GT Engineering is member of the Technical Committees of the following Standards: IEC 61508 series, IEC 61511-1, IEC 62061 and ISO 13849-1.
We support users in the reliability assessment of Safety and Production Systems. From a Hazard and Operability (HAZOP) study, we move to a Layer of Protection Analysis (LOPA) to a SIL allocation.
We can than help designing each Safety Instrumented System and calculate the Sil level reached.
We organise training on all the aspects of Functional Safety and we are lecturers on Functional Safety at Brescia University and at the Milan Polytechnic (School of industrial and Information Engineering).
Random Failures, Systematic Failures and the Systematic Capability
In Functional safety, Failures are classified as either random (in hardware) or systematic (in hardware or software).
The Failure Rate λ
The failure rate is the basis of the Functional Safety theory.
Reliability of a Safety Function in Low Demand mode.
In low demand mode, the Reliability of a safety function is defined with the parameter PFDavg.
IEC 61508 divides the requirements into four safety integrity levels, SIL1, SIL2, SIL3, and SIL4, with SIL4 being the most reliable and SIL 1 being the le...
Reliability Models used to estimate the PFH
In high demand mode, the two standards, ISO and IEC, use different models to come to the estimation of the unreliability function.
IEC 62061 uses the Reliability Block Diagram method and it assumes the systems (Architectures) as non-repairable. ISO ...
Route 2H
Route 1H uses both the Failure rates and the SFF Parameter.
The Simplified Approach
The numerical quantification of the probability of failure of a subsystem can never be attained exactly, but only by approximation with the aid of statistical methods or other estimations are possible.
Any validated and recognized method can be us...
Category B
Subsystems of Category B must use Basic Safety Principles, where applicable and detailed in ISO 13849-2, and shall be designed according to relevant standards. That should guarantee that they can withstand expected operating stresses and influences o...
Type A and Type B Components
Components used in a Safety Function can be classified as Type A or Type B.
The Maclaurin series and the Failure in Time (FIT)
Mathematically, it can be shown that certain functions can be approximated by a series of other functions. In particular, ex can be developed as a so called Maclaurin series:
Weibull Distribution
It is now clear that, in Functional Safety, the failure rate of any component has to be constant: the issue are components subject to wear, like contactors and solenoid valves, since their failure rates are usually not constant. Therefore, the expone...
Category 1
In Category 1, the same requirements as those for Category B applies; moreover, well-tried safety principles should be followed, if applicable. Additionally, Category 1 is the only one requiring the use of well-tried components. The Safety-related Bl...
Limits od the SFF parameter
There is another issue linked to the use of the SFF parameter. Considering the definition of SFF, the safety of a component can be enhanced by making the dangerous failure rate lower, and the safe failure rate higher, assuming the total failure rat...
