[2025 Grounding System] Should the metal enclosures of field instruments, instrument protection boxes, wiring boxes, etc. be provided with protective grounding?

The following reply is for reference only (not the content in the attachment). Proper grounding can both suppress external interference and reduce the impact of the equipment on the surrounding environment. An incorrect grounding, on the other hand, can introduce interference; in severe cases, it may even prevent the measuring equipment from functioning properly. Therefore, the grounding issue must be carefully considered during design, and proper grounding is also essential when the instruments are installed and in operation. The grounding of instrumentation equipment includes protective grounding, operational grounding, anti-static grounding, and lightning protection grounding, among others.

Safe grounding involves establishing a good electrical connection between the metal parts of a system that are not normally energized (such as metal enclosures) and the ground, in order to protect measuring equipment and human safety. The reason is that field equipment is usually powered by high-voltage electricity; under normal circumstances, the casing and other components are not charged. When a fault occurs and causes a short circuit between the power supply’s live wire and conductive metal parts such as the instrument casing, these metal parts or casings become charged. If there is no proper grounding, a high voltage difference is created between these charged parts and the ground. If a person accidentally touches these charged parts, electric current can flow through the body, posing a risk of electric shock. Therefore, it is necessary to ensure a good connection between the instrument’s metal enclosure and ground, so that the enclosure and ground are at the same potential.

Working ground is a type of grounding designed to ensure the reliable operation of instruments and those connected to them, as well as to maintain accuracy in measurement and control. It is divided into equipment logic grounding, signal circuit grounding, and shielding grounding; in other explosion-proof systems such as those in the petrochemical industry, there is also intrinsically safe grounding. Grounding of the signal circuits, such as grounding the negative terminals of various transmitters, and grounding the negative terminals of digital signals. Shield grounding refers to the grounding of the shield layer of analog signals. Intrinsic safety grounding is the grounding of intrinsic safety instruments or safety barriers. In addition to suppressing interference, this grounding is also one of the measures that gives instruments intrinsically safe properties.

The electrostatic shielding of the instrumentation system is a key measure to ensure equipment safety and improve measurement accuracy. Electrostatic accumulation can generate electric sparks; if flammable gases are present in the environment (such as oil and gas, dust), a spark with an energy exceeding the minimum ignition energy (e.g., only 28 mJ for methane) can trigger an explosion. Through reliable grounding, the accumulated charge is discharged to the ground via a low-impedance path, preventing discharge caused by potential differences. The absence of an anti-static grounding system can lead to various hazards, with impacts affecting equipment safety, human health, and the production environment among other aspects. Grounding reduces the risk of sparks to zero by dissipating static electricity. Electronic components are sensitive to static electricity; a static voltage of over 2000V can break down CMOS circuits, resulting in the malfunction of the instrument.

Lightning protection and grounding of instrumentation equipment are essential requirements for safe production and compliant operations. Instruments can be directly damaged by lightning strikes, affecting the company’s production. The electric field in thunderclouds induces overvoltages (reaching several thousand volts) on signal lines, which can cause malfunction or damage to equipment in the control room. The electromagnetic pulse generated by lightning strikes can increase instrument signal errors by 30%-50%, while these errors can be kept within 5% after grounding.

● Protective grounding primarily ensures the safety of equipment and people； ● Working ground ensures the stability of the normal operating voltage in electrical systems, preventing failures and avoiding hazards ； ● Anti-static grounding prevents damage caused by static discharge ； ● Lightning protection grounding safeguards electrical equipment from damage caused by lightning strikes. Different types of grounding methods vary in terms of implementation. In actual design and installation processes, the appropriate grounding method and devices should be selected based on specific circumstances, and design and operations must be carried out in strict accordance with relevant safety standards.

1. HG/T 20513-2014 \"Code for Grounding Design of Instrument Systems\": 3.1.3 For field instruments, transmitters, local switches, etc. powered with voltages below 36V, protective grounding is not required unless there is a specific need for it. Explanation of the provisions: 3.1.3 The regulations regarding safe voltage levels vary from country to country. In our country, 36V and 12V are commonly used, while in some foreign countries 50V and 25V are specified ； Some companies in Japan stipulate that electrical meters with voltages below 60V do not require protective grounding. According to these specifications, field instruments, transmitters, local switches, etc. powered at less than 36V do not require protective grounding unless there are specific requirements. Due to the highly complex installation conditions on site, the metal enclosures of field instruments powered at less than 36V may also come into contact with other power sources at higher voltages than 36V; in such cases, the enclosures of these instruments should also be protected by grounding.

2. SH/T 3081-2019 \"Code for Design of Grounding of Instrumentation in Petrochemical Industries (under revision)\": 4.1.4 In non-explosive environments, the metal enclosures, metal protection boxes, and metal wiring boxes of field instruments with a supply voltage below 36 V do not require protective grounding; however, those that may come into contact with equipment operating at voltages higher than 36 V must be provided with protective grounding. 4.1.5 In explosive hazardous environments, the metal enclosures, metal protection boxes, and metal wiring boxes of field instruments in non-intrinsically safe systems shall be provided with protective grounding, while those of field instruments in intrinsically safe systems need not have protective grounding. 4.1.6 The metal enclosures, metal protection boxes, and metal wiring boxes of field instruments used for lightning protection shall be provided with protective grounding. Explanation of the provision: 4.1.4 This clause pertains to field equipment. Our country once specified that the safe voltage should not exceed 36V; later, different safe voltages were set for various working environments. The current Chinese standards specify the voltage levels that will not cause harm to humans under various environmental conditions and in different fault scenarios (see GB/T 3805-2008 (Limits for Extra-Low Voltage CELV)). For ease of differentiation and implementation in engineering, this specification still limits it to 36V. 4.1.6 The field instruments referred to in this clause are those for which instrument lightning protection works are carried out in accordance with the provisions of SH/T 3164.

3. SH/T 3164-2021 \"Code for Design of Lightning Protection Engineering in Petrochemical Instrumentation Systems\": 9.3.1 The grounding of field instruments shall be protective grounding, and it shall comply with the provisions in Table 9.3.1; this can be achieved using enclosure grounding terminals or by means of natural grounding during installation.

4. SH/T 3551-2024 \"Code for Construction and Acceptance of Petrochemical Instrumentation Projects\": 10.3.1 In explosive hazardous environments, the metal enclosures, metal protection boxes, and metal wiring boxes of field instruments in non-intrinsically safe systems shall be provided with protective grounding, while those of field instruments in intrinsically safe systems need not have protective grounding. 10.3.4 The metal enclosures, metal protection boxes, and metal wiring boxes of field instruments used for lightning protection shall be provided with protective grounding.