Instrument grounding issue
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For example, there is a ground connection on the housing of the pressure transmitter, and another ground connection on the terminal board – do these two ground connections serve the same purpose? Are they all protective earthings? So how should the working ground be connected? Is it done via a two-wire system to the control side, and then one of the two wires at the control room is used to connect to ground?
On the terminal board, the cables are generally grounded within the DCS cabinet for shielding purposes, thereby ensuring proper signal transmission. The outdoor enclosure is usually connected to pipes or equipment, mainly to prevent the accumulation of charge or to ensure that lightning currents can be conducted away from it.
The electrical grounding is on the internal circuit board of the instrument, while the grounding of the instrument housing is a safety grounding.
Generally, we ground the instrument shielding wire at one end only, connecting it to the cabinet side; the grounding connections are made at the enclosure of the instruments on site and at the points provided in the conduit
How is grounding done on the other side of the cabinet? Are the two signal wires of the instrument connected to the terminal board on the control room side, and then from there connected to the grounding busbar?
Could you explain how to connect the working ground? I know that generally, the working ground for field instruments is connected to the control room’s ground system, but I’m not sure exactly how to do it. In a two-wire connection for instruments, there isn’t a wire dedicated to grounding. If the grounding is done at the ports inside the cabinet, isn’t it required that each instrument be grounded separately?
How is the ground connection on the terminal board made? Is it connected to the control room through a cable along with the signal lines?
There are regulations regarding grounding; please check them by yourself.
This is a very good question; there are too many regulations at the moment, and they are too chaotic.
Under normal circumstances, it is not necessary to ground the terminal block; since the voltage is below 24V, grounding the shielding layer of the cable is sufficient for eliminating signal interference
The shielding wire of the transmission cable is connected to the working ground bar in the DCS cabinet