1-of-2 and 2-of-3 terminology

Hello guys,
I need clarification of few techical terms.
I often read about 1-of-2 and 2-of-3 systems but it is not always clear what that means.
In my experience "1-of-2 fault tolerant" means that if one component fails, system will continue to work without disturbances. On the other hand, "1-of-2 fail safe" means that in case if one component fails, other will make sure system reaches "safe state" which in practice usually means shutdown.

Fault tolerant 2-of-3 is same like 1-of-2 case, but in case "2-of-3 fail safe", if one component fails, system will continue to work, only after two component fail, then system will reach safe state.

Can you confirm this?

Thanks

Hello Pandiani,

In our application it is called 2oo3 (two out of three) voting, it means if two sensors/detectors are detecting something, it is confirmed that the condition is true.

We also apply in our application, 2oo4, 2oo5 etc, and it is commonly used in the safety instrumented system (SIS).

For example, if you have 4 gas detectors in one zone, and you apply 2-o-o-4 voting, then if one of your detectors is alarming (i.e. detecting gas) then the voting system will not process the signal, because the 2-o-o-4 voting is not reached. And if another gas detector is alarming (i.e. detecting gas) then the voting system applied in your logic will process the signal for the executive action (i.e. activate general alarm, shut down your equipment and etc.)

It is also applied to the process shutdown system (pressure, temperature, flow, level etc) with your transmitter as the object of the voting system.

Hope this is what you means.

Regards,
Ade

In your 1oo2 (one out of two) the key thing is the usage.

In 1oo2 fault tolerant, you would ignore a "bad" instrument, (broken wire, out of range etc...) you basically have an installed spare, so that the plant will continue to run if you have a bad signal.

In 1oo2 safety systems, a bad signal will cause a shut down, you have two devices to increase the probability that at least one of them will cause the system to shut down, WHEN IT NEEDS TO BE SHUT DOWN. So, when one signal goes "bad" you have to shut down because you do not know if your single device is reliable.

So you agree with my understanding of meaning "1-of-2 fault tolerant" and "1-of-2 fail safe".

I needed to be sure about this terminology. I often hear people talking about 1-of-2 but that is not enough. On must mention 1-of-2 falut tolerant or fail safe to avoid confusion.

In my world, 1oo2 is ALWAYS used for fail safe.
We never use 1oo2 fault tolerant, we just use "fault tolerant".

What we now have is:


1oo2 = safety system
Fault Tolerant= redundant