fail safe PLC & A class instrument

Hi Every body

I would like to ask you for a help on the following:

1) what does it mean by fail safe & fail safe PLC?

2)what are the new regulation of A class instrument (instrument that cause the ESD for the plant) and how often it is required to do reqular maitemance for them. In addition where can i find the any useful documentation

3)the fire and gas system is it consider to activates the ESD or it just for alarms to operator.

Your help is highly a preciated

Thank you and best regards

Elkawafi

I "assume" ESD means electrostatic discharge, if so there is a magaine specific to that: http://www.esdjournal.com/
Tutorial on ESD: http://www.midwestesd.org/aboutesd.html

This describes the term Fail-safe: http://en.wikipedia.org/wiki/Fail_safe

This link will take you to Siemens support for the S7-400 Fail safe plcs, you can get manuals etc and should be able to see wiring examples etc: http://support.automation.siemens.com/-snm-0135109872-1116758784-0000025094-0000013271-1119614215-enm-WW/llisapi.dll?func=cslib.csinfo&lang=en&siteid=cseus&aktprim=0&startNode=4000024&nodeID0=10805207&basisview=4000003&viewLevel=6&wttree=cs&foldersopen0=-175-176-177-228-298-324-&jumpto=324