Which is better, interlock 2-of-3 or 3-of-3?

Of course, two out of three is better than a tie

Of course, it’s better to go with two out of three, as the information is more accurate

In our company, interlocks are designed to be reliable with a three-out-of-two configuration

This post was last edited by ZhiGE on 2019-1-15 at 20:15. 1. According to 6.4.3 of GB 50770-2013, when a system needs to balance high security and high availability, a two-out-of-three logic structure is advisable; such circuits are generally of SIL3 level, and two-out-of-three is a logical judgment mechanism； 2. Three-out-of-three is a method for selecting an analog value, and it is typically used in important control loops, such as the speed control loop of turbines ； If one fails, use the average or the higher value ； Two faults, single control. Generally, interlocks are not triggered using the three-out-of-three principle. 3. Therefore, there is no superiority or inferiority.

Phase 3, stage 2 involves two out of three judgments reaching the interlock value, which is used for interlocking. Three-way selection means choosing the middle value among three, and it is used for display or calculation; it is generally not used for interlocking.

Two-out-of-three refers to a voting system used in digital control, where three redundant signals of the same signal are used for voting, and the condition is considered met if two of them satisfy it. It is commonly used to protect control loops. The three-way averaging method involves using three redundant inputs for analog signals, with the average value of these being taken as the value to be used in practice; this approach is often applied in main control loops. Options include taking the average value, the highest value, the lowest value, the mean value, or any one of the three values. The two-way averaging method is employed to ensure accuracy and reliability in judgments, thereby preventing incorrect operations. The protection logic (digital control) generally uses a two-out-of-three voting system; a three-out-of-three system is used less frequently. The choice depends on the operating conditions. Usually, when there are significant deviations among three measurement points located in the same area, it is necessary to identify the problem and address it safely and quickly.

Three in a row? Is the acquired value a continuous signal? In the 2oo3 loop involved in interlock, three consecutive signals are fed into the SIS; when the interlock threshold is reached, a digital signal of 0 is generated to be used in the 2oo3 interlock evaluation. This is the evaluation based on these three digital signals. Out of three attempts, what is the outcome? These are usually control and regulation loops, and have nothing to do with interlocks.

Here we use two out of three or two out of two

For interlocking, “three-out-of-two” or “two-out-of-two” is commonly used; this is a voting procedure, and it usually involves digital signals； When used for logical operations, redundant inputs are often handled using the \"majority of three\" (MOO3) method. Note that here, M does not merely refer to the intermediate value; it can also represent the highest or lowest value among the three, or the average of the remaining values after discarding those with large deviations. In some cases, two faulty values can be excluded to leave just one remaining value. Even if all three sensors are faulty, the value from the last normal reading can be used for logical operations. Therefore, there is a fundamental difference between majority voting and majority with tie-breaking.

Since 2-out-of-3 interlock is more suitable for practical applications, 3-out-of-3 voting is better suited for critical controls; for example, some systems use the final decision from 3-out-of-3 voting within the SIS to trigger actions, while the average value obtained from this voting is fed to the DCS for control purposes