Hi,

I'm trying to justify our handling of safety analogue inputs in to a PLC. We check for above and below range (outside fo 4 -20mA), but is anyone aware of a standard where the exact requirements for this are defined? I beleive there is something in the German NAMUR specifications.

Thanks,

Daniel

Hello mccted,

Some questions:

If the signals perform a safety function, should they be handled by a PLC?

Have the Safety Requirements and Safety Instrumented Function been defined?

if they have, is there a resultant SIL?

Is the PLC rated to this SIL?

Have you considered appropriately SIL rated dedicated trip-amps?

I'm not being obtuse but interested in what it is you're controlling.

Yes it is part of a SIF to a SIL 3 rated PLC (German manufacturer starting with S!), all the documentation is done pretty much with IEC 61511 in mind.

I was just trying to check what I had put about the bounds checking, as I had some comments from auditors about NAMUR requirements for bounds checking. I've got the usual things from the IO channel driver library block defined to cover checking and all I can find out on NAMUR is that it needs to check the signal is within 3.6mA to 21mA. I was wondering if there was any guidance or something in an EN / BSi standard I could look at as I don't have access to any NAMUR information or much knowledge of it. Really so I can add anything else or feel happy I have covered everything I need to.

The inputs are only from simple transmitter, no use of HART or such like.

Hi again mccted,

The NAMUR recommendation is:

NE 043 Standardization of the Signal Level for the Failure Information of Digital Transmitters - (57 KB) - 104422680003.02.2003
02/03/2003 AK 3.1 - Signal Processing

available free from the NAMUR portal http://nena.namur.de/index.php?id=6

hope this helps

Thanks for your help chartassuk , much appreciated.