Fault safety of the SIS system
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Seeking help: SIS systems generally require fail-safety functionality. I would like to know in which standards this requirement is specified, and particularly where the statement following appears: \"The Safety Systems shall be normally energized. A ‘1’ logic signal is considered to be a normal state, and a ‘0’ logic signal is an abnormal (trip or failure) state. Output signals shall go to a ‘0’ (de-energized) state on power failure or on component failure.\"
Standard of the Petrochemical Industry of the People’s Republic of China SH/T 3018-2003 (Code for Design of Safety Instrumented Systems in Petrochemical Industries) 3.5 The safety instrumented system shall be designed to be fail-safe. The subsequent requirements are specified in our own design standards.
There are currently two methods for interlock design: the first is where 0 represents a normal signal and 1 represents a shutdown signal; the second is where 0 represents a fault signal and 1 represents a normal signal. The second method is clearly safer, as under normal conditions the relay is powered on, and if it loses power due to damage or other reasons that affect its lifespan, the system will shut down. The first type obviously cannot be detected when the relay fails, and it may be impossible to shut down the machine when shutdown is necessary. Foreigners prefer the second option because they place more emphasis on safety. In China, the first method is probably used more often, as people there don’t like to jump out of the car. The cause of the accident must be investigated in the case of a vehicle jump.
I know all of this. The reason is that my understanding of this statement differs from that of the foreigners; they believe that all inputs and outputs should be fail-safe, including analog alarm signals. They require that the alarm signal be “1” in normal conditions and “0” in abnormal conditions. I would like to see the original specifications, especially those from IEC or ISA. I searched on my own and couldn’t find that phrase in the specifications. Please help everyone.
Although both 0 and 1 can be used for parking. However, stopping at 0 is an international convention. Especially for action-priority ESD systems. For the vast majority of Di and Do modules that support fail-safe operation, their safety status is also 0, rather than 1!
That's how it is. The difference we understand now lies in the analog input.