I. Overview
The DCS control room is mainly used to house control cabinets , auxiliary cabinets, and operator stations, and is the control core of process industry plants. Therefore, when designing the control room, it is necessary to consider the space and accessories for installing cabinets and operator stations, the system operating environment, anti-static flooring, system grounding, and other aspects.
II. System Operating Environment
The control room design must ensure good ventilation and lighting; air conditioning is recommended to maintain the system's operating temperature within a suitable range. The control room must also be located away from strong electromagnetic interference. Specific environmental requirements are as follows:
III. Antistatic Flooring
To prevent static electricity from damaging system components, it is recommended to install a standard anti-static floor in the control room, typically 30mm to 50mm high. Steel cable trays can be laid under the anti-static floor to facilitate the routing of signal cables connecting to the DCS from the field, while also providing electromagnetic shielding.
IV. System Power Supply
The DCS system requires two independent power supplies: voltage range of 220VAC±10%; frequency range of 50±1Hz; each power supply must provide a power supply capacity of no less than AC220V20A (please modify the parameters according to actual needs).
The power cable entering the distribution box should be >= 6mm², and the power cable inside and out of the distribution box should be >= 2.5mm².
V. System Grounding
The grounding principle of the control system is equipotential grounding. The connection point of the control system on the grounding network should be at least 10 meters away from the connection point of lightning protection grounding, high current or high voltage equipment. When using a separate grounding method, the separate grounding body should be at least 10 meters away from other electrical professional grounding bodies and at least 20 meters away from the independent direct lightning protection grounding body.
1. General Requirements for Cable Specifications:
The conductors of the grounding system should be multi-strand copper core insulated wires or cables.
Each grounding busbar in the grounding system can be made of copper strips with a cross-section of 25mm × 6mm.
Each grounding junction plate in the grounding system should be made of copper plate with a thickness of not less than 6mm, and the length and width dimensions should be determined as needed.
The protective grounding busbar inside the cabinet should be reliably electrically connected to the cabinet.
The working grounding busbar and working grounding junction plate should be fixed with insulating brackets.
All connections in the grounding system should be secure, reliable, and ensure good conductivity. Connections between grounding wires, grounding trunk lines, main grounding trunk lines, and grounding busbars/grouting junction boxes should use copper lugs and galvanized steel bolts, and should be secured with anti-loosening devices or welded. Switches or fuses are strictly prohibited from being connected to any grounding connections.
The cross-sectional area of the grounding wire can be selected according to the following values based on the number of connected instruments and the length of the grounding wire:
(1) Internal grounding wire (connection between the grounding part inside the cabinet and the working (protective) grounding busbar inside the cabinet): not less than 1mm2
(2) Grounding branch line (connection between working (protective) grounding busbars in each cabinet and working (protective) grounding main board): not less than 4mm2
(3) Grounding trunk line (connection between working (protective) grounding junction plate and main grounding plate): not less than 10mm2
(4) Main grounding line (connection between main grounding plate and grounding electrode): not less than 16mm2
(5) Surge protector grounding wire: not less than 2.5mm²
(6) The grounding system is marked with green or alternating green and yellow colors.
The wire diameter of the grounding wire should meet the following requirements: the wire diameter of the grounding branch line from the working ground copper busbar (protective ground copper busbar) to the main working ground copper busbar (main protective ground copper busbar) should not be less than 4mm2; the wire diameter of the grounding main line from the main working ground copper busbar (main protective ground copper busbar) to the main grounding copper busbar should not be less than 10mm2.
The diameter of the main grounding conductor from the main grounding busbar to the grounding connection point should not be less than 16mm².
2. Grounding resistance requirements
The grounding resistance requirement for DCS is no more than 4 ohms in general locations; and no more than 1 ohm in substations, power plants, and places with large electrical equipment.
3. Grounding pole number requirements
General recommendation: Use a common grounding grid with equipotential bonding for grounding.
If there is no common grounding, the number of grounding electrodes is calculated based on the local soil resistivity.
If the grounding electrode is a single ground stake, it is recommended to use four 2m long 50×50mm hot-dip galvanized angle steels, arranged in a 5m square, driven into the ground at least 600mm from the ground surface. Then, weld them together with 4×40mm hot-dip galvanized flat steel (surfacing welding is recommended), with a weld length of at least 10cm. The weld joint should be treated with red lead or asphalt oil for corrosion protection. The grounding wire should be 4×40mm hot-dip galvanized flat steel (or a conductor ≥16mm²), with a grounding test point pre-installed. The connection point should use standard flat copper, with gas welding used at the connection between the flat copper and flat steel. At least three Ф8 connection holes should be pre-installed on the flat copper, and appropriate stainless steel or copper screws should be provided.
(1) If the user provides the following three grounding copper busbars: main protective grounding copper busbar, main working grounding copper busbar, and main grounding copper busbar, then all protective grounding busbars in all cabinets of the control system should be connected to the main protective grounding copper busbar, all working grounding busbars in all cabinets should be connected to the main working grounding copper busbar, and then the main protective grounding copper busbar and the main working grounding copper busbar should be connected to the main grounding copper busbar. From this grounding copper busbar, the main grounding line can be led to the common grounding grid or a separate grounding electrode, as shown in the figure below:
(Note: The shared grounding grid is also known as the electrical grounding grid.)
