Thank you very much for your help! I really appreciate that! To be honest with you this forum is only one place where I can obtain answers for my questions.
If you don’t mind I would like to ask one more thing which refers to detection methods and protection devices.
As I’ve previously written I’m performing FMECA analysis of HVAC system. The mission of the system is to provide constant rate (-/+) of temperature to the room (hypothetically). I prepped severity classification:
1. Failure does not affect the temperature
2. Failure affect the temperature (+/-) but mission can be accomplished
3. No cooling (wrong temperature) – with warring
4. No cooling (wrong temperature) – without warring

If detection method exist then severity level is 3 but if it doesn’t – 4.

And now… we have PLC (parent, not covered by the analysis) controller and if temperature falls to much or go to high the PLC temperature sensors (also, not covered) send information to the PLC that there is a failure (but we don’t know exactly where). Is it a detection method? If yes, then each part in the system have detection method (because each failure can results in wrong temperature) but… when sensor sent information to PLC that there is wrong temperature the mission is actually failed (or its going to fail in few hours).

The question is – how to treat detection method in my case?
a) Detection as an indication of particular failure for example: evaporator – leakage is visible or,
b) Detection as an effect for example: evaporator – temperature in the room is low (by the sensor).

And another question…
We have two temperature controllers – first maintain the temperature and the second one protects against low temperature (if something go wrong he takes control and turns the heater) – this are not a redundant devices. Each PLC has Duty Cycle 100%.
This two have failure mode “malfunction” (among the others) .

Cm=λp x α x β x t
PLC1: [FR] x [FM “malfunction”] x [beta = 1 ] [DC 100%] – effect: “wrong temperature”
PLC2: [FR] x [FM “malfunction”] x [beta = 1 ] [DC 100%] – effect: “wrong temperature”

When PLC 1 is “malfunction” (it can cause low temperature) PLC2 start to works (he protect against low temperature – he control the heater in evaporator). PLC2 DC is 100% (he works all the time) but he control heater only in case when PLC is “malfunction”. So in case of PLC2 criticality number of “malfunction” failure mode should be lower (because it not control the heater all the time but only then when PLC1 is “malfunction”).
In my opinion in PLC2 beta value should be expression of percent of time when he actually control the heater for example 1% of time. So beta value should be 0,01.
Is it correct?
Regards!