I would like to ask how far a 4-20mA signal can be transmitted?
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I would like to ask how far a 4-20mA signal can be transmitted? Many of the signals at the site are located about 900 meters away from the control room. Is it necessary to set up a remote I/O station for such a long distance? I asked some colleagues, and they said that transmitting 4-20mA over 900 meters is no problem at all. But I’m worried that if the signal can’t be sent, it will cause big problems.
Network reference: The 4-20mA transmission has no requirement regarding length, but it does have requirements for the load resistance. For example, for a meter with a operating voltage of 24V and a load capacity of 750 ohms, its minimum operating voltage is (24 – 0.020*750) = 9V; in other words, it is necessary to ensure that the voltage applied to the secondary meter is not less than 9V. If the load on the meter is 250 ohms when in use, the maximum allowable resistance of the cable is 750–250 = 500 ohms. As long as the cable resistance does not exceed 500 ohms, the length of the cable doesn’t matter. At the same time, you also need to consider the operating voltage; if the voltage is low, the allowable load resistance must be reduced. Here is a calculation formula: RLmax = (Vs – Vmin) / 0.02, with the unit being ohms. Here, RLmax represents the maximum allowable load resistance; Vs is the supply voltage, and Vmin is the minimum voltage required for the secondary instrument to function properly.
This post was last edited by denghl on 2011-1-24 at 18:24. This issue can be analyzed from the following aspects: 1. The impedance of the cable. The load-carrying capacity of a transmitter is within a certain range; for two-wire transmitters, the actual maximum load that can be handled by the transmitter can be calculated based on the supply voltage ； For four-wire transmitters, the load capacity is clearly specified in the manual. The sampling resistance of the DCS card is generally 250Ω, and this value cannot be ignored. Using the method above, the maximum length of the cable can be calculated. 900 meters is more than sufficient for practical use. 2. Insulation performance of the cable. If the cable insulation is poor, current may leak, resulting in a discrepancy between the transmitter’s output and the DCS display. This problem is also rare in actual production. 3. It is necessary to consider remote stations, especially when there are a large number of them, as the cost of cables can be quite substantial. This requires further economic calculation.
The double issue is an old one; I’ve done detailed calculations on this thread, please take a look: http://bbs.hcbbs.com/viewthread.php?tid=710746
The load capacity of the safety barrier also needs to be taken into account, as well as the capacitive and inductive reactances of the capacitors and inductors along with those of the wires
Theory suggests that signals can be transmitted to infinity, but this requires the use of impedance-free wires, namely superconducting materials, which do not exist yet. As a workaround, very thick wires can be used; for example, wires with a diameter of 1 meter. I estimate that it’s no problem to transmit signals over several hundred kilometers using such wires.
Let’s calculate the costs. It’s better to use remote IO or cables; using cables over a distance of 900 meters is risky, so it’s recommended not to do so. If cables are used, avoid those with a diameter of 1.5 mm and opt for ones with a diameter of 2.5 mm instead. Use MTL for the safety barrier; if it were me, I wouldn’t consider that option and would opt for remote IO with fiber optic transmission instead.