1. Scope
This standard specifies the classification, methods, system structure, grounding resistance, and design principles of instrument system grounding. It aims to ensure safe and reliable grounding for instrumentation and control systems to prevent electrical hazards and interference.
2. Grounding Classification and Methods
2.1 Protective Grounding
Protective grounding is implemented to prevent electric shock and equipment damage caused by leakage currents. The following guidelines apply:
All exposed conductive parts of instrumentation and control systems must be connected to the protective grounding system.
Metal panels, consoles, enclosures, cabinets, and racks housing instruments or control systems should have protective grounding.
If an instrument is in good electrical contact with a grounded metal panel, enclosure, or cabinet, additional protective grounding is not required.
In non-hazardous environments, protective grounding is not required for field instruments with supply voltages below 36V, unless they might come into contact with higher voltage systems.
In hazardous environments, non-intrinsically safe field instruments must be grounded, while intrinsically safe systems may not require protective grounding.
Instruments requiring lightning protection should always be grounded.
Grounding should be done locally to the nearest grounding grid.
Metal cable trays and conduit should be grounded using grounding clamps and connected to the grounding grid at multiple points, with a maximum spacing of 30 meters.
Cable trays and conduits should be grounded at the point of entry into buildings.
2.2 Functional (Working) Grounding
Functional grounding ensures proper operation of instrument signal circuits. The following principles should be followed:
Instrument signal circuits requiring grounding should be connected to the working grounding system.
Non-isolated signals should use the DC power negative terminal as the working ground reference.
Isolated signals do not require grounding; circuits must be electrically insulated from other circuits.
Protective and working grounding must not be combined before reaching the grounding busbar.
Signal circuits should follow single-point grounding to avoid ground loops.
Multi-point grounding should be avoided unless isolation devices are used to break ground loops.
2.3 Intrinsically Safe System Grounding
Systems using isolated safety barriers do not require grounding.
Systems using Zener barriers must be connected to the working grounding system.
The grounding busbar for Zener barriers should be connected to the DC power negative terminal.
2.4 Shielding Grounding
Proper shielding grounding helps prevent electromagnetic interference:
The inner shield of signal cables should be grounded at the control room end.
Cables with existing natural grounding at field devices should not be grounded again at the control room end.
Multiple cable shields should be collected and grounded at a common terminal.
Spare cable shields and cores should be grounded at the control room end, but unnecessary grounding should be avoided for cables installed in metal conduits.
2.5 Electrostatic Discharge (ESD) Grounding
Equipment prone to static charge accumulation should be connected to the protective grounding system.
If already connected to protective or working grounding, additional ESD grounding is not required.
Control rooms and cabinets should have anti-static flooring and workbenches connected to the grounding system.
2.6 Lightning Protection Grounding
Surge protection devices should be directly connected to the cabinet grounding busbar or structural grounding grid.
3. Grounding System Structure
3.1 Grounding Principles
Each instrument or device should have an independent grounding conductor connected to the grounding busbar, avoiding series connections.
Cabinets should be independently connected to the grounding grid.
Equipotential grounding principles should be followed.
The grounding system should be integrated with the electrical grounding system.
TN-S power supply system should be used, with PE conductors connected to the grounding panel.
Branch centralized grounding structures should be implemented where necessary.
In installations with numerous grounded instruments, multiple grounding busbars should be considered.
4. Grounding Connections
4.1 Grounding Conductors
Conductors should be multi-stranded copper insulated wires or cables.
Recommended conductor sizes:
Indoor instruments: 1 mm² – 2.5 mm²
Field instruments or junction boxes: 2.5 mm² – 4.0 mm²
Cabinet busbars: 4.0 mm² – 6.0 mm²
Main grounding conductors: 10 mm² – 25 mm²
Grounding grid connections: 25 mm² – 70 mm²
Grounding conductors should be identified using yellow-green stripes or green insulation.
4.2 Conductor Routing
Grounding conductors should be as short and straight as possible.
Loops and coils should be avoided.
Parallel grounding paths should be used for redundancy.
Conductor joints should be welded or bolted with anti-corrosion treatment.
4.3 Grounding Busbars and Summation Boards
Grounding busbars should be at least 25mm x 6mm copper bars.
Grounding summation boards should be made of copper, with a minimum thickness of 6mm.
Protection grounding busbars should be securely connected to the cabinet.
Working grounding busbars should be insulated from the cabinet.
All grounding connections should be mechanically secure and corrosion-resistant.
5. Grounding Resistance and Connection Resistance
5.1 Grounding Resistance
The frequency grounding resistance of instrument and control systems should not exceed 4Ω.
5.2 Connection Resistance
The connection resistance of all instrument and control system grounding should not exceed 1Ω.
Conclusion Implementing the above grounding standards ensures the safety, reliability, and effectiveness of instrument and control systems. Regular inspections and adherence to these guidelines will enhance the long-term performance of the system.