Impact of safety relays on SIL calculations
safety relay, SIL calculations Impact of safety relays on SIL calculations 2022-02-24 Impact of safety relays on SIL calculations 573

First of all, for system level, SIL caculation is the result of a whole SIF(safety instrumented function), not a single component (such as relay, barrier, sensor, etc) . Relay accounts for a very little proportion, it won't effect much on the SIL caculation. 

Secendly, for components level, HFT and SFF decides component's SIL. If SFF is enough high, low HFT still can reach the high SIL grade.

Thirdly, for CHENZHU safety relay, it is two parts:power part and contact part. Powr part is 1oo1,HFT=0 and SFF 90%<99%, located in SIL3. Contact part is 1oo3, HFT=2 adn SFF<60%, located in SIL3. So, the whole relay is SIL3.

As the conclusion, safety is a component of a SIF. If it is well certificated, SIL is the only indicator for its safety performance. And we can just provide the lamda parameters to customer to caculate the whole SIF's SIL. Component's HFT can be ignored.

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