Safe failure fraction

The fault tolerance requirement for PE logic solvers is based on the SIL and the SFF. ANSI/ISA-84.00.01-2004-1 uses the following definitions  

Safe Failure - Failure that does not have the potential to put the safety instrumented system in a hazardous or fail-to-function state 

Safe-Failure Fraction - The fraction of the overall random hardware failure rate of a device that results in either a safe failure or a detected dangerous failure. 

In other words, the safe-failure fraction is a measure that indicates the probability of a subsystem failure being either safe or detected by diagnostics. The measure is applied to each major subsystem in a safety instrumented function (sensor, logic solver, final element) separately. 

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The extent to which faults lead to a safe condition or can be detected by diagnostics so that a specified action can be taken. This capability is termed the safe failure fraction of the device ... 

Instrumentation in safety applications (SIS) utilises vendor information on diagnostics and safe failure fraction (SFF) as well as performance information collected from the applications to calculate the probability of failure on demand (PFD). 

Results of the evaluation typically include a number of safety integrity and availability measurements. Most important, the average probability of failure on demand (PFDavg) and the safe failure fraction (SFF) is calculated for low demand mode. Probability of failure per hour is calculated for high demand mode. From charts, the SIL level that the... 

If these restrictions are met, only one transmitter or valve is needed for a S1L2 SIF to meet this requirement. Alternatively the charts of lEC 61508 may be used for field devices. Given the lack of definition as to what "prior use" really means, the authors prefer to use the tables from lEC 61508 which are more flexible, provide at least the same level of "exception" for products with sufficient design quahty and are clearly justifiable. The disadvantage of these charts is that the safe failure fraction must be calculated for the field devices. 

The use of this chart requires the calculation of a measure called the Safe Failure Fraction (SFF). This chart is completely equivalent to the same chart in lEC 61508. 

Safe Failure Fraction Hardware Fault Tolerance ... 

To determine the architectural requirements, the SFF number is calculated. This applies to each SIF subsystem, i.e., sensor, logic solver, and final element. To calculate the Safe Failure Fraction for the pressure switch we must first calculate and. ... 

Component Failure rates (1/hr) Architectural Constraint Type Safe Failure Fraction... 

The required SIL is shown with the relationship between hardware fault tolerance (HWFT) and safe failure fraction (SFF) for two types in Table 4. 

ANSI/ISA-84.00.01-2004-1 requires the use of minimum fault tolerance (i.e. device redundancy) to ensure that adequate protection is provided. The required fault tolerance is related to the device complexity. It is important to note that the device s safe failures tend to drive the process toward the safe state, whereas the safe failure fraction is based on the safe failures and the dangerous detected failures. Thus, there is an implicit assumption in the safe failure fraction that the requirements of ANSI/ISA-84.00.01-2004-1, Clause 11.3, are met. Refer to ISA-TR84.00.04-1, Clause K.4, for more information concerning the safe failure fraction. 

Clauses 11.3 and 16. Othen/vise, the safe failure fraction becomes excessively optimistic. The owner/operator should account for this when developing operating and alarm response procedures. Refer to ISA-TR. 00.04-1, Annex B, for more guidance on operator response as part of an SIF. 

The random hardware failure results are also used to calculate the Safe Failure Fraction (SFF) of the device and the PFDavg or Frequency of Failure for continuous-mode devices. The PFDavg provided by the manufacturer is based upon specific proof-test intervals and the mean time to repair specified by the manufacturer. For different assumptions, the user can use the fundamental failure rates provided and determine a PFDavg for their specific application. Details on SFF and PFDavg are included in Annex K. 

The owner/operator should determine the failure rates of the SIS devices, diagnostic coverage, test intervals, redundancy, safe failure fraction, etc., and evaluate each specific SIS to validate its performance. 

There have been changes to the way in which Route 1h is applied and together with changes to the definitions of safe and dangerous failures, some differences in the calculation of safe failure fraction may arise compared to the method specified inIEC 61508/Editionl. 

No claims shall be made in the safety manual, in respect of the hardware fault tolerance or the safe failure fraction or any other functional safety characteristic that is dependent on knowledge of safe and dangerous failure modes, unless the underlying assumptions, as to what constitute safe and dangerous failure modes, are clearly specified. ... 

Safe failure fraction-. Safe failure fraction (SFF) is a term from the lEC 61508 and lEC 61511. 

As per lEC, safe failure fraction is the ratio of the (total safe failure rate of a subsystem plus the dangerous detected failure rate of the subsystem) to the total failure rate of the subsystem. ... 

Reliability, probability of failure on demand and safe failure fraction (lEC). 

Part 2 of the standard also has annexure such as control of failures during operation (Annex A), avoidance of systematic failures during different phases of the life cycle (Annex B), and diagnostic coverage and safe failure fraction (Annex C). 

Failure rate safe failure fraction and diagnostic coverage with their relationship. For symhol legend refer to Table VII/1.3.1-1. Standard followed lEC 61508 2010. 

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