[2026 Grounding System] It is strictly prohibited to mix the signal ground of on-site instruments with the equipment protection ground
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It is strictly prohibited to mix the field instrument signal ground (SG) with the equipment protection ground (PE); the main reason for this is that their functions are entirely different: the signal ground provides a stable reference zero potential for low-voltage signals, while the protection ground is intended solely for dissipating fault currents and ensuring personal safety； Mixed connections can introduce ground potential differences, power-frequency interference, common-mode noise, and may even lead to equipment malfunction, signal distortion, or electric shock hazards.
1. The signal ground (SG) serves as a measurement reference, providing a “zero potential” reference for low-voltage circuits such as sensors, transmitters, and DCS systems (e.g., 4–20mA). It requires low noise, single-point grounding or equipotential bonding to prevent the formation of ground loops. If connected in parallel with protected areas, since such areas are often subject to equipment leakage currents, lightning strikes, or operational transient currents (which can reach several hundred amperes), instantaneous or steady-state voltage differences (which can be several volts or even higher) are generated at the connection points. These differences, when coupled directly into high-impedance signal circuits, can cause data drift, fluctuations, false alarms, or communication disruptions.
II. The protective earth (PE) serves as a safe discharge path; its only purpose is to reliably direct the fault current from the equipment’s enclosure to the ground in the event of insulation failure, thereby triggering the circuit breaker to trip (in accordance with standards such as GB/T 16895 and IEC 60364). This path may carry short-circuit currents or harmonic currents; although the grounding resistance is required to be ≤4Ω, a voltage drop still occurs (e.g., 10A × 4Ω = 40V). If it comes into contact with the signal ground, this voltage will be applied directly to the reference terminal of the instrument circuit, which can either degrade the integrity of the analog signals or even destroy the isolation components in the input stage (such as optocouplers and isolated amplifiers).
III. Formation of ground loops and common-mode interference: If there is multiple grounding points or different potential references in the system after mixing, closed loops (ground loops) can be formed. These loops allow electromagnetic interferences such as power-frequency signals (50/60 Hz) and inverter harmonics to be coupled into the signal lines, resulting in alternating-current ripple superimposed on the DC signal. This has a significant impact, especially on low-level signals such as thermocouples and RTDs. At this point, even being “grounded” turns it into an interfering antenna.
IV. Risk of failed safety mechanisms: If a protected area is used as a signal reference, it may be affected by the current in the signal circuit (usually
V. Standards specify isolation requirements. In accordance with the \"GB 50093-2013 Code for Construction and Quality Acceptance of Automation Instrumentation Projects\" and \"GB/T 14048.1\", the instrumentation system should employ separate grounding systems and equipotential bonding (without mixing of grounds). The signal ground should be connected to a dedicated signal grounding bar (with ultimate connection to the main grounding grid), while the protective ground should be connected to a separate protective grounding bar. These two grounds are connected at only one point, namely at the main grounding point (such as the building’s grounding electrode), and no signal currents or fault currents shall flow between them.
Mixed wiring may seem to \"simplify grounding,\" but it actually undermines the stability of signal references and the safety protection mechanisms; it is a common cause of signal abnormalities, equipment damage, and safety risks in industrial settings. The correct approach is: physical separation, single-point equipotentiality, and separate grounding (single-ended/dual-ended as required), strictly adhering to the principles of \"functional isolation and separate grounds but shared current paths\".
For reference only: 1. Specifications for the grounding design of instrument systems (HG/T 20513-2014) 3.1.3 Field instruments, transmitters, local switches, etc., powered by voltages below 36V do not require protective grounding unless there are specific requirements. If contact with a power supply above 36V is possible, protective grounding should be implemented.
2. Code for Design of Grounding of Petrochemical Instruments (SH/T 3081-2019) 4.1.4 In non-explosive environments, the metal enclosures of field instruments powered at less than 36V do not require protective grounding; however, such grounding is necessary when they may come into contact with high-voltage equipment. 4.1.5 In explosive hazardous environments, the metal enclosures of field instruments in non-intrinsically safe systems shall be provided with protective grounding. 4.1.6 The metal enclosures of field instruments used for lightning protection shall be provided with protective grounding.
3. Code for Lightning Protection Design of Petrochemical Instrumentation Systems (SH/T 3164-2021) 9.3.1 The grounding of field instruments shall be protective grounding; the specific requirements are detailed in Table 9.3.1 below.
4. Construction and acceptance specifications for engineering projects (SH/T 3551-2024) 10.3.1 In explosive hazard environments, the metal enclosures of field instruments that are not intrinsically safe shall be provided with protective grounding. 10.3.4 The metal enclosures of field instruments used for lightning protection shall be provided with protective grounding.