Why do chemical process instruments need to be grounded? ? ? ?
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1. Classification of instrument grounding: The grounding in instrument automation systems can be divided into two main categories based on their function: protective grounding and operational grounding. 1.1 Protective grounding: Also known as safety grounding, its primary purpose is to ensure the personal safety of workers and the stable operation of system equipment. These can be further divided into protective grounding (in the narrow sense), anti-static grounding, and lightning protection grounding. Normally, electrical meters and control systems have some bare metal parts that are not energized and are exposed in areas where people can easily reach them. This section will develop dangerous voltages in the event of a system failure or abnormal operation; at such times, we need to consider whether it is necessary to connect these dangerous voltages to the ground in order to prevent safety incidents. The safety grounding done for these reasons is referred to as protective grounding (in the narrow sense). All materials, whether solid or liquid, are composed of atoms arranged in various ways. An atom consists of a positively charged nucleus and negatively charged electrons outside the nucleus. If two previously uncharged substances come into contact, charge transfer usually occurs at their common boundary. When separated, each surface will carry an equal amount of charge with opposite polarity. A conductive object will become charged if placed in the electric field generated by other charged objects, or near a conductor with a high potential. Due to reasons such as the contact and separation of objects, electrostatic induction, dielectric polarization, and the adhesion of charged particles, the positive and negative charges on an object become unbalanced or the charge distribution becomes uneven, resulting in a charged appearance on a macroscopic scale. When a certain charging factor causes the rate at which static electricity is generated on an object to exceed the rate at which it dissipates, static electricity accumulates on the surface of the object. Once the energy of the static electric field reaches a certain level, it can break down the surrounding medium and lead to discharge. At this point, it becomes necessary to consider whether it is necessary to connect the static voltage to the ground in order to prevent safety accidents. The safety grounding done for these reasons is known as static grounding.
Lightning is a discharge phenomenon in the atmosphere, and there is a certain chance that it will strike the exposed parts of instruments and control systems. Powerful lightning surges can damage these instruments and equipment as well as pose a threat to human safety. At this point, we need to consider directing lightning into the ground through grounding wires to prevent safety accidents. The safety grounding done for these reasons is known as lightning protection grounding. Lightning protection grounding includes external lightning protection grounding and internal lightning protection grounding. External lightning protection grounding is the responsibility of the electrical engineering team and is outside the scope of this discussion. Internal lightning protection grounding includes the shielding grounding of cable cabinets and the grounding of surge protectors.
1.2 Working Grounding: Working grounding is implemented to ensure the accuracy and reliability of data from automatic control instruments; it can be further divided into circuit grounding, shielding grounding, and intrinsically safe grounding. Instrument system signals are divided into isolated signals and non-isolated signals: Isolated signals are in an insulated state, and their power supply is also independently isolated; therefore, there is no need for circuit grounding, such as on the circuit boards inside the instrument equipment ； Non-isolated signals require a unified signal reference point, usually the negative pole of the DC power supply. The setup of shield grounding is intended to prevent high-frequency electromagnetic interference signals from affecting the circuit signals inside the equipment, thereby avoiding unreliable data from control instruments. Typically, the cable shielding layer, shielded twisted pairs, the shielding grounding terminals on instruments, and the spare cores of ordinary multi-core cables without a shielding layer should all be grounded for shielding. Three necessary conditions are required to cause an explosion, all of which are essential: the energy needed to ignite the explosion, air or oxygen, and a combustible gas (dust). Intrinsic safety instrument systems limit the energy required for ignition: even in the event of a short circuit or electric spark, it is not sufficient to ignite the surrounding flammable and explosive gases (dusts). Intrinsic safety grounding refers to the grounding of those components of intrinsically safe instruments that require grounding for their safety functions. A safety barrier is a current-limiting and voltage-limiting device placed between intrinsically safe circuits and non-intrinsically safe circuits, to prevent dangerous energy from the non-intrinsically safe circuits from entering the intrinsically safe circuits.
