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Entry type: FAQ Entry ID: 23493978, Entry date: 06/26/2006

How high are the MTBF (Mean Time Between Failure) values for the SIMATIC Panel PCs 677 and 877?

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MTBF means Mean (Operating) Time Between Failure.
There is a common misunderstanding that the MTBF is the expected minimum - guaranteed - service life (or time to failure) of parts and devices.
However, a device consists of many separate components that are not necessarily independent. Each of these components has its own MTBF and failure probability that can change in time and is subject to different dependencies. Here the environmental conditions such as temperature fluctuations, vibrations etc. have a great influence.

Calculation procedure
In the simplest case you calculate the total MTBF of a device as follows:


Example: The calculation for two components with an MTBF of 15,000 hours and 30,000 hours is:


Such a calculation would however only give a theoretical statement about the MTBF.

In order to obtain practical values for the calculation we add to the suppliers' specifications (e.g. IEC 60050(191)) values gathered in the field by Siemens Repair Center and by production.

All results are standardized to 24 hours operation at 40°C (according to standard EN/IEC 61709 and Siemens standard SN 29500 with 50% partial load). For specifications for 8 hours/day or 4000 hours/year you must include the relevant additional values for inactive times (also switching on/off etc.) in the calculation.

Failure probability calculations are only possible to a certain extent because of the different "behavior" of the components – e.g. the failure probability of the processor is constant within the MTBF time period, whereas the brightness of the display's backlights decreases in the course of time and the backlight MTBF means the reaching of a specific residual brightness. 

Here and also generally you must always remember that all information about MTBF is related to specific environmental conditions (see above). Deviations or if the approved temperature range, mounting angle, shocks and vibrations are exceeded as well as other influences (also dirt or incorrect operation of the keys on the panel front, with hard objects for example) during transport or operation lead to a restriction of the MTBF (see the relevant device-specific manual for details).

If the temperature range is exceeded this doesn't say anything about the behavior of the components. Material properties can change outside this temperature range (example: Evaporation processes or adhesive bonds that dissolve).

An operating time of 1% are estimated for optical drives in the MTBF values. Maintenance parts are included in the calculations. You are advised to replace them at regular intervals. This is necessary in order to ensure a high level of system availability.

MTBF tables of Panel PCs 677 and 877 (MTBF in years)

Panel PC

Front type

Operation 8 hours/day
2920 hours/year

4000 hours/year

Operation 24 hours
8760 hours/year

677 Touch 14,4 11,4 5,8
Pushbutton controls 15,9 12,5 6,4
877 Touch 14,0 11,0 5,6
Pushbutton controls 15,3 12,1 6,1

With regard to different fronts a distinction is only made between touch and pushbutton control fronts. Differences arising from different screen diagonals are negligible.

This FAQ will be adapted in line with technical changes and new products.

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