When Di adopts 24VDC loop voltage and the threshold voltage of Di module reaches 18vdc or 17vdc, di card considers that there is a signal and that the field contact is closed. The detection voltage of judgment 1 signal is generally 16.8vdc-28.8vdc, and it is considered as 0 when it is lower than 16.8vdc. The judgment voltage of 1 signal of some product brands is only 15Vdc. But in fact, it is possible that the on-site contacts are not combined at all, and the induced voltage on the line can reach more than ten VDC, which is what we usually call the false signal caused by interference. The contact is not closed, but there is a high induced voltage. When the threshold voltage of Di card is reached, it is considered that there is a 1 signal.
For Di cards with 48VDC loop voltage, the threshold voltage of 1 signal is 36vdc-52.8vdc, which will be slightly lower by some manufacturers, but it will not be too low. It is rare that the induced voltage of the field line reaches more than 30VDC, which greatly reduces the occurrence of false signals caused by interference. Therefore, the power industry adopts 48VDC as the loop voltage of Di card instead of 24VDC and 36VDC, which is the result summarized according to the field practice and application experience. Then why not choose 60VDC or 110VDC? Choosing a reasonable threshold can solve the main problem is the appropriate solution, so the query voltage of Di card is 48VDC.
The 24VDC Di signal circuit also has the problem of signal attenuation. Now the concept of centralized control room is popular, and tens of thousands of cables are connected to the centralized control room. The mutual interference is very serious. The cable laying distance is virtually longer. The longer the cable, the easier it is to attenuate, because the line resistance will increase a lot. Theoretically, the 24VDC line can support a distance of 500m, but in fact, the field problems are more complex. In many cases, the voltage has attenuated obviously when the cable is laid for three or four hundred meters. In addition, if the index of the cable itself is not enough, the attenuation will be more serious.
As a result, the original signal is closed and can send a high enough voltage back; As a result, the contact is closed due to attenuation, but the voltage is not high enough, and di considers it as a 0 signal. Properly increasing the query voltage of DI can solve the problem of signal attenuation in long lines. After years of summary and experiment, it is found that 48VDC is very suitable. When the line is laid for 1km, it can also ensure that the signal can be detected.
To sum up, 48VDC is selected as the loop voltage of Di card of DCS control system, which is better than 24VDC. At the same time, it needs the equipment related to Di to support 48VDC.