Safety integrity level

The concept of safety integrity levels (SILs) was introduced during the development of BSEN 61508 (BSI 2002) as a measure of the quality or dependability of a system which has a safety function – a measure of the confidence with which the system can be expected to perform that function. It is also used in BS IEC 61511(BSI 2003), the process sector specific application of BS EN 61508 ways.
In particular they recognise that many such functions are only called upon at a low
frequency / have a low demand rate. Consider a car; examples of such functions are:
· Anti-lock braking (ABS). (It depends on the driver, of course!).
· Secondary restraint system (SRS) (air bags).
On the other hand there are functions which are in frequent or continuous use; examples of
such functions are:
· Normal braking
· Steering
The fundamental question is how frequently will failures of either type of function lead to
accidents. The answer is different for the 2 types:
· For functions with a low demand rate, the accident rate is a combination of 2 parameters
– i) the frequency of demands, and ii) the probability the function fails on demand (PFD).
In this case, therefore, the appropriate measure of performance of the function is PFD, or its reciprocal, Risk Reduction Factor (RRF).
· For functions which have a high demand rate or operate continuously, the accident rate is the failure rate, λ, which is the appropriate measure of performance. An alternative measure is mean time to failure (MTTF) of the function. Provided failures are
exponentially distributed, MTTF is the reciprocal of λ.
These performance measures are, of course, related. At its simplest, provided the function can be proof-tested at a frequency which is greater than the demand rate, the relationship can be expressed as:
PFD = λT/2 or = T/(2 x MTTF), or RRF = 2/(λT) or = (2 x MTTF)/T
where T is the proof-test interval. (Note that to significantly reduce the accident rate below the failure rate of the function, the test frequency, 1/T, should be at least 2 and preferably ≥ 5 times the demand frequency.) They are, however, different quantities. PFD is a probability– dimensionless; λ is a rate – dimension t-1.