Up until the release of the IEC 61511 2nd Edition in 2016 the standard allowed users to reduce subsystem minimum hardware fault tolerance requirements by one for devices selected on the basis of prior use (which is synonymous with proven in use). Taking this reduction can be achieved easily in Vertigo from the Sensor or Final Element Details form. This article provides instructions on how to take this reduction and when it should be used.
From the Sensor or Final Element details form, selecting Prior Use or Proven in Use and the device selection basis will display a checkbox to right of the device selection basis drop-down menu labeled “reduce fault tolerance”. This is shown below for a Sensor.
Checking this checkbox will result in a reduction of the minimum fault tolerance requirements for any subsystem that this instrument is associated with if all other devices in that subsystem are also marked to take a prior use fault tolerance reduction. Note that in Vertigo a subsystem is not fully defined until all instruments of a given type (Sensor, Logic Solver or Final Element) have been defined for an IPF. Prior use minimum fault tolerance reductions can only be applied once to a subsystem of a IPF regardless of the number of instruments in that subsystem.
There are a couple of things to keep in mind when apply fault tolerance reduction for prior use.
- Checking the Reduce Fault Tolerance checkbox will only have an effect if the Fault Tolerance Calculation Mode setting for your study is set to IEC-61511 – 2003. This setting, located on the study settings form, allows you to select between the 1st and 2nd editions of IEC 61511 when performing fault tolerance calculation. IEC 61511 2016, the 2nd edition, does not allow for reducing fault tolerance requirements through prior use and therefore Vertigo will ignore this checkbox when performing calculations in this mode.
- If all devices in a subsystem are configured to take a prior use fault tolerance reduction credit than Vertigo will not attempt to calculate the maximum achieved SIL based on the architectural constraint requirements of IEC 61508. By default, Vertigo will calculation the maximum achievable SIL for a subsystem by applying the minimum fault tolerance requirements of the applicable version of IEC 61511 (as defined by the Fault Tolerance Calculation Mode Setting) and the architectural constraint requirements of IEC 61508 2010. Whichever method results in a higher maximum achievable SIL will be applied. Opting to apply fault tolerance reduction through prior use will override this behavior and ignore the architectural constraint requirements of IEC 61508.
- Per IEC 61511 2003, There are additional requirements beyond prior use alone that must be satisfied to claim a fault tolerance reduction for devices selected based on prior use. These requirements are:
- the device allows adjustment of process-related parameters only, e.g., measuring range, upscale or downscale failure direction, etc.;
- the adjustment of the process-related parameters of the device is protected, e.g., jumper, password;
- the function has a SIL requirement less than 4.
In addition to complying with the requirements of device selection via prior use, you should also confirm that the above requirements are also being achieved before taking a fault tolerance reduction credit.