Emerson Exchange 365

S Series AI Charm failure

I have a field powered scale sending a 4-20ma signal back to an S series AI charm located in a RIO cabinet less than 50 feet from the field device. Measuring the signal from the field device I have 4.8ma but when I terminate the field wiring to the charm base and push in the AI charm I get an open loop indication. I measured the MA signal and when the charm is pushed in the signal drops from 4.8ma to 0.  When I use my Fluke 707 in the source mode to generate the same 4.8ma signal and terminate it to the same charm baseplate points I get a good signal back to DeltaV.  What would cause the 4.8ma signal from the scale to drop to 0.00ma but from the same 4.8ma signal from the meter functions as it should? 

Shawn Connor

• 7 Jan 2021 12:33 PM
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6 Replies

• RickV

  

  • 7 Jan 2021 12:52 PM

  Analog Input CHARMS can be wired two ways, as a 2-wire field device and a 4-wire field device. It sounds like you have a 4-wire device. Verify DeltaV configuration for the CHARM Analog Input IO point and ensure it is downloaded. Verify that you have a standard CHARM Term block, verify positive is on terminal 2 and negative is on terminal 4 for a 4-wire device.

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• Shawn Connor

  

  • 7 Jan 2021 1:07 PM

  Good morning RickV

  The field device is hooked up to terminals 2 and 4 (2 being positive and 4 being negative) and when the charm is pushed in the signal drops from 4.8ma to zero. For troubleshooting purposes I did reverse the wiring to check for polarity issues. When I source the same signal value from a Fluke 707 to terminals 2 and 4 I get a good signal back to DeltaV and can simulate a full 4ma to 20ma range. The 4-20ma signal from my field device is dropping to 0 at the Charm but not my meter.

   

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• Gary Law

  

  • 7 Jan 2021 2:24 PM

  In reply to Shawn Connor:

  When you say the signal drops to 0mA - Measured how ?
  What happens if you put a 250 ohm resistor across the wires from the device - se the CHARM terminal and wire the resistor across the same
  term 2 & 4 , but DO NOT plug the CHARM in yet. Measure across the resistor. Are you seeing 1.2V ?
  
  Remove the resistor , plug in CHARM and take same Voltage measurement - internally the CHARM is approx 250 ohm (not exact) , so you should see similar voltage.

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• Cedric Dawnhawk

  

  • 7 Jan 2021 5:26 PM

  Have you checked that the output of your field instrument is floating or at least the negative (not the positive) is grounded? Terminal 2 of the AI terminal block is connected to the negative of the DeltaV 24V field supply which is probably grounded.

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• Cedric Dawnhawk

  

  • 7 Jan 2021 5:28 PM

  Sorry that should have said terminal 4 of the AI terminal block.

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• Andre Dicaire

  

  • 7 Jan 2021 6:16 PM

  In reply to Cedric Dawnhawk:

  The AI terminal 4 connects to the deltaV DC Common which is to be grounded as per installation requirements. 4 wire transmitters must offer a floating 4-20 mA signal if they are to be connected to a DeltaV AI HART channel, CHARM or multi Channel Card. If the field device is grounded, and the CIOC is properly installed, i.e. grounded, you would damage the internal PC board circuitry on the signal common side from the ground loop current.
  
  In the DeltaV CHARMS, the CHARM circuits have sacrificial failure points on all four connections to the Terminal Blocks. If you had a ground loop condition that resulted in high current on the Common, the CHARM would fail with an open circuit, protecting the rest of the system from this excessive current.
  
  So if you do have a ground loop, you would not observe this when you read the signal with a meter. But if the CHARM's internal connection to the Common has open circuited, you would see a an open circuit fault as the transmitter's signal would be 0 amps.
  
  However, if the CHARM still reads correctly when simulating a signal across terminals 2 and 4, that tells me the CHARM internal protection has not open circuited.
  
  Curious. If you connect an Ammeter from the transmitter negative wire to the DeltaV DC Common, do you read a current? (disconnected from the CHARM Terminal), If so, the transmitter is not isolated. If this polarity is counter to the 4-20 mA, would this negate the signal? But in any case, if it show current flowing between commons, you cannot connect this transmitter without an isolator.

  Andre Dicaire

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