Powering Discrete Inputs

[ControlConcepts]

What are your views on powering plc inputs either from the field device or from the plc cabinet. Example being you kill the power on an mcc bucket if the plc input is powered from the bucket you'll have no other power source in it, if it were powered from the plc cabinet and you killed the power to the mcc bucket there still would power present in the mcc bucket. I looked through the NEC on this and couldnt really find, does NEC have anything on this situation?

 

[gcaudill]

Control or signal power that originates from any other source other than a source controlled by the local disconnect switch should be treated as a secondary source. At the minimum I would make signs or plaqards that indicate that if the local disconnect is open there may still be voltage present.

 

[mcgowan]

most of the systems i have worked on use relays to overcome this.. The MCC bucket steps down to 110. This is run to a relay cabinet in the MCC board. The relay is turned on and off by the PLC (Usually at 24vdc. This allows stock of standard parts no matter what voltage plc you are using. 600-120v xformer off the shelf.. 600-24vdc..good luck getting it a day or two! If the voltage output of the plc changes due to upgrade conversion then change the relay and you are done.. This also eliminates the dangers of two voltages in the bucket...also gcaudill is right..if thats what you have the Canadian code tells us we MUST post sighns that two voltages from seperate sources are present..