What are two-wire, three-wire, and four-wire systems in instrument wiring?

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“The ‘wiring system’ refers to the number of wires used for powering the instruments and transmitting standard signals. The terms for multiple-wire systems came into use after the invention of two-wire transmitters. This is the result of the widespread use of electronic amplifiers in instruments; the essence of amplification is a process of energy conversion, which relies on an power supply. Therefore, the first to appear were four-wire transmitters, in which two wires are used for power supply, while the other two wires are used to transmit the amplified signal after conversion (such as voltage, current, etc.). ◆Meters powered by AC220V must be of the four-wire type. A four-wire system for signals means that the power supply circuit and the signal circuit are separate from each other, with an isolation circuit used to separate them; the standard 4-20mA signal is utilized in this setup. So the signal must be isolated and is usually not grounded. The power supply can be either DC24V or AC220V. ◆When the power supplied via a two-wire system is not sufficient to power the instrument, a three-wire system is used, with a 4-20mA standard signal. The three wires in this three-wire system are: one for power supply (usually DC24V+), one for the signal, and one common wire for both power supply and the signal, which serves as the voltage reference point and the current loop. ◆Signals that are not of the 4-20mA standard are not considered two-wire systems. A two-wire system transmits a 4-20mA standard signal while using two wires for power supply. A signal that does not meet this standard is not called two-wire. The minimum power supply capacity for a two-wire system is: 4mA×24V – power consumption of the receiving instrument – line losses. ◆In actual field operations, what are the main power supply methods for four-wire, three-wire, and two-wire systems? ①Four-wire power supply is mostly AC220V; there are also four-wire transmitters that use DC24V for power supply, as shown in the figure below. Their power supply is generally AC220V, but some use DC24V as well. The output signal can be 4-20mA with a load resistance of 250Ω, or 0-10mA with a load resistance of 0-1.5KΩ ； Some also have mA and mV signals, but the load resistance or input resistance varies depending on the design of the output circuit. ②Three-wire power supply is mostly in the form of DC24V three-wire transmitters, as shown in the figure below. By three-wire system, it means that one wire is used for the positive terminal of the power supply, another wire for the positive terminal of the signal output, and one common wire for both the negative terminal of the power supply and the negative terminal of the signal. Its power supply is typically DC24V; the output signal can be 4-20mADC with a load resistance of 250Ω, or 0-10mA with a load resistance of 0-1.5KΩ ； Some also have mA and mV signals, but the load resistance or input resistance varies depending on the design of the output circuit. ③The two-wire power supply for DC24V two-wire transmitters is as shown in the figure below: it operates on DC24V, outputs a signal of DC4-20mA, and the load resistance is 250Ω. The negative terminal of the 24V power supply has the lowest potential; this terminal serves as the common signal line. For intelligent transmitters, an FSK modulated signal based on the HART protocol can also be added to the 4-20mA signal. Due to the different working principles and structures of various transmitters, different products have been developed, which in turn determines the two-wire, three-wire, and four-wire wiring configurations for transmitters. ◆How to choose between four-wire, three-wire, and two-wire instruments: ① When the power consumption is greater than 10W and high accuracy is required, four-wire instruments are generally chosen. The use of four-wire instruments originated with the emergence of the DDZ-II type electric unit combination instruments; these instruments operate on AC220V voltage, and later, four-wire transmitters that provide a signal in the range of 0-10mA became widely used. Due to factors such as complex conversion circuits and high power consumption, many instruments still prefer the four-wire system. The most common examples are Coriolis mass flow meters and electromagnetic flow meters, which still use the four-wire system. Mass flow meters require power for vibration, while electromagnetic flow meters need power for excitation; the power consumption of both types of meters exceeds 10 watts, which is why a four-wire system is necessary. ②To reduce the use of non-safe voltage for power supply, three-wire instruments are generally employed. To meet the power requirements of these instruments, the power supply to the transmitters is changed from AC220V to low-voltage DC; for example, the power can be sourced from a DC24V power supply unit, thereby reducing the use of non-safe voltage. Thus, three-wire transmitter products are available. ③With low power consumption and cost-effectiveness, two-wire systems are generally chosen for such instruments; they use 4-20mA signals, allowing the instruments to operate in a two-wire configuration. Currently, two-wire transmitters are widely used in China. In summary, for users, the selection process should take into account the actual conditions of their own organization, such as the standardization of signaling systems, explosion-proof requirements, specifications for receiving equipment, and investment considerations, in order to make an informed decision. It should be noted that for the 4-20mA signals output by three-wire and four-wire transmitters, since their output circuit principles and structures differ from those of two-wire transmitters, it is important to consider whether their negative terminals can be connected to the negative wire of a 24V power supply or shared with the ground. If necessary, isolation measures such as distributors or safety barriers can be employed to enable sharing of the power supply and ground with other instruments, while also preventing the occurrence of additional interference.