Wiring of the pressure switch
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Dear sea friends: Our factory has a pressure switch with normally open contacts, used for low-level interlock. We connect it in this way: the field instrument is connected to the DI card in the DCS remote cabinet, and then to the PLC cabinet in the cabinet room. During today’s circuit testing, when components were disconnected or shorted on the field panels, no indication appeared on the PLC ； But when the two wires that go from the DCS to the PLC were shorted at the remote cabinet, a display appeared on the PLC. Finally, we found two spare terminals and resolved this issue by connecting them directly to the PLC, rather than using the DCS DI card. I would like to ask everyone: why does this situation occur? Could it be that there is a short circuit on the DCS card? Please explain in detail.
Is the setting on the DCS correct? The low-level interlock should have certain conditions, right? Is it sufficient to just provide a short circuit? On the PLC, it should be just a switch, right?
The DI card is a digital input card and cannot be used as a DO card. The field signal is connected to the DI cards in the DCS remote cabinet; by using the DCS settings, the output can be made to change depending on whether the field circuit is connected or not. It can then be connected to the DI cards in the PLC cabinet located in another cabinet through DO cards, thereby enabling a connection. However, this approach requires one DI channel and one DO channel each, which is a waste of resources. It is simpler to use an intermediate terminal, as this reduces the number of fault points
It is best for the pressure switch to first activate the relay, with the normally open and normally closed contacts of the relay being connected respectively to the DI card of the DCS and the PLC.
Reply to 1# God of Soul Killing: Regarding the short circuit or disconnection at the site you mentioned, does the PLC show no display, or does the display remain unchanged? If there is a display, is it normal or does it show an alarm?
Reply to 5# jun7203: Thank you for your reply; the PLC shows no change.
Reply to 3# kaidaju: But it’s hard to understand why similar problems haven’t occurred with the other switches!
Typically, the operating voltage for DI cards in PLCs and DCS systems is between 10V and 30V. With the wiring method used by the original poster, the DCS cards and PLC cards share a 24V supply. If the internal resistance of the DCS cards is high, the actual operating voltage of the PLC cards may drop below 10V, which is why there is no response. Generally, to send the signal to both the PLC and the DCS at the same time, one could consider using relays with NC and NO contacts on the 4th floor, but this actually changes the way the signal functions. Or use a one-in-two-out isolation barrier.
You need to determine what signals the PLC and DCS use. Generally, there are two types of TTL signals and opto-isolated signals. It needs to match; otherwise, it won’t be captured.