The Concept of Grounding

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I. Concepts of “ground” and “grounding” 1. Ground (1) Electrical ground The earth is a substance with extremely low resistance and very high capacitance; it has the ability to absorb an infinite amount of charge, and it can maintain its potential unchanged even after absorbing large amounts of charge. Therefore, it is suitable to be used as a reference potential in electrical systems. This “ground” refers to an “electrical ground”; it is not the same as a “geographical ground,” but it is included within the concept of a “geographical ground.” “The scope of “electrically” depends on the composition of the ground structure and the condition of contact between the ground and the charged body. 　　(2) Earth potential: One or a set of conductors that are in close contact with the ground and form an electrical connection are called grounding electrodes; round steel or angle steel are commonly used, while copper rods or copper plates can also be employed. Figure 1 shows a round steel grounding electrode. When the current I flowing into the ground spreads in a hemispherical pattern toward the earth through the grounding electrode, due to the spherical shape of this hemisphere, the radius becomes smaller the closer one is to the grounding electrode and larger the farther away one is; as a result, the resistance is higher near the grounding electrode and lower farther away from it. Tests have shown that at a distance of 20 meters from a single grounding electrode or contact point, the hemispherical surface is already large enough; in practice, there is little resistance left, and no voltage drop occurs any longer. In other words, the potential there is nearly zero. This electrical ground with a potential of zero is called the “ground potential”. If the ground electrode is not a single wire but consists of multiple wires, the shielding factor increases, and the aforementioned distance of 20 meters may increase as well. The dissipation zone in Figure 1 refers to the area of soil where a significant potential gradient is generated as current flows through the grounding electrode into the ground. Geopotential refers to the soil area outside the dissipation zone. In areas where the grounding electrodes are densely distributed, it is difficult to find an electrical ground with a potential of zero. (3) Logically, the transmission of currents in various circuits within electronic devices and the conversion of information require a reference potential; this potential also helps to prevent the intrusion of external electromagnetic field signals. This potential is often referred to as the “logical level”. This “ground” does not necessarily refer to a geographical location; it could be the metal casing of electronic devices, their bases, the ground wires on printed circuit boards, or the main grounding terminals and grounding conductors within buildings ； Logically, it can be in contact with the ground or not, but “electrically” it must be in contact with the ground. 　　2. Grounding 　Connecting a certain part of an electrical power system or electrical device to a ground electrode through a grounding wire is called “grounding”. “An “electrical installation” is a combination of several interconnected electrical devices within a certain space. “\"Electrical equipment\" refers to any device used for power generation, transformation, transmission, distribution, or consumption, such as motors, transformers, electrical appliances, measuring instruments, protective devices, wiring materials, etc. In a power system, the point where grounding occurs is generally the neutral point, but it can also be a point on a phase line. The grounded part of an electrical device is the exposed conductive part. “An “exposed conductive part” is a conductive part in an electrical device that can be touched; it is not charged under normal conditions but may become charged in the event of a fault, and generally refers to the metal enclosure. Sometimes, for safety reasons, the external conductive parts of the device are connected to a ground wire for grounding. “The \"external conductive parts\" of a device can also be referred to as external conductive elements; they do not belong to the electrical equipment itself, and typically include metal pipes for water, heating, gas, and air conditioning, as well as the metal structures of buildings. External conductive parts may introduce a potential, usually the ground potential. A ground wire is a conductor connected to the grounding electrode. A grounding device refers to the collective term for grounding electrodes and grounding wires. 　　Any current exceeding the rated current is called overcurrent. The overcurrent that occurs due to faults with negligible impedance between different potential points under normal conditions is called short-circuit current; for example, the current generated by a metallic short circuit between a phase wire and the neutral wire is known as single-phase short-circuit current. The current generated by insulation failure is called fault current, and the fault current that flows into the ground is called ground fault current. When the enclosure of an electrical device is grounded and its insulation is damaged, causing the phase wire to come into contact with the metal enclosure, this condition is known as \"earth fault,\" and the current that results is called \"earth fault current.