1、 Basic requirements for DCS system grounding
DCS system grounding is to ensure that when there is a problem with the signal, power supply, or DCS system equipment entering the DCS system, the effective grounding system can withstand overload current and quickly introduce the overload current to the ground. The grounding system can provide a shielding layer for DCS, eliminate electronic noise interference, and provide a common signal reference point (i.e. reference zero potential) for the entire control system.
When problems occur in the grounding system (such as excessive grounding resistance, multi-point grounding, grounding wire breakage, or grounding wire contact with high voltage or high current equipment), it can cause electric shock injury to personnel and equipment damage. It is understood that some DCS systems often “crash” (or unknown cause of “crash”), mostly caused by poor grounding system or problems. Therefore, a sound, reliable, and correct grounding is the key to the safe, reliable, and good operation of the DCS system.
2、 DCS grounding classification
In general, DCS control systems require two types of grounding: protective grounding and working grounding (logical grounding, shielding grounding, etc.). For systems equipped with safety barriers and explosion-proof measures, such as those used in the chemical industry, intrinsic safety measures are also required.
2.1 Cabinet Grounding (CG)
It is a protective measure taken to prevent the accumulation of static charges in the equipment casing and avoid personal injury. All operator cabinets, on-site control station cabinets, printers, terminal cabinets, etc. of the DCS system should be grounded for protection. The protective ground should be connected to the electrical professional grounding network in the factory area, with a grounding resistance of less than 4 Ω.
2.2 Logically
Also known as machine logic ground or host power ground, it is the common ground of the negative end of the logic level inside the computer and also the output ground of the+5V power supply. For example, the positive and negative 5V and positive and negative 12V terminals of the CPU. Need to connect to a public grounding electrode.
2.3 Shielded Ground (AG, Analog Grounding)
Also known as analog ground, it can shield the interference received during on-site signal transmission to improve signal accuracy. The shielding layer of signal cables in the DCS system should be grounded through shielding. The cable shielding layer must be grounded at one end to prevent the formation of closed circuit interference. The metal armor of armored cables should not be grounded as shielding protection, but must be grounded with copper wire mesh or aluminum shielding layer. Connect to the public grounding electrode.
2.4 Intrinsically Safe
An independent grounding system should be set up with a grounding resistance of ≤ 4 Ω. The grounding system of intrinsically safe grounding should remain independent, and the distance from the electrical grounding network or other instrument system grounding network in the factory area should be at least 5m.