Ye Xiangdong: The instrument does not necessarily have to be grounded

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Regarding instrument grounding, with the official implementation of the new grounding standard \"SH∕T 3081-2019 Specifications for the Grounding Design of Petrochemical Instruments\", the principles behind instrument grounding have become clearer, the issues related to it have been more clearly defined, and its implementation has become simpler. Does the instrument have to be grounded? Expert Ye Xiangdong is here to answer everyone’s questions. Instruments do not necessarily need to be grounded; the electronic circuits themselves do not require grounding. Instrument grounding is only done for the external circuits related to auxiliary functions, and it is possible for instruments to operate without being grounded. For example: mobile or portable instruments, instruments with insulated casings, etc. Auxiliary functions related to grounding can also be implemented using other methods; therefore, it is not necessary for the instrument to be grounded. Meters should not have their own separate grounding system. If a meter does have its own separate grounding system, the fault current from a 220V AC power supply must flow through the ground in order to return to the power supply source. Due to the resistance of the ground and variations in ground potential, the leakage protection device cannot function in a timely and accurate manner to provide protection. Since most instruments are powered using the TN-S wiring system, if a separate grounding device is installed for the instruments, two grounding systems will exist within the same instrument setup. Uneven potentials in the ground or backflow of potential from one of the grounding devices can cause injury to personnel, as well as instrument failures or damage. Even if the instruments are not powered using the TN-S system, it is still convenient to connect them to the electrical system’s grounding system. Therefore, the instrument should not have a separate grounding device. In intrinsically safe installations, a separate grounding is not required. To understand this, we must first start with the basic concept of intrinsically safe instruments. An intrinsically safe system consists of intrinsically safe instruments as well as instruments that are related to them in terms of intrinsic safety. Under normal operation and specified fault conditions, intrinsically safe circuits do not create conditions that could lead to an explosive atmosphere. The intrinsically safe instrument system is safe both for human safety and for preventing the formation of explosive atmospheres; therefore, intrinsically safe field instruments are not grounded. Meters that already have protective grounding do not require additional static grounding; the purpose of static grounding is to discharge charges and reduce the accumulation of static electricity. The discharge resistance of static electricity to ground is typically in the range of 10⁴Ω to 10⁶Ω, while many relevant standards and specifications specify a resistance of 100Ω for static grounding. Therefore, instruments and equipment that are equipped with protective grounding or working grounding already have the function of static discharge, and no additional anti-static grounding is required. In recent years, control devices such as DCS, PLCs, and PCs have been equipped with anti-static features as well as connectors that protect against voltage or current surges, which enables effective prevention of damage caused by static electricity. There is thus no longer a need for external anti-static protection for the finished instruments; neither do the instruments nor the control systems require such protection, making anti-static grounding less important as well. It is safe to share the instrument working ground and protection ground with the electrical grounding system; for current to flow, a circuit must be formed. To apply a voltage to something, there must be a common point; the potentials on the electrical grounding network are equal for instruments, and thus no voltage exists. Birds standing on power lines do not get electrocuted, simply because they are on a single wire – this is the principle of equipotentiality. The instrument circuit is not part of the electrical circuit or the path to ground, so the current flowing through the grounding grid in the electrical equipment cannot flow through the instruments. The level of the ground potential in the equipotential system also does not generate interference currents in the instruments. Therefore, using the same grounding device for instrument operation grounding, protective grounding, and electrical purposes is safe, reliable, and convenient. Related Reads ◆ Ye Xiangdong’s Q&A on Instrument Protection Grounding ◆ Explanations of the 16 Most Important Instrument Grounding Issues of Interest to Instrument Professionals