What is the difference between four-wire and two-wire instrumentation? Does two-wire system require power supply?

In the four-wire system, the power supply for the instrument is separate from the signal line, while in the two-wire system, the power supply and the signal line are shared.

Four-wire instruments have two power supply wires and two signal transmission wires, whereas two-wire instruments combine both power supply and signal transmission on two wires. Two-wire instruments require power supply, and usually the power supply and the signal share the same pair of wires. .

Since it’s an electric instrument, it naturally needs power! In a four-wire instrument, the signal wires and power wires are separated; there are two signal wires and two power wires. It is called a four-wire instrument because there are four wires in total. In a two-wire instrument, the signal and power share two wires. Hence, it is called a two-wire instrument. For field instruments, four-wire instruments require that a power supply be brought to the field, and the power output must meet the instrument’s operational requirements (i.e., it must be sufficiently high). Therefore, four-wire instruments cannot be intrinsically safe. Two-wire instruments can achieve intrinsically safety, as the signal and power share the same two wires; a safety barrier can be installed to separate the safe area from the hazardous area, and they can together form an intrinsically safe circuit with intrinsically safe instruments.

Even single-wire instruments require power supply. Two-wire instruments use the same circuit for both power supply and signal transmission. Since the power consumption of these instruments is very low, even a current of less than 4 mA is sufficient to enable them to function properly; therefore, the power from the signal circuit can be used for this purpose. Electrical instruments cannot function without electricity.