Functional Safety FAQs

What is a Safety Integrity Level (SIL)?

“A safety integrity level is one of four levels, each corresponding to a range of target likelihood of failures of a safety function. Note that a safety integrity level is a property of a safety function rather than of a system or any part of a system.”

What is the difference between low demand and high demand or continuous mode of operation?

“Low demand mode, as defined in 3.5.16 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.
High demand or continuous mode, as defined in 3.5.16 of IEC 61508-4, is where the frequency of demands for operation made on a safety-related system is greater than one per year. Continuous is regarded as very high demand.”

Source: IEC International Electrotechnical Commission
http://www.iec.ch/functionalsafety/faq-ed2/ampcontrolgroup.com1300