I think your question is relating to powering a field transmitter. If
the DCS powers the field transmitter, it is typically providing 24Vdc to power the 4-20mA and/or Hart Loop. In other words, 24Vdc power and
4-20mA output signal share the same two wires.
If the transmitter is "field power" as in the typical case of a flow
meter or analyzer, then power is landed on the field instrument and its 4-20mA output is sourced back to the DCS. In other words, the field instrument has separate terminals for power and its output 4-20mA signal.
In many cases, 4-20mA isolators and splitters are used to help convert
sourced 4-20mA signals from field powered devices into an "output loop
powered" signal that is "DCS powered" and can be interfaced with the DCS analog input cards. Without the isolator in between for these cases, you get what's called "bucking power supplies" and things won't work.
Below are three Acromag links that may be very helpful for you. They
include an e-newsletter that explains the need for isolators/splitters
along with solutions. Also found below are links to an 8 minute video
on isolators and a link to application notes.
Donald Lupo
Director of Sales & Marketing - Process
Corp. Tel. 248-624-1541 x 1860
Group Tel. 248-295-0880
Corp. Fax 248-624-9234
Corp Web http://www.acromag.com
Acromag is an ISO9001:2000 and AS9100 certified manufacturer.
Basically the source of the loop power, i.e. from the DCS I/O card or a source of power local to the field device. Something like a temp transmitter doesn't need much power, so it can be powered from the 4/20mA signal. A complicated level gauge or flowmeter probably needs far more current and so will be powered from a local supply.
To add to the previous posters, be aware that some DCS modules will only allow field or DCS powered on a per module basis depending on which termination module is used, DeltaV for example.
DCS powered and loop powered are same. To quote as an example Deltav input modules come with two types of termination blocks, either a 2 wire type or a four wire type. Both connections are essentially of two wires only but term two wire OR loop powered refer to transmitters which draw power from 24VDC present across two wires coming from Input card. Same two wires takes 4-20mA output from field devide to the DCS system.
In a four wire or field powered system, DCS connections only take 4-20mA process variable signal to the control system. Transmitter power, either a 24 VDC or 220 VAC is given via a separate connection. Hence the name four wire type.
Important: Instruments used in hazardous areas utilize a hybrid of both systems. The transmitter can be either a two wire or four wire type. But the input card is always a four wire type because of the barrier used between hazardous and safe zones. Since barrier is powered by power bus separately so Input cards from DCS sees it as a 4 wire transmitter without seeing the field device.
These barriers also come in HART or FB compatable mode so that Asset management softwares like Emerson AMS have no difficulty .
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