Analog inputs - System or field powered

What is the difference between system and field powered? Is this the same as sink or sourced ?


I hope you are referring the 4-20 mA field transmitters power supply.

The field instruments can be powered in two ways, mainly:

  1. From Marshalling Cabinets (control room)
  2. From Power Supply Cabinets (depends on their location, field or control room).

From Marshalling Cabinets (control room)

In general, we see two wire transmitters in the field like pressure transmitters, temperature transmitters, level transmitters and flow transmitters.

These transmitters use two-wire for powering-up the transmitter and use the same two-wire to send us the value of respective process variable (flow, temp., level, pressure, etc.) to the DCS or PLC systems.

The below article help you out to understand the basic concept of two-wire transmitter.

Read: Working of 4-20 mA Transmitters

So these two-wire transmitters are powered directly from marshalling cabinets.

if you don’t have the marshalling panel then field cables directly layed from system cabinets which contains all processor cards, IO cards and respective barriers. (this is only for very small/compact installations).

From Power Supply Cabinets

Some field instruments need higher voltage/current ratings for operating, and we find these transmitters are four-wire. Two-wire for power supply and another two-wire for sending the signal (process variable) to the DCS or PLC systems. Example: Magnetic Flow Meters

We will use the power supply cabinets to connect the required raing MCB’s to the field devices. These power supply cabinets can be installed in control room or field, depends on Plant design.

The below article help you out to understand the basic concept of transmitter wirings.

Read: Types of 4-20 mA Transmitters Wiring

Sink or source concept is different.

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Thank you very much for the information. Are there any pros or cons on using system vs field? Is there an advantage to one over the other or is it strictly chosen based off the type of transmitter?

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It depends on the field instrument only.

In general, we choose 2 wire transmitters only. some field device may require power supply like 230 VAC then they will come with 4 wire.

4 wire transmitters requires separate cable laying for power supply, so needs more cables, cable trays, man power, equipment and high installation cost when compared to 2 wire transmitters.

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