Distance requirements for transmitting field instruments to the DCS

As long as the resistance value of the cable is within the specified limits, it can be used. However, when the distance is too great, the cost of the cable becomes very high; therefore, in many cases, the control cabinet is placed nearby, with optical fibers used to transmit signals to the control room

Hello, if cable costs are not taken into consideration, is the transmission distance of this signal determined by the cable resistance (length)? Thank you.

For normal signal cable transmission, the transmission distance of ZR-DJYVP does not exceed 600 meters

For 1.5mm2 shielded cables, with a current of 4~20mA, the maximum length is around 1000 meters. In fact, the upper limit is not just that. If you’re interested, you can search online; there are many similar articles. Teach you how to calculate the theoretical usage length. When calculating, pay attention to several aspects: 1. Different wire diameters and materials result in different impedances. 2. For example, if the cable length is 500 meters, the impedance = 500 x 2 x the impedance value per meter. “2” is because the cable has two cores, so the total length is twice as much. 3. The rated voltage is 24VDC, but in practice fluctuations are allowed; the specifications also allow a range of ±5%. So it should be calculated as 24x0.95. 4. The minimum power requirements for each meter vary, so it can be calculated that 1 meter can cover 1000 meters, while another meter may require a different amount to cover the same distance of 1000 meters. 5. It is necessary to use conservative values (considering 0.7 to 0.8 times the theoretical value), as after all, these are all calculated based on theoretical values. In practice, issues such as shoddy cable construction, interference, and the presence of terminals in the wiring can have various effects.

To transmit a 4–20mA signal losslessly over a two-wire circuit, Ohm’s law must be satisfied. That is, the following condition must be satisfied: (Excitation voltage – Minimum operating voltage allowed by the transmitter) ≥ Output current × Total resistance of the current loop. When the output current I is 20 mA, or 0.02 A, the equation becomes an equality; thus, the total resistance of the current loop is equal to (Excitation voltage – Minimum operating voltage allowed by the transmitter) ÷ 0.03. This calculated value is denoted as r, that is, r = (Excitation voltage – Minimum operating voltage allowed by the transmitter) × 50, with the unit being ohms. This value of r is referred to in the industry as the load resistance of the current signal, and it represents the maximum load capacity of the current signal.

We know that wires also have resistance, and with resistance, voltage division occurs. According to Ohm’s law V=IR, the voltage drop across the cable equals the circuit current multiplied by the cable resistance; the maximum circuit current is 20 MA (with a practical maximum of 21.5 mA). The minimum operating voltage permitted by the field instruments is generally 10V; therefore, the maximum resistance R of the cable is (24-10)/0.020 = 700 ohms. According to GB/T 3596-2008 for cable conductors, the maximum DC resistance at 20 degrees for the first type of conductor with a nominal cross-sectional area of 1.5 square millimeters is 12.1 ohms per kilometer; this corresponds to 1.21 ohms per 100 meters. Therefore, the maximum allowable cable length is 700/12.1 = 57.85 kilometers. Note that the cable length here refers to the total length of the circuit; the distance from the field instrument to the clip should be divided by 2. Even so, in practical applications this distance is likely not reached.

For 4–20mA current signals, there is no attenuation. However, the longer the cable, the greater the line resistance, which leads to a drop in voltage; if the voltage drops below the operating level required by the instrument, it will not be able to function properly. So it’s not that any length will do

I’ve used it in practice; it’s powered by a safety barrier, uses 1.5 mm shielding wire, and works fine over 500 meters – I haven’t tried it for longer distances. Of course, differences may exist due to varying environments

Shielded cables within 500 meters are all problem-free.