Output checking

How does output checking work ? NFPA87 calls for output checking in chapter 8. What is output checking ? Thanks

Basically it means using some sort of feedback that the action the output was to perform, actually did. So, if you tell a valve to shut, then you need some form of indication that it did indeed shut (limit switch on valve, pressure switch downstream, flow sensor, etc...). Since the NFPA87 is concerned with fluid heaters, safety of the fuel source is a concern. When you tell a fuel valve to shut you want to ensure it did to prevent unexpected fuel leakage and possible explosion or fire. Output checking in this case would cause a alarm or further shutdown measures to occur if an output device fails to operate as instructed.

Thanks for your reply Mark, that makes a lot of sense. I am involved with upgrading an obsolete PLC on a thermal oil heater. One of our objectives is to ensure that the new PLC will comply with the new NFPA code. In order to comply with the output checking stipulation we may need to add some instrumentation which will likely increase our scope. Thanks for your help Mark