\" 　　3. Contact voltage In Figure 2, when the insulation of electrical device M is damaged and causes a short circuit to the casing, the short-circuit current flowing through the grounding electrode is Id. If the grounding resistance of the grounding electrode is Rd, then the voltage with respect to ground generated at the grounding electrode is Ud = Id·Rd; Ud is commonly referred to as the fault voltage, and the corresponding potential distribution curve is curve C in Figure 2. Under normal circumstances, the impedance of the ground wire can be ignored, and the potential appearing at M is thus Ud. When a person is in the dispersed area, as indicated by curve C, the geopotential at that location is Uφ. At this time, if a person comes into contact with M, the fault voltage generated by this contact is Ut = Ud - Uφ. A person stands on the ground, and the resistance between the shoe and sock of one foot and the ground is Rp. When the person touches M, the two feet are in parallel, and their combined resistance is Rp/2. Under the action of Ut, Rp/2 is connected in series with the human body’s resistance RB; thus, the current flowing through the human body is IB = Uf/(RB + Rp/2), and the voltage applied to the human body is Ut = IB·RB = Uf·RB/(RB + Rp/2). In this case, when the insulation of an electrical device is damaged, the contact voltage Ut that appears between the hand touching the electrical device and the feet touching the ground is related to the distance between M and the grounding electrode. As can be seen from Figure 2, as M gets closer to the ground electrode, Uφ increases, whereas Uf decreases; accordingly, Ut also decreases. When a person is outside the range of the dispersed area, Uφ = 0; at this point, Uf = Ud and Ut = Ud·RB／(RB+Rp／2), with Ut representing the maximum value. Since the position where a person stands within the exposure area is related to Uφ, the contact voltage is usually calculated under the conditions of standing 0.8 m horizontally from the electrical device and 1.8 m vertically above the electrical device with the hand in contact with it. If the electrical device is located outside the dispersion area, the aforementioned horizontal and vertical distances need not be considered when calculating the contact voltage Ut. 　　4. Step voltage 　 When a person walks in a contaminated area, as shown by curve C in Figure 2, the potential of one foot is Uφ1, while the potential of the other foot is Uφ2; thus, the fault voltage resulting from stepping is Uk = Uφ1 - Uφ2. Under the influence of Uk, the body current IB flows from the resistance Rp of one foot, through the body’s resistance RB, and then to the resistance Rp of the other foot; thus, IB = Uk／(RB + 2Rp). At this time, the voltage applied to the human body is Ut = IB·RB = Uk·RB／(RB+2p). When the insulation of an electrical device is damaged, the voltage Uk that the human body experiences under stepping conditions within the discharge area is known as step voltage. The step length of an average person is about 0.8 m; therefore, the step voltage Uk is calculated based on the potential difference over a horizontal distance of 0.8 m above the ground. As can be seen from Figure 2, the closer a person is to the ground electrode, the larger Uφ1 becomes. When one foot is on the ground electrode, Uφ1 = Ud; at this point, the fault voltage Uk generated by stepping is at its maximum value, namely Ukm in Figure 2, and correspondingly, the step voltage value is also at its maximum. Conversely, the farther a person is from the grounding electrode, the lower the step voltage. When a person is outside the discharge area, both Uφ1 and Uφ2 are equal to zero; thus Uk = 0, and no step voltage is generated. 　　5. Leakage resistance, grounding resistance, and impulse grounding resistance: The ratio of the voltage of the grounding electrode with respect to ground to the grounding current flowing into the ground through the grounding electrode is called leakage resistance. 　　The ratio of the voltage between the grounded part of the electrical equipment and ground to the grounding current is called the grounding resistance of the grounding system; it is equal to the sum of the resistance of the grounding wire and the dissipation resistance. Generally, since the resistance of the ground wire is very small and can be ignored, the ground resistance can be considered equal to the dissipation resistance. 　　