Instrument selection and cable selection

This post was last edited by hanhaigang128 on 2024-8-7 at 11:15. The decision is generally made after taking into account various factors such as the power supply for the instrument, the type of signal, and the requirements regarding which signals need to be displayed, controlled, or used for interlocking in the PLC or DCS. For example, a 2-wire pressure transmitter can use 2 wires – these 2 wires serve both as the power supply lines and the signal lines； Electromagnetic flowmeters are generally of the 4-wire type: 2 wires are used for power supply, and 2 wires are used for the current signal or the 485 bus. Some electromagnetic flowmeters also come with a relay output (this signal can be used if alarm outputs are required, etc.). There are also 2-wire electromagnetic flowmeters available on the market, which use just 2 wires – these wires serve both as power supply wires and signal wires ； Some electric actuator circuit boards have terminals for on/off commands, feedback on whether the on/off operation has been completed, as well as signals for remote control and ESD connection; the DCS or PLC should connect as many of these signals as it needs. Electrical contact pressure gauges have terminals for the common terminal, low-pressure alarm, and high-pressure alarm. Depending on the requirements, some may need only a low-pressure alarm signal, some may require a high-pressure alarm signal, and others may need both high-pressure and low-pressure alarm signals – it depends on the specific signal requirements.

The original poster hasn’t been to the workshop in person, I guess. For ordinary thermometers and pressure gauges, since they are low-power instruments, they only require two wires; the power is supplied by the DCS. The instruments receive energy from the DCS, start working, measure temperature or pressure, and then send the signals back to the DCS by adjusting the current level in the circuit. If the power is greater than 0.1W, the signal and power of the instrument must be separated. The power supply is not provided by the DCS; instead, it comes from a 24V power source. Two power cables are connected to the instrument, after which the instrument starts to operate. The resulting process values are then sent as 4-20MA signals to the DCS, for which another two cables are required. Generally, four wires combined together form a four-core wire. Two of the four wires are connected to the 24V power supply, and the other two are connected to the DCS; together, they are connected to the instruments. If it is a cut-off valve, there are six wires. Two are the DO output wires, and four are the DI input wires for open-position feedback and closed-position feedback. Of these six wires, two are connected to the relay amplification board of the DO card, and four are connected to the two channels of the DI card. Together, they are connected to the shut-off valve; the DO output wire is connected to the electromagnetic coil of the shut-off valve, which is used to drive it open or closed (the shut-off valve can be operated by air to open or close). The other four wires are connected to the limit switches that provide feedback on when the valve is in the open or closed position.

Such issues require consideration of the electrical and signal specifications of the instrument. For two-wire instruments, a two-core twisted pair is sufficient; the wire diameter needs to be calculated based on the wiring length, especially when HART protocol communication is used. Three-wire instruments can use a three-core cable, or two two-core cables. For four-wire instruments, power supply and signals should be separated as much as possible, by using two different types of two-core wires. For such specific issues, it is necessary to make a judgment based on the particular circumstances

The cable type is determined based on the instrument type and the design provided by the design institute. There are also those that use a main cable, with one cable serving multiple instruments