Safety barriers are mainly divided into Zener safety barriers and isolated safety barriers. When a fault occurs, thanks to the reliable isolation unit inside the isolated safety barrier, a potential is generated with respect to ground; however, no current can flow from this reliable isolation unit into the hazardous area. Therefore, a dedicated intrinsically safe grounding is not required on the intrinsically safe circuit side of the safety barrier. Zener-type safety barriers should be grounded intrinsically, while isolated safety barriers do not require intrinsic grounding. Zener-type safety barriers utilize a fast fuse and a current-limiting resistor in series within the circuit loop, along with a voltage-limiting Zener diode in parallel, to limit energy flow and ensure safe energy regulation when connecting instruments in the hazardous area to those in the safe area. If a Zener-type safety barrier is not grounded, and a power distribution fault within the safe area results in a high potential with respect to ground (such as the phase wire at 220V AC) appearing across the safety barrier, the Zener diode will only limit the voltage Uo between the wires of the safety barrier; it cannot limit the potential of any single wire with respect to ground. This potential can then be introduced into the hazardous area. If the field instruments do not have proper insulation from ground, a short circuit to ground may occur, generating a strong ground current. Since such high potentials and ground currents are not restricted, sparks can easily be generated, posing a danger. If the safety barrier is properly grounded, when the same fault occurs, the Zener diode limits the potential difference with respect to ground; as a result, the fault current can only flow within safe limits, thereby ensuring safety in the hazardous area.
2. Grounding principle: In accordance with the \"SH/T 3081-2019 Code for Design of Grounding of Instrumentation in Petrochemical Industries\" and the \"HG/T 20513-2014 Code for Design of Grounding of Instrumentation Systems\", the principle for grounding design at present is to employ an independent equipotential connection structure, in order to prevent potential differences from affecting the normal operation of the instrumentation system, while also taking into account the value of the grounding resistance. Therefore, the grounding of chemical process instrument control systems should comply with the following requirements: (1) Each individual instrument, device, and control system shall be connected to the ground bus using a separate grounding wire; no form of series connection is allowed ； (2) The grounding of various instruments, equipment, and control systems should be carried out in a categorized and consolidated manner, with the ultimate connection being made to the grounding grid, that is, a structured approach to centralized grounding by categories ；
(3) The grounding of various instruments, equipment, and control systems shall, in accordance with the principle of equipotential grounding, form an equipotential grounding connection network； (4) The instrument grounding shall share the same grounding device as the electrical system grounding, and shall be connected to the electrical system’s ground plate ； (5) The grounding connection resistance of the instrument system (the resistance value between the grounding terminal and the main ground plate) should not be greater than 1Ω, and the grounding resistance of the instrument system should not be greater than 4Ω. (ISA-RP12.06 \"Implementation of Grounding for Instruments in Hazardous Areas – Part 1: Intrinsic Safety\" specifies that the connection resistance between the safety barrier grounding bus and the neutral point of the AC power supply shall be less than 1Ω. The grounding documentation proposes a method of repeatedly connecting two grounding wires in order to measure the resistance of the grounding path, rather than the resistance of the connection to ground. The grounding resistance of the instrumentation and control systems refers to that of the grounding devices in the low-voltage power distribution system of the electrical engineering department; it is determined in accordance with the relevant standards and specifications of this field, and under normal circumstances should not exceed 4Ω. The grounding resistance calculated based on the power-frequency AC current passing through the grounding electrode is called the power-frequency grounding resistance. )
3. Grounding Methods 3.1 Protective Grounding 36V is considered a safe voltage – it is a voltage level that does not cause immediate death or disability in humans, and it represents the \"safe ultra-low voltage\" for which continuous contact is permitted under normal environmental conditions. When the human body is exposed to a voltage of 36V or less, death from electric shock does not occur, regardless of the magnitude of the current. When the human body is exposed to a voltage of over 36V, a current of 90-100mA can be fatal. Therefore, field instruments, equipment, control systems, etc., powered by more than 36V must be provided with protective grounding. The correspondence between various currents and the degree of harm to the human body is shown in the table below:
In non-explosive hazardous environments, the metal enclosures of field instruments, metal protection boxes, and metal wiring boxes with a supply voltage of less than 36V do not require protective grounding； In explosive hazard environments, where the presence of flammable and explosive gases is possible, the metal enclosures, metal protective boxes, and metal wiring boxes of field instruments in non-intrinsically safe systems must be provided with protective grounding; whereas such enclosures, protective boxes, and wiring boxes for field instruments in intrinsically safe systems do not require protective grounding. Generally, anti-static grounding serves two purposes: 1. It is implemented to prevent the harmful effects of static electricity on fuel tanks, natural gas storage tanks, pipelines, and similar systems ； 2. Measures taken by the electronics industry to reduce damage caused by electrostatic discharge, etc. Since this article discusses the grounding of instrument systems, the first purpose is not covered. Although the energy generated by static electricity is very small (usually not exceeding the millijoule range), it can produce high static voltages, which may cause damage to voltage-sensitive semiconductor devices. Therefore, anti-static grounding should be provided in the control rooms where devices such as automatic control systems are installed, in the cabinet rooms, in the computer rooms for process control, as well as on the static electricity collection bars of metal and non-metal cable trays. Instrumentation equipment that already has protective grounding and working grounding does not require anti-static grounding.