Modes of operation are used in IEC 61508 to describe two types of safety function carried out by E/E/PE safety-related systems. The modes are relevant when relating the target failure measure of a safety function to be implemented by an E/E/PE safety-related system to the safety integrity level. IEC 61508 relates the safety integrity level of a safety function to:

• the average probability of failure to perform its design function on demand (in the case of low demand mode – see table 2 of IEC 61508-1), or
• the probability of a dangerous failure per hour (in the case of high demand or continuous mode – see table 3 of IEC 61508-1). The probability of a dangerous failure per hour is sometimes referred to as the dangerous failure rate (i.e. dangerous failures per hour).

Low demand mode, as defined in 3.5.12 of IEC 61508-4, is where the frequency of demands for operation made on a safety-related system is no greater than one per year and no greater than twice the proof test frequency.

 

High demand or continuous mode, as defined in 3.5.12 of IEC 61508-4, is where the frequency of demands for operation made on a safety-related system is greater than one per year or greater than twice the proof test frequency. In the context of this definition, continuous is regarded as very high demand.

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