# Two out of three (2oo3) Logic

Two out of three logic is called the triple module redundancy or triple mode redundancy, in which three independent inputs are connected to a system and the output, comes out based on the maximum number of voting.

## Why two out of three logic use?

Every industry uses more than one source of sensing and source of action instrument to make its system reliable and efficient. Its main objective is that if a sensor fails or malfunctions, then the system will not be false trip as the other two sensors or the final control element are working. So that false tripping does not occur and faulty instruments can be repaired or replaced without interrupting the process.

We understand 2oo3 logic with an example:-

Furnaces are used in every industry, which are used for different purposes. Different fuels are burnt in different furnaces and after the burning; the waste gases released from it are discharged through a fan. The pressure of the furnace is increased by the flue gases emanating from the fuel burn, which is let out by a fan. If the fan stops working, the pressure of the furnace will increase, which can lead to a very dangerous accident.

Therefore, to avoid this accident and install transmitter for furnace pressure monitor and pressure switch for safe shutdown.

Pressure switches are installed in the furnace to avoid the furnace from accident. Fuel valves are operated by taking the NC contact of the pressure switch. If the furnace pressure increases, the fuel valve will be closed which will not increase the draft pressure of the furnace and the furnace safe shutdown will occur.

If the pressure of the furnace does not increase but the pressure switch gives contact change, then the furnace will be tripped but this tripping will be a false tripping. This will reduce the reliability of the furnace.

Redundant systems are used to avoid such false tripping. We will learn three types of redundant system.

1. One out of one logic
2. Two out of two logic
3. Two out of three logic

## One out of one(1oo1) logic

In one out of one logic, logic is made by only one instrument.

Which can understand with example?

For example, a pressure switch has been placed in the furnace which changes the contact when the furnace is pressurized, thereby closing the fuel valve and causing the furnace to trip. But if the pressure switch is faulty in any condition, then either the pressure switch will change the contact causing the furnace to false trip or the contact will not change and the furnace will not trip even if he pressure of the furnace increases, causing a dangerous accident. As can be understood from the animation above.

## Two Out Of Two(2oo2) Logic

Two out of two logic uses two sources of input or two sources of output. In two out of two logic, when both inputs are the same, the output is generated.

Two out of two logic switches are installed in the furnace. Its NC contact is used in logic. We will understand logic through relay logic and Boolean logic.

### 2oo2 RELAY logic:-

The logic created by using relay is called relay logic. To make two out of two logic, the wire is connected according to the logic diagram given below.

#### Wiring:-

In logic, NC contact of pressure switch is taken. When the pressure of the furnace increases, the NC contact of the pressure switch will change and open. The NC contact of the pressure switch has been used to make fail safe.

In the logic above, the sequence relay R1 and R2 are wired by the NC contacts of the pressure switches PS1 and PS2. The relay R1 and R2’s NO contact are wired in parallel to relay R3.

#### Working:-

Normal operation: – In normal operation, the PS1 and PS2 remain healthy so that the Relay R1 and R2 are energized by its NC contact. The R3 also remains energized and the fuel valve is also open and the furnace normal operates.

Condition1:- If the furnace pressure increase in actual.

If the furnace pressure increases then the PS1 and PS2 will actuate and the contacts will change from NC to OPEN. Relay R1 and R2 will be de-energized and its contacts that were energized to relay R3 will be de-energized. The contact of relay R3 will shut off the fuel valve causing the furnace trip.

Condition2:- If one of the pressure switch becomes faulty.

If one of the pressure switch is faulty or the wire breaks, one of the relay R1 or R2 will be de-energized but the relay R3 is energized using parallel to R1 or R2. That’s why relay R3 remains energized even when a pressure switch fails and the furnace avoids tripping.

### 2oo2 Boolean logic:-

Connect the PS1 and PS2 to the input of the OR gate. When the pressure of the furnace increases, both the inputs will zero and the furnace will trip. This can be understood from the truth table given below.

Demerit of 2oo2 Logic:-

If either of the two pressure switches changes the set point due to any reason and the pressure of the furnace is increased due to any reason, but one pressure switch does not change the contact due to fault, then the furnace trip will not occur and the fatal accident It is possible.

2oo3 is used to protect the plant from such an accident.

## Two out of three(2oo3) Logic

The logic created by using relay is called relay logic. To make Two out of three logic, the wire is connected according to the logic diagram given below.

### Wiring:-

In drawing, relay R1, R2 and R3 are wired by NC contact of PS1, PS2 and PS3. And using logic of R1, R2 and R3, logic is formed. Relay R4 is wired by connecting all of them in parallel to R1 and R2, R2 and R3, and R3 and R1 in series, which is the final relay.

### Working:-

Normal working:­-

Out of total three inputs in 2oo3 logic, two inputs will be the same, only then will the desired output be obtained.

For example: -If the furnace has three pressure switches installed, the furnace will not trip until two of the three pressure switches are activated. Its working is understood with the condition given below.

Condition 1:- If the furnace pressure increase in actual.

If the pressure of the furnace will increase in actual and the calibration of the three pressure switches is fine, then the three pressure switches will change contact from NC (Normally closed) to OPEN state. The relay R1, R2 and R3 will be de-energized and R4 will also be de-energized (according to the drawing) and the fuel valve will be closed and the furnace will be tripped.

Condition 2:- If one pressure switches becomes faulty.

If the set point of one of the three pressure switches is changed, then the pressure switch will not activate even if the pressure of the furnace increases, but the activation of two pressure switches will close the fuel valve and the furnace will shut down safely. It can be understood with drawing given below.

Suppose the calibration of PS3 is out.

It is shown in the drawing above that when PS1 and PS2 are activated then they will change contact from NC (Normally Closed) to OPEN state so that relay R1 and R2 will be de-energised and R3 will be energized. According to logic relay R4 will be de-energized and the fuel valve will be close and the furnace will trip.

## 2oo3 Boolean Logic:-

Let us first make logic to understand 2oo3 in Boolean logic. To become logic, use three AND Gate and One OR Gate.

Normal working: ­–  In normal operation, the three pressure switches are NC (1), so the logic output is 1. This is explained in the truth table.

Condition1:- If the furnace pressure increase in actual.

If the pressure of the furnace will increase in actual and the calibration of the three pressure switches is fine, then the three pressure switches will change contact from NC (1) (Normally closed) to OPEN (0)state. The output of logic will be 0 and furnace will be trip.

Condition2:- If one pressure switches becomes faulty.

If the set point of one of the three pressure switches is changed, then the pressure switch will not activate even if the pressure of the furnace increases, but the activation of two pressure switches will close the fuel valve and the furnace will shut down safely. It can be understood with drawing given below.

Suppose the calibration of PS3 is out. Now truth table