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Old January 14th, 2020, 06:22 PM   #1

PhilNoName is offline
Join Date: Jan 2020
Location: italy
Posts: 1
MTBF for a redundant system configuration siemens s7-400H

Hi all,
I'm strugglin with mtbf calculation for my system, that is a siemens sistem H with double central rack and double et200M redundant on profibus dp.
I've got tech pdfs and mtbf excel sheet from arcticle 16818490
From s7-400H manual where in appendix A Siemens state 230y as mtbf (redundant).

Question 1: IS that for the first case (Fault-tolerant CPU in stand-alone mode)?I mean is that number calculated with only two components, ps and cpu like in figure?

I've tried to calculate mtbf for these components and I've got a similar result (from 244 to 306, because of different order numbers).
(This done with the expression LambdaSYS = sum on Lambda of all components, as explained in FAQ

My sistem in this calculation take 288y, with factor 20 (547y in double rack UR1)

Question 2: How can I calculate periphery mtbf? It is a separate calculus to connect with central rack mtbf result or I have to calculate central rack modules with periphery modules in the same operation?

Question 3: in the last figure, for Redundant I/O the table below lists some components but order numbers don't match with mine, so how can I calculate factor? these number are to be sum or multiplicate?

Anyway I tried to calculate mtbf for the periphery rack, composed of 2x IM 153-2, 1x repeater485, 1x SM 321, 1x SM 322, 3x SM 331, and 1x SM 332.

I've put only one IM153-2 in the calculation for single rack (so eight components)

Question 4: is that correct?

I found two old manuals for s7-400 fault tolerant systems and they report specific factors for periphery calculation...
they are:
factor 1 for cpu reundancy, one periphery (One-sided, distributed I/O)
factor 3-12 for cpu reundancy, one periphery, 2 channels

factor 65 for cpu reundancy, redundant periphery (Redundant one-sided I/O)

factor 70 for cpu reundancy redundant periphery (Redundant switched I/O)
the question is: if the first case with factor 1 is for cpu redundancy, how can I calculate for single cpu rack?

do I have to sum (or multiply or something else) the factor for central rack with factor for periphery?

Thanks for attention
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