Assessing safety-related system architectures against the integrity targets

The choice of failure rate data, the assessment of the common cause proportion, and the assumptions concerning diagnostic coverage and repair times all contribute to the final calculation of the probability of failure on demand. The main benefit of reliability assessment is in showing the major contributions to failure and in comparing alternative design and maintenance proposals. Having decided that a design proposal meets the appropriate safety integrity level (SIL) target, the principles of ALARP (as low as reasonably practicable) still apply. Improvements must still be considered, and the increment of integrity compared with its cost to decide if that cost is grossly disproportional to the benefit.