Question:
Is there always a separate voltage supply required for SM301/SM302?
Why must the voltage supply meet the SELV/PELV requirements?
Answer:
A separate voltage supply for the module and a separate voltage supply for the safe output of the SM301/SM302 is only required if the safety module or the safe output of the SM301/SM302 and the 9400 are to be wired with electrical isolation.
If this is not the case, the module (and the safe output) can be connected to the same voltage source as the controller.
However, it is absolutely necessary to use a voltage supply that meets the SELV/PELV requirements.
The main reason for using SELV or PELV in safety control systems is the defined error behaviour.
When using a PELV supply unit according to EN60204, you can expect a maximum voltage of 60 V in case of an error. When using a PELV supply unit according to IEC/EN 60950, you can also expect a voltage of 60V, but for 0.2 s even a voltage of 120 V DC if an error occurs. I.e. if component manufacturers do not set any requirements on the supply unit, you will have to think about any voltages that may occur in the event of an error or you will have to make an FMEA for the supply unit used and keep the errors occurring under control. In general, the analyses must be repeated for each supply unit.
Therefore, for the sake of simplicity, manufacturers prescribe PELV or SELV because of the known error model although there does not exist any standard that directly prescribes PELV or SELV for safety control systems.
