What is 1oo1, 1oo2, 2oo2, and 2oo3 voting?

It is a typical Safety Instrumented Function (SIF) Solution.

What is the best method to determine the safety architecture?
It is the fault tolerance of various components that determines the architecture of an safety instrumented function (SIF). It is possible to achieve a higher level of safety by utilizing the system redundancy.

The number of devices will depend on the reliability of each component defined in its FMEDA (Failure Modes, Effects and Diagnostic Analysis).

There are three most common architectures

1. Simplex or 1oo1 ( 1 out of 1) voting

2. Duplex or 1oo2 or 2oo2 voting

3. Triplex or 2oo3 voting.

1. Simplex or 1 out of 1 (1oo1) voting

A 1001 voting system is typically used for low-level security applications and comprises a single channel. In the event of a loss of safety function or shutdown of the process, the issue is immediately resolved.

2. Duplex or 1oo2 or 2oo2 voting

As the way to improve the security integrity performance of 1001 based security systems, the 1002 voting principle was developed. A failure on one channel does not affect the other’s to provide safety function, and can still be performed by other channel.

It is unfortunate that this concept does not improve the rate of false trips. more over worse. Unfortunately, this concept does not improve the rate of false trips. Furthermore, the chances of a false trip nearly double.


A single, non-redundant security system has the main disadvantage of causing a trip instantly. As both channels must fail for the system to shutdown, channel duplication in 2 out of 2 reduces false trips.

However, the system has the disadvantage that the probability of failure on demand is twice likely as a single channel.

3. Triplex or 2oo3 voting

2oo3: there are three channels for this voting, two channels must be ready for operational to meet the security needs. in order to effectively implement 2003 voting principle, microprocessors should be physically separated.

In the meantime, it requires three separate modules to handle these operational functions.
Having a higher level of diagnostics is one of the advantages of most recent systems. A disadvantage of 2oo3 voting-based security systems that their probability of failure on demand is three times higher than that of 1oo2-based systems.

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