在《SIS系统03：SIL等级代表的含义是什么？》这篇内容中，我们通过低要求模式和高要求或连续操作模式下安全完整性等级所对应的故障概率。那么，大家有疑问了：什么是低要求？什么又是高要求或连续操作呢？有没有中要求呢？
在操作模式里面，分为两种：低要求模式；高要求或连续操作模式。没有中要求。 low demand mode: where the safety function is only performed on demand, in order to transfer the EUC（Equipment Under Control） into a specified safe state, and where the frequency of demands is no greater than one per year; or
NOTE The E/E/PE safety-related system that performs the safety function normally has no influence on the EUC or EUC control system until a demand arises. However, if the E/E/PE safety-related system fails in such a way that it is unable to carry out the safety function then it may cause the EUC to move to a safe state. high demand mode: where the safety function is only performed on demand, in order to transfer the EUC into a specified safe state, and where the frequency of demands is greater than one per year; or continuous mode: where the safety function retains the EUC in a safe state as part of normal operation。
这段定义来源于IEC61508-4-2010.
低要求模式：按需执行安全功能时，能将EUC（受控设备）转换到指定的安全状态，并且每年的要求频率不超过一次。
高要求模式：按需执行安全功能时，能将EUC（受控设备）转换到指定的安全状态，并且每年要求频率大于一次。
连续模式：安全功能将EUC保持在安全状态，作为正常操作的一部分。
在IEC61511-2016中： low demand mode: mode of operation where the SIF is only performed on demand, in order to transfer the process into a specified safe state, and where the frequency of demands is no greater than one per year. high demand mode: mode of operation where the SIF, is only performed on demand, in order to transfer the process into a specified safe state, and where the frequency of demands is greater than one per year. continuous mode: mode of operation where the SIF retains the process in a safe state as part of normal operation.
低要求模式：仅在需求时执行SIF的操作模式，以便将过程转换到指定的安全状态，并且每年的要求频率不超过1次。
高要求模式：SIF只在需求时执行，以便将过程转换到指定的安全状态，以及要求频率大于每年一次的操作模式。
连续模式：SIF将操作模式作为正常操作的一部分保持在安全状态。
这里的分界线是要求频率，小于1为低要求，大于1为高要求。
那么，连续模式呢？
不妨仔细阅读一下SIL等级代表的含义是什么？你会找到答案。
