This article explains how to apply the Prior Use (also referred to as Proven in Use) hardware fault tolerance reduction in Kenexis® SIS, and when it is appropriate to do so.

Background

Prior to the release of IEC 61511 2nd Edition in 2016, the standard permitted users to reduce the minimum hardware fault tolerance requirement for a subsystem by one, provided the devices were selected on the basis of prior use. This reduction acknowledges that a device’s real-world operational track record can serve as evidence of its reliability, potentially relaxing the redundancy requirements that would otherwise apply.

Applying the Reduction in Kenexis® SIS

Taking this reduction is straightforward in Kenexis® SIS. The option is available directly from the Sensor or Final Element Details form, and the steps for enabling it are demonstrated in this article.

Read on for full instructions on how to apply the prior use reduction and guidance on the circumstances under which it should — and should not — 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 Kenexis® SIS (formerly 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 Kenexis® SIS (formerly 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 Kenexis® SIS (formerly Vertigo™) will not attempt to calculate the maximum achieved SIL based on the architectural constraint requirements of IEC 61508. By default, Kenexis® SIS (formerly 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:
    1. the device allows adjustment of process-related parameters only, e.g., measuring range, upscale or downscale failure direction, etc.;
    2. the adjustment of the process-related parameters of the device is protected, e.g., jumper, password;
    3. 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.