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Impact of removing field power with master control relay

In my past experience on PLC systems, it was fairly common to remove power to one or more of your DO cards via a master control relay when the application was machine control.  If someone pulled an e-stop, then some or all DO's lost their power, and the status of the master control relay is used to logically move all the DO's to the de-energized state.

I'm being asked to do that for a DCS (although this machine application is still commonly executed on a PLC/PAC).  Before knowing this requirement, the selected IO was M-Series DO 24VDC High Side.  If the 24VDC field power is switched off upstream of field power connector to the 8-wide, wouldn't I get a flood of integrity errors?  I know there are various masking capabilities of module and block errors, but is it possible to prevent the operator from seeing bad status, integrity alarms, etc?

Maybe the better question is... if you had this requirement, how would you handle it?

Many thanks...

K Chad Thompson

• 15 Jun 2022 3:15 PM
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• fkitzmann

  

  • 15 Jun 2022 4:07 PM

  Doing this kind of E-Stop interlock in the DeltaV DCS is tricky and can lead to alarm floods as you adequately indicated.
  It does not help that you have to work with an M-series DO card. Is it possible to install a set of interposing relays that would allow you to shut off power to the field devices without removing power from the M-series DO card directly?
  This would at least prevent system alarms in the DeltaV.
  The status of the Master Relay would have to be used to either disable module level alarms for affected devices or engage logic that would prevent an alarm flood and put the modules in an interlock state.
  I would honestly stay away from changing too much within the block error and module error masks, just because if you are using a lot of class-based modules it will affect other modules as well for which you might not want those changes active.
  I guess it depends how your system is setup and want kind of programming philosophy is used.
  
  Does this help?

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