3-out-of-2 principle

With a two-out-of-three mechanism, I want to ensure both the security of the system and its continuous operation – preventing it from stopping frequently. To that end, three detection elements are used; three signals are generated, and an interlock action is triggered only when two of these signals reach the specified interlock level

For redundant sensors, a two-out-of-one logic structure should be employed when system safety is the primary concern; a two-out-of-two logic structure should be used when the system’s usability is of primary importance; and when both system safety and usability need to be ensured, a three-out-of-two logic structure is generally appropriate!

There’s something wrong with the way the original poster phrased this. Are you referring to 2003? This depends on industry standards, national standards, or IEC standards. Of course, using this method for ordinary control is good from a safety perspective, but it’s not practical in reality

In aspects such as compressor speed and oil pressure, the two-out-of-three principle is generally adopted. This principle is used in all critical components, and it is applied in equipment interlocks to ensure the safe and stable operation of important equipment and to avoid unnecessary shutdowns.

2OO3 is generally used in safety systems, in environments with specific SIL level requirements. It is employed in special applications such as ESD, PSD, EMS, HIPPS, etc., where SIL3-level solutions are required; the design of such solutions is often handled by design firms and EPC companies. Not all critical components need to be treated in this way – it depends on the design standards. Of course, there’s no harm in using it, but who will cover the costs?

The boiler drum is equipped with a 2-out-of-3 or 1-out-of-3 safety system to ensure the safety of critical equipment

Under what circumstances is the 3-out-of-2 rule generally used? It is generally used in SIS systems or important units. 1. Application in SIS systems; SIS systems are subject to SIL safety integrity level requirements based on the IEC61508 standard, and are classified into SIL1, SIL2, and SIL3. In petroleum and chemical plants, in accordance with the safety requirements of such facilities, professionals in process engineering, automation control, and safety conduct evaluations of the plants. Corresponding mathematical calculations are also performed to determine the SIL level of safety for that plant. The highest is SIL3. Only in aerospace is a safety level of SIL4 required; SIL4 is not seen in the petrochemical industry. Once the SIL is determined, it is necessary to consider how the on-site instrumentation, equipment, and system software and hardware should be configured to meet the required safety level. If the safety level is low or no requirements exist, there is no need to design it as 1oo1, 1oo1D, 1oo2, 1oo2D, 2oo3, 2oo4D. If high safety levels are required, and 1oo1, 1oo1D, 1oo2, and 1oo2D are not sufficient to meet the SIL requirements, then hardware redundancy in the field instrumentation and systems is used to ensure compliance with those SIL requirements. Redundancy of the individual field instrument equipment or redundancy of the individual system hardware is meaningless; only when both are redundant can the SIL requirements be met. To determine which of 1oo2, 1oo2D, 2oo3, and 2oo4D to use, rigorous calculations using formulas are required first. Otherwise, it will result in considerable unnecessary investment. 2. Critical units: For example, in large compressor units, the temperatures of the bearing shells, shaft vibration, and shaft displacement are monitored using a three-out-of-two design with on-site instruments, to prevent the occurrence of false readings that could affect the operation of the unit. This is designed for the safety of the crew.