To reduce the ground resistance, multiple individual grounding electrodes are often connected in parallel as a metal body to form a composite grounding electrode or a group of grounding electrodes. Since the burial distance of each individual grounding electrode is usually equal to its length and far less than 40 meters, when current flows into these individual grounding electrodes, it is restricted by one another, which hinders the dispersion of the current. In other words, it is equivalent to increasing the resistance of each individual grounding electrode. This phenomenon of the influence current dispersing is known as shielding, as shown in Figure 3. 　　Due to the shielding effect, the dissipation resistance of the grounding electrode array is not equal to the parallel value of the dissipation resistances of the individual grounding electrodes. At this time, the dissipation resistance of the grounding electrode array is Rd = Rd1／(n·η) Equation (1) Where: Rd1 —— the dissipation resistance of a single grounding electrode; n —— the number of individual grounding electrodes; η —— the utilization factor of the grounding electrodes, which depends on their shape, as well as the number and location of the individual grounding electrodes. The grounding resistance mentioned above refers to the value measured under low-frequency conditions with low current density, or the value calculated using steady-state formulas. This is very different from the operating condition of the grounding device used to introduce the lightning current during a lightning strike. Since lightning current is a very powerful shock wave, its amplitude is often as high as tens of thousands or even hundreds of thousands of amperes. As a result, the current density flowing through the grounding device increases, and it is affected by the inductance arising from the characteristics of current surges. In this case, the grounding resistance is referred to as surge grounding resistance, or simply surge resistance. Due to the increased current density flowing through the grounding device, spark discharges occur in areas such as air gaps in the soil and the air layer between the grounding electrode and the soil. This leads to a decrease in the resistivity of the soil as well as an increase in the contact area between the soil and the grounding electrode. As a result, by increasing the size of the grounding electrode, the impulse resistance value was reduced. 　　For longer strip-shaped grounding devices, due to the effect of inductance, once a certain length is exceeded, the impulse resistance no longer decreases. This critical length is known as the effective length. The lower the soil resistivity and the shorter the lightning current pulse, the shorter the effective length becomes. 　　Due to various factors, the impulse resistance of the grounding device when lightning current is introduced is a function of time. The time it takes for the lightning current in the grounding system to reach its amplitude IM is delayed compared to the time when the potential of the grounding system reaches its maximum value UM. However, in engineering, given the amplitude IM of the inrush current and the inrush resistance Rds, the amplitude UM of the inrush voltage as the inrush current flows through the grounding electrode can be calculated as UM = IM·Rds. Since, in reality, the maximum values of voltage and current occur at different times, the amplitude calculated in this way is often slightly higher than the actual amplitude; this provides a safety margin, so it remains applicable in practice. II. The functions of grounding -------------------------------------------------------------------------------- The main functions of grounding are to prevent electric shock to people, damage to equipment and circuits, fires and lightning strikes, static electricity damage, and to ensure the proper operation of power systems. The details are explained as follows. (1) Preventing electric shock to the human body 1. Mechanism of electric shock: The electric current generated by an electric shock passes through the body of a human or animal, causing pathological physiological effects such as muscle contraction, difficulty breathing, increased blood pressure, the formation of cardiac excitation waves, atrial fibrillation and brief cardiac arrest without ventricular fibrillation, ventricular fibrillation, and ultimately death; therefore, protective measures must be taken. When a person or livestock comes into contact with the live parts of electrical equipment, it is called direct contact. When a person or livestock comes into contact with a live metal enclosure in the event of a fault, it is called indirect contact. Electric shocks caused by direct and indirect contact are referred to as direct electric shock and indirect electric shock. To prevent electric shock, it is necessary to first understand the mechanism of electric shock, and then take appropriate protective measures against direct electric shock, indirect electric shock, and those that combine both, in order to ensure the safety of people, animals, and equipment. 　　