SIL stands for Safety Integrity Level, translated as Safety Integrity Level. SIL was first proposed in the IEC61508 functional safety standard issued in 1998, and it is a classification of functional safety levels. IEC61508 divides SIL into four levels, namely SIL1, SIL2, SIL3, and SIL4. The level of SIL that safety related systems should reach is determined by risk analysis, which involves analyzing the severity of risk consequences, exposure time and frequency, probability of unavoidable risks, and probability of unexpected events occurring. The higher the level, the lower the probability of dangerous failure.