while doing the I/o allocation in PLC or DCS we keep 20% spare space in I/O modules. my question is that why we keep 20% spare? why not 10% or 30% or more

 

I don't think there is a good answer to this question. Like many decisions of this nature, someone has to decide something. 20% is a safe bet. You know you are going to add stuff over time, and you want some spares built into the system as well. I have seen everything from 5 to 20% spare I/O points, usually with the provisio that there be some minimum number of each type of I/O used.

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Bob

 

A lot can depend on what kind of organization is to be the end user of the panel. Some folks buy stuff and never modify/add/change etc. Others are always adding/modifying and continuously evolving. Obviously the latter would justify going with more spare space and I/O in order to be safe. We try to shoot for a MINIMUM of 25-30% spare space in a new panel...usually by year end...its full !