE) devices, including all elements of the system such as power supplies, sensors and other input devices, data highways and other communication paths, and actuators and other output devices.

 

Recommended in-depth study:

 

Redundancy:

The existence of more than one means for performing a required function or for representing information.

 

Mean Time to Failure:

Expectation of the mean time to failure, the MTTF is normally expressed as an average value of expectation of the time to failure

 

Probability of dangerous Failure per Hour (PFH):

Average probability of dangerous failure within 1 h.

 

Probability of dangerous Failure on demand (PFD):

Average probability of dangerous failure on demand.

 

Safety Function:

Function to be implemented by an E/E/PE safety-related system or other risk reduction measures, that is intended to achieve or maintain a safe state for the EUC, in respect of a specific hazardous event.

 

Recommended in-depht study:

 

Safety Instrumented Function (SIF):

Safety function with a specified safety integrity level which is necessary to achieve functional safety and which can be either a safety instrumented protection function or a safety instrumented control function.

 

Safety Instrumented System (SIS):

Instrumented system used to implement one or more safety instrumented functions. An SIS is composed of any combination of sensor (s), logic solver (s), and final elements(s).

 

Safety Integrity:

Probability of a SIS or its subsystem satisfactorily performing the required safety-related control functions under all stated conditions.

 

Systematic Safety Integrity:

Part of the safety integrity of a safety-related system relating to systematic failures in a dangerous mode of failure.

 

Systematic Capability:

Measure (expressed on a scale of SC 1 to SC 4) of the confidence that the systematic safety integrity of an element meets the requirements of the specified SIL, in respect of the specified element safety function, when the element is applied in accordance with the instructions specified in the compliant item safety manual for the element

 

Hardware Safety Integrity:

Part of the safety integrity of a safety-related system relating to random hardware failures in a dangerous mode of failure.

 

Safety Integrity Level (SIL):

Discrete level (one out of a possible three) for specifying the safety integrity requirements of the safety-related control functions to be allocated to the SIF, where safety integrity level three has the highest level of safety integrity and safety integrity level one has the lowest.

 

Diagnostic Coverage (DC):

Ratio of the detected failure rate to the total failure rate of the component or subsystem as detected by diagnostic tests. Diagnostic coverage does not include any faults detected by proof tests.

 

Mode of Operation:

Way in which a safety function operates, which may be either:

  1. Low demand mode, where the safety function is only performed on demand, in order to transfer the EUC into a specified safe state, and where the frequency of demands is no greater than one per year; or
  2. High demand mode, where the safety function is only performed on demand, in order to transfer the EUC into a specified safe state, and where the frequency of demands is greater than one per year; or
  3. Continuous mode, where the safety function retains the EUC in a safe state as part of normal operation

 

Fault Tolerance:

Ability of a functional unit to continue to perform a required function in the presence of faults or errors.

 

Failure:

Termination of the ability of a functional unit to provide a required function or operation of a functional unit in any way other than as required.

 

Random Hardware Failure:

Failure, occurring at a random time, which results from one or more of the possible degradation mechanisms in the hardware.

 

Systematic Failure:

Failure, related in a deterministic way to a certain cause, which can only be eliminated by a modification of the design or of the manufacturing process, operational procedures, documentation or other relevant factors.

 

Dangerous Failure:

failure of an element and/or subsystem and/or system that plays a part in implementing the safety function that:

  1. Prevents a safety function from operating when required (demand mode) or causes a safety function to fail (continuous mode) such that the EUC is put into a hazardous or potentially hazardous state; or
  2. Decreases the probability that the safety function operates correctly when required.

 

Recommended in-depth study:

 

Safe Failure:

Failure of an element and/or subsystem and/or system that plays a part in implementing the safety function that:

  1. Results in the spurious operation of the safety function to put the EUC (or part thereof) into a safe state or maintain a safe state; or
  2. Increases the probability of the spurious operation of the safety function to put the EUC (or part thereof) into a safe state or maintain a safe state.

 

Common Cause Failure:

Failure, that is the result of one or more events, causing concurrent failures of two or more separate channels in a multiple channel system, leading to system failure.

 

No Part Failure:

Failure of a component that plays no part in implementing the safety function.

 

No Effect Failure:

Failure of an element that plays a part in implementing the safety function but has no direct effect on the safety function.

 

Safe Failure Fraction (SFF):

Property of a safety related element that is defined by the ratio of the average failure rates of safe plus dangerous detected failures and safe plus dangerous failures.

 

Failure Rate:

Reliability parameter (λ(t)) of an entity (single components or systems) such that λ(t).dt is the probability of failure of this entity within [t, t+dt] provided that it has not failed during [0, t].

 

Probability of Dangerous Failure on Demand (PFD):

Safety unavailability (see IEC 60050-191) of an E/E/PE safety-related system to perform the specified safety function when a demand occurs from the EUC or EUC control system.

 

Average Probability of Dangerous Failure on Demand:

Mean unavailability (see IEC 60050-191) of an E/E/PE safety-related system to perform the specified safety function when a demand occurs from the EUC or EUC control system.

 

Safety Lifecycle:

Necessary activities involved in the implementation of safety-related systems, occurring during a period of time that starts at the concept FASE of a project and finishes when all of the E/E/PE safety-related systems and other risk reduction measures are no longer available for use.

 

Recommended in-depth study:

 

Functional Safety Assessment:

Investigation, based on evidence, to judge the functional safety achieved by one or more E/E/PE safety-related systems and/or other risk reduction measures.

 

Functional Safety Audit:

Systematic and independent examination to determine whether the procedures specific to the functional safety requirements to comply with the planned arrangements are implemented effectively and are suitable to achieve the specified objectives.

 

Proof test:

Periodic test performed to detect dangerous hidden failures in a safety-related system so that, if necessary, a repair can restore the system to an “as new” condition or as close as practical to this condition.

 

Safety Manual for Compliant Items:

Document that provides all the information relating to the functional safety of an element, in respect of specified element safety functions, that is required to ensure that the system meets the requirements of IEC 61508 series.

 

For further information:

Do you want to learn more about Functional Safety?

What does HARA mean for ISO 26262?

The HARA method The HARA method aims at identifying and categorizing hazardous events of items, and also at specifying safety goals according to ISO 26262 and ASILs (Automotive Safety Integrity Levels) related to the prevention or mitigation of the associated hazards in order to avoid unreasonable risk. This means that the combination of a hazard […]

Read more

A brief introduction to ISO 26262

ISO 26262 Standard Application It covers the implementation of functional safety through electrical and/or electronic (E/E) systems, and presents a specific lifecycle for items used in the automotive sector. Thus, it provides a reference for the automotive safety life cycle and supports the adaptation of activities to be performed during the lifecycle phases, i.e. development, […]

Read more
Byhon Logo bianco

Subscribe to our newsletter to stay up to date on Functional Safety and Industrial Cyber Security news and events

ISCRIVITI
close-link
Send this to a friend