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A Two Wire or Loop Powered4- mA signal system requires small amount of power for functioning of the Instrument, which is supplied by the minimum4 mA current in the circuit; whereas, when more power is required for functioning of the Instrument a Four Wire or Externally Powered4- mA signal system is used, where two wires are used for signalling and two for powering the instrument up.
For 2 wire instruments , both power(24VDC) and signals(4-20ma) are transmitted through 2 wires only .(passive instruments)
For 4 wire instruments additional power(230VAC) is to be provided for the instruments separately and Signal is transmitted through other 2 wires . (active instruments)
In case of 2 wire circuit the same 2 wires are used to supply power to device & take output from device.
In case of 4 wire circuit 2 separate wires are used for Power supply & 2 separate for output.
the two wires are used for operation of instruments while fours wires are usually used when the instrument has two transmit signal to RTU or PLC , well i don't have too much knowledge but i come across instrument in Well automation like FDi, GDP, LIT, TIT, PIt which needa four wires the two wires were of power while the other two were for signal transmission one(third) for normal conditions while the fourth was to be set when the instrument senses abnormal situation it sent an alarming signal to the RTU to close the well.
Siri don't understand your question can you explain it
The major difference between 2 and 4 wire systems is that for 2 wire systems power and signal are carried on the same set of wires. Also known as loop powered devices.
For 4 wire systems a separate set of wires supplies device power and a separate set is used for signal.
The advantage of 2 wire systems is half the wire runs are required. The disadvantage is relatively low impedance capability that may limit the type and number of devices you can drive with the signal as well as distance that the signal can be transmitted before circuit loading occurs.
The advantage of 4 wire systems is higher signal side impedance, so better loading and signal transmission distance characteristics. The disadvantage, more wire.
In a 2 wire transmitter, one wire is the power source (+) and other one is the return Power (-) + Signal (mA).
In a 3 wire transmitter, two wires are used as a power supply (+) and (-) and the third one is used as Signal (mA).
In a 4 wire traansmitter, two wires are used for power supply and the other two wires are used for Signal (mA)
