Ok, so you can add another CPU for more redundancy and security. Safety PLC's do that. But the one which takes the cake is definitely Pilz. The ended up using three of the same CPU in one controller. Does that at that point really increase failsafe performance or is it just overengineering for getting a certification?

If the norm is double CPU, triple CPU failing to double CPU during operation means switching from overredundant to redundant(norm) mode. You will have enough time to switch/fix processing unit.

My issue is, how does a CPU fail? Electrical overcharge?

@aviophile in industrial environnements often because of the elements. Heat, humidity, power surges, dust and organic material creeping in, mechanical vibrations of machinery that in the long run could loosen soldering (if made on the cheap or made in china).

When I was young, like 20 years ago, I have worked at various big companies on the port. One was for loading and storing food products like corn powder. That stuff is everywhere and we had to box in the PLC controlling the machinery in an airtight box with suspension mechanisms and proper grounding or else it would break down a production line generating hundreds of thousands of euro's a day.

Three CPUs in one device sounds unusual - the normal way is one CPU, but three redundant devices. That way, you have more protection against common mode failures.

Or is it actually three different computers in one housing, and each computer also with its own power supply stage?

@Fast-Nop yeah three cpu in one box means eventually others willsuffer from the same fate too.

@NeatNerdPrime sounds like a customer we had back when the former company still imported computers.

We got one for repair and when opening it up it was totally caked with some packed powder.

Apperently it was powdered limestone since the computer sat in a digger ;)

Two CPUs mean you know when a calculation is wrong, but you don't know which one is correct. With three, you can trust the result if two of the three agree.