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CHARM Terminal Block Wiring for 3-Wire Transmitters

I have an MSA X5000 gas detector that is considered to be a 3-wire transmitter.  I know that I can use the 3-wire fused injected terminal block to make a clean and certain connection to the 4020mA AI CHARM.  However, this would be a one off setup as the other IO in the system is using all standard terminal blocks.  Does anyone have experience wiring a 3-wire transmitter to a standard terminal block and using an external 24VDC power supply?  This particular transmitter will be on an SIS CHARM.  I will have one 3-wire flowmeter on a DCS CHARM with the same question.

The sketch below looks like it would work.  But I don't know if the ground inside the CHARM will drain loop current so that none returns to the external 24VDC you're showing. Or does it get sort of split between both paths?  The 24VDC I'm showing is an external supply.

Greg Bayne

• 16 Feb 2022 1:36 PM
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4 Replies

• Tadeu Batista

  

  • 16 Feb 2022 6:41 PM

  Hello Greg,

  Looking at the actual 3-Wire AI Terminal Block wiring, terminals 3 and 4 are coming straight from the Injected Power, the only terminal being used is Terminal 2, receiving S (signal) from the transmitter, there's no need for you to go thru Terminal 4 as you're pointing out in your scheme.

  

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• Greg Bayne

  

  • 16 Feb 2022 6:59 PM

  In reply to Tadeu Batista:

  Thanks for the quick reply! My sketch is an attempt to show how it might be wired using a Standard Terminal Block. Are you thinking I could get away with no connection back to Terminal 4? Just connect the bottom transmitter wire (DC-) directly back to the external 24VDC power negative?

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• Andre Dicaire

  

  • 16 Feb 2022 9:24 PM

  In reply to Greg Bayne:

  The 3 wire terminal shows that the DC Common from the transmitter being grounded via the Injected Power DC Common reference, with the 4-20 mA CHARM only having the signal. Since the CHARM is referenced to the same DC common as the injected power circuit, the AI CHARM sees the expected signal across its load resistor.
  
  The CHARM Baseplates DC Common is sized to a maximum of 10 amps of return current. Typically the maximum current this bus will see is with 96 DO High side CHARMS all driving maximum 100 mA, or 9.6 Amps. AI and DI CHARMS draw much less. If you start connecting 3wire devices with the common connected to the AI CHARM common, and these device draw 700 mA or so, you could end up with more return current on the common than the DC Common was design for, or allowed by electrical code.
  
  That is why the 3 wire AI terminal block grounds the transmitter to DC Common via the Injected Power terminals. Since this is connected back to the same DC common, the AI CHARM common is not connected.
  
  With a 3 wire device there is one common wire that must be grounded and this cannot be to the common of the AI CHARM, so you end up with either 3 wires to the 3 wire AI Terminal block, or one wire to the standard Terminal block. Either way, this special case device is not wired to the AI CHARM terminal block exactly like the other 2 wire devices.
  
  
  You would have to use a 4 wire device to have the 4-20 mA wiring make use of a signal/Common connection. Note that his too is different that standard single ended 4-20 as these all use terminal 1 and 2, not 2 and 4. Yes it uses the standard TB, but wiring instructions are different. If you think about it, using the standard TB for this 3 wire device probably exposes you to human error when some thinks the wires should be on 1 and 2, just like all the other devices.
  
  The only way you can wire this AI like all others if is the device were 4 wire and the 4-20 mA accepted loop power from the AI Channel. Then, you could wire power and signal on separate pairs and use the AI loop power.
  
  By using the 3 wire AI Terminal, you clearly indicate to any instrument tech that his channel is using a three wire device and the instrument cable shield can be referenced at the CHARM slot providing shield protection all the way to the CHARM.

  Andre Dicaire

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• Greg Bayne

  

  • 16 Feb 2022 10:41 PM

  In reply to Andre Dicaire:

  Thanks Andre for the very informative reply. It sounds like we might be able to "get by" using an un-orthodox wiring method using the standard TB, but it would bring risks, and be confusing for initial installation and future troubleshooting efforts.
  
  Greg Bayne

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