When we estimate the reliability of a safety instrumented function, we separate between “low demand” functions and “high demand” or “continuous demand” functions. These are all safety critical functions but their nature is different in terms of how frequently they must act on the system of study.
Consider for example the braking system on a train – the brakes need to work every time they are used – for every curve, for every station. The train driver will activate the brakes several times every hour. Obviously, this is a “high demand” system. As an example of the opposite, think of systems where we are monitoring some process and only acting if the system detects a dangerous state. A common example of this is an over-temperature trip on a heating system; if the temperature becomes too high in the system, the system shuts off power to the heater through a circuit breaker (assuming it is an electrical heater). Nobody will design a system such that its intention is to overheat, so this function would only need to activate when a specific scenario hits. Whether this is a “low demand” or “high demand” function depends on how often the function must work – and this again depends both on the “intrinsic frequency” of overheating, and other protection measures that may exist such as independent alarms or special operator training and procedures.
If you think of the stove guard installed in your kitchen that monitors overheating in the range area, what would the demand rate be? If we assume you are a 25-year old person and normally functioning as long as you are sober, you would not forget to turn off the plate on the oven more than once per year. In addition to this, you may get really drunk 10 times per year, and you cook something some of those times, with a higher probability of forgetting – say this also happens once per year – then you have an initial event rate of 2 times per year. Is this the demand rate on the stove guard function? It depends. Do you have any other measures that help you reduce the fire risk?
Typically you would have a smoke detector with alarm, and possibly the stove guard would also give you a pre-alarm. The smoke detector would be completely independent from the stove guard – and if it goes off you would react to it. This takes down the demand on the stove guard if you look at it solely as a way to stop fires from occurring (smoke comes before fire). We now assume that the smoke alarm works 1 out of 10 times and that you or a (sober) friend would always react correctly in this situation – it is normally easy to identify the smoke coming from the kitchen. Then we have reduced the demand on the stove guard to 2 x 0.1 = 0.2 times per year. This is safe to put in the bracket “low demand”. We did not count the pre-alarm on the stove guard itself on purpose, because it can have common cause failures with the core functionality of the stove guard – if one fails, the other one fails too.
The next natural question to ask in this connection is: “how reliable must the stove guard be”? We may conservatively assume that every 10th time there is a real demand on the guard and it does fail there will be a fire that can kill you and destroy the house. This risk is quite severe, and say you would only allow your house on average to burn down every 10.000 years, statistically speaking. This is your “acceptance criterion”. That is, you accept 0.0001 fires per year due to this potential source. We know the demand is 0.2 times per year – what is the allowable probability of failure on demand for the stove guard? This would be 0.0001 / 0.02 = 0.005. This means that we should require the system to have SIL 2 performance with a PFD of minimum 0.005 with this acceptance criterion, if we have a system developed in accordance with IEC 61508.
As a side note – the Norwegian research institute SINTEF has tested some stove guards. They tested 7 different types, and concluded that only 3 of them worked well. The reliability of the devices also depend on installation (location of sensors). This means that close to SIL 3 performance seems unreasonable to expect for the solutions on the market today. The SINTEF report is found at the Norwegian Directorate for Civil Protection’s website.