Abstract
PDS is a method used to quantify the safety unavailability and production loss for Safety Instrumented Systems (SIS). The method has recently been updated, resulting in a new PDS method handbook and an accompanying data handbook. In this paper a brief introduction to the updated PDS method is given. Focus has been put on the treatment of systematic failures. Whereas IEC 61508 explicitly states that only random hardware failure shall be quantified, in PDS we attempt to quantify the systematic failures as well, the main argument for this being that we want to predict the actual performance of the system/component in the field as accurate as possible. Systematic failures are modelled as comprising two main categories; systematic failures detectable during testing (e.g. a detector left in bypass mode at the last test) and systematic failures normally not detected during testing, but occurring only upon a true demand (e.g. a software error introduced during update of the program logic).