Ambient Temperature Limits of decentralized IO

Hello Nic A:

If your remote I/O station is subject to harsh ambient conditions, you have to take these in consideration when you develop your design. Failure of the IM 155-5 PN ST or any of the I/O modules connected to it due to excessive temperatures could createcontrol issues in the long term.

Since your CPU will be placed in optimal conditions, you can use the standard part number, with standard temperature range,  without problem.

As an alternative to "regular" modules, which as you have seen operate well within a range of -25 to 60 Celsius, Siemens proposes a range of modules from the SIPLUS extreme design. They operate identically to the regular module they are based on, with additional robustness, on temperature range, on deleterious atmospheres (chemicals and dust in the air), in high altitudes... Look at the following presentation for your IM 155-5, offered with SIPLUS specifications (ambient operating temperatures from -40 to 70 Celcius):

SIPLUS extreme - Delivery Release for SIPLUS ET 200MP IM 155-5 PN ST

https://support.industry.siemens.com/cs/ww/en/view/109766720

Using the IM 155-5 in SIPLUS design without doing the same for the I/O modules would be inconsistent, a standard analog input module failing at 60 Celcius would still be a problem for the distributed I/O station. The ET 200MP I/O modules are also available in SIPLUS extreme format, with a similar temperature range (the preceding link will show you more information). 

The attached screenshot will show you the temperature differences supported by a IM 155-5 and a ET 200MP module, in standard and Siplus variations. Take into account that SIPLUS extreme modules are a bit more expensive that regular modules when you start analyzing your design.

Hope this helps,

Daniel Chartier