4. Grounding method:
The grounding of instruments and control systems should use a common grounding system for equipotential bonding; if equipotential grounding cannot be met, protective grounding and working grounding can be classified and summarized and grounded separately.
Each working ground should not be mixed with the protective ground before being aggregated.
The shielding layer of the signal shielded cable should be grounded at a single point, and different connection methods should be adopted according to the different situations of the signal source and the receiving instrument.
When the signal source is grounded, the shielding layer of the signal shielded cable should be grounded on the signal source side; otherwise, the shielding layer of the signal shielded cable should be grounded on the signal receiving instrument side.
Instrument cable trays and instrument cable protection pipes should be connected to the grounding busbar for lightning protection of the electrical system at the entrance to the control room.
The grounding wire of the instrument signal surge protector (SPD) in the control room should be connected to the working ground busbar, and the grounding busbar of the surge protector should be connected to the working grounding main board or the main grounding plate.
Surge protectors for powering instruments in the control room should be connected to the PE line in the TN-S grounding system of the power supply.
The surge protectors of the field instruments should be connected to the grounding busbar for field lightning protection of the electrical system.
For multi-core cables without shielding that are laid outdoors in lightning-prone areas, the spare core should be connected to the shielding ground; for shielded cables with grounded shielding, or cables laid in steel pipes or metal cable troughs, the spare core does not need to be grounded.
5. Grounding connection method:
Each grounding branch line of the instrument and control system protective grounding should be connected to the protective grounding junction board, and then connected to the main grounding plate through the protective grounding junction board via the grounding trunk line;
Each grounding branch line of the instrument and control system's working ground should be connected to the working grounding main board, and then...
The working grounding junction plate is connected to the main grounding plate via two separate working grounding trunk lines;
The grounding of instrument signal common points, and the grounding of non-isolated inputs of DCS, PLC, SIS, etc., should all be connected separately to the grounding connection terminal block or working ground busbar, and then connected to the working ground through the grounding branch line.
Summary board;
When the shielding layers of multiple signal shielded cables are grounded, it is advisable to first connect the shielding layers of each signal shielded cable to the working ground busbar, and then connect them to the working ground main line to the working ground main board.
The negative terminal of the DC power supply must be connected to the working ground busbar of the cabinet. If there is no working ground busbar, it should be connected to the working ground main board via the working ground branch line.
If the length of the grounding trunk line exceeds 10 meters or there are strong magnetic field devices nearby, shielding measures should be taken, such as protecting the grounding trunk line with steel pipes and connecting the steel pipes together; or using shielded cables, and the shielding layer of the steel pipe or shielded cable should be grounded at one end.
If the grounding trunk line runs outdoors and the distance exceeds 10 meters, it should be double-shielded, with the inner layer grounded at a single point and the outer layer grounded at both ends, or a grounding point should be set every 30 meters to prevent interference from lightning electromagnetic pulses.
The following are the detailed grounding requirements:
Working ground:
Each working grounding branch line of the control system should be connected to the working grounding aggregation board, and then connected to the main grounding plate through two separate working grounding branches. Each working grounding should not be mixed with the protective grounding before being aggregated.
All grounding connections, including all grounding wires, grounding trunk lines, and grounding busbars, should be insulated before being connected to the main grounding plate, except for normal connection points.
The final connection between the working ground and the grounding electrode or grounding grid should be wired separately from the main terminal block.
The negative terminal of the DC power supply must be connected to the working ground busbar of the cabinet. If there is no working ground busbar, it should be connected to the working ground main board via the working ground branch line. Multiple working ground buses may be provided as needed.
Signal shielding ground:
The shielding layer of the signal shielded cable should be grounded at a single point, and different connection methods should be adopted according to the different situations of the signal source and the receiving instrument. When the signal source is grounded, the shielding layer of the signal shielded cable should be grounded on the signal source side; otherwise, the shielding layer of the signal shielded cable should be grounded on the signal receiving instrument side.
When the shielding layers of multiple signal shielded cables are grounded, it is advisable to first connect the shielding layers of each signal shielded cable to the working ground busbar, and then connect them to the working ground main line to the working ground main board.
Safety protective grounding:
Protective grounding (also known as safety grounding) is grounding installed for personal safety and the safety of electrical equipment. All metal casings of electrical equipment such as control system cabinets, control panels, instrument cabinets, distribution cabinets, and relay cabinets, as well as normally non-energized metal parts of control equipment, that may potentially carry dangerous voltages due to various reasons (such as insulation failure), should be protected with grounding.
All grounding branches of the DCS control system's protective grounding should be connected to the protective grounding aggregation board, and then the protective grounding system should be connected to the aggregation board.
The grounding junction plate is connected to the main grounding plate via the grounding trunk line.
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