(1) Composition of human body impedance: The magnitude of the electric shock current is determined by the contact voltage and the human body impedance. Human body impedance is primarily related to factors such as the current path, skin moisture level, contact voltage, duration of the current flow, contact area, contact pressure, temperature, and frequency. The composition of human body impedance is shown in Figure 4. If two electrodes are placed in two parts of the human body and the skin beneath the electrodes is removed, the impedance between these two electrodes is the internal impedance Zi of the human body. The impedance between the electrodes on the skin and the conductive tissue beneath the skin is the skin impedance ZPl and ZP2. The vector sum of Zi, ZP1, and ZP2 is the total impedance of the human body, ZT. The characteristics of these impedances are described as follows: Figure 4 Composition of human body impedance. ① Internal impedance Zi of the human body: According to IEC measurement results, Zi is primarily resistance with only a small amount of capacitance, as shown by the dashed line in Figure 4. Its value is mainly determined by the current path and generally has little relation to the contact area; however, the internal impedance increases when the contact area becomes as small as a few square millimeters. ②Skin impedance ZP1, ZP2: ZP1 and ZP2 consist of a semi-insulating layer and small conductive elements (such as resistance-capacitance networks formed by pores), as shown in Figure 4. When the contact voltage is 50 V or less, the skin impedance value changes significantly depending on factors such as the surface contact area, temperature, and breathing ； At 50–100 V, skin impedance decreases significantly ； As the frequency increases, skin impedance decreases as well ； When the skin is damaged, its impedance can be considered negligible. ③ Total human impedance ZT: ZT consists of resistive and capacitive components. When the contact voltage is 500 V or less, the ZT value is primarily determined by the skin impedance value ； The higher the contact voltage, the less the relationship between ZT and skin impedance ； When the skin is damaged, the ZT value approaches the impedance of the human body. ④The initial resistance of the human body, Ri, is the resistance value at the moment when contact voltage appears; at this point, the capacitance of the human body has not yet been charged and the skin impedance can be disregarded. This resistance value is referred to as the initial resistance of the human body. This value limits the peak of the short-duration pulsed current. When the current path is from hand to hand or from hand to foot and the contact area is large, the 5% distribution rank (i.e., the minimum initial resistance value observed in 5% of people) Z5% can be considered to be equal to 500Ω. 2) The relationship between human body impedance and contact conditions is generally classified into the following three categories: ① Condition 1: Dry or moist areas, dry skin, and high-resistance surfaces; in this case, the human body impedance value is: Z1 = 1000 + 0.5Z5% (Ω), where 1000 represents the random value of the resistance between shoes/socks and the ground, in ohms; 0.5 takes into account the dual contact between both hands and both feet; Z5% refers to the 5% distribution rank, meaning that 5% of people exhibit this minimum resistance value, in ohms. ② Condition 2: Moist areas, moist skin, and low-resistance surfaces; in this case, the human body impedance value is: Z2 = 200 = 200 + 0.55% (Ω) ； 200──Lower value of surface resistance, ignoring the resistance of shoes and socks, Ω 　③ Condition 3: When submerged in water, the resistance of the skin and that of the surrounding medium can be ignored. 　　The safe voltage values under various conditions are specified differently by different countries, as shown in Table 1. 　　Table 1 shows the safe voltage for alternating current. IEC specifies that the safe voltage for direct current (without ripple) is: in Condition 1, not more than 120V ； In Condition 2, it is not greater than 60V. Safe voltage includes the voltage relative to ground in grounded systems, or the voltage between phases and between poles in ungrounded and imperfectly grounded systems. 　　 2. Electric shock effect 　　(1) Electric shock effect of alternating current 　　After years of experimental research, IEC believes that ventricular fibrillation is the main cause of death resulting from electric shocks. A cardiac cycle, as shown in Figure 5, consists of the excitation phase P, the excitation propagation phase R, and the excitation recovery phase T. The numbers in Figure 5 indicate the order of excitation propagation. During the recovery period from excitation, there is a relatively small phase known as the vulnerable period. During this phase, the cardiac muscle fibers are in a state of uneven excitation; if stimulated by an electric current of sufficient amplitude, the ventricular fibers experience fibrillation, as shown at point X in Figure 6. The effects on the electrocardiogram and blood pressure are illustrated by the curves in Figure 6. At this point, ventricular fibrillation and a drop in blood pressure occur; if the electric current is strong enough, it can lead to death. 　　The minimum current value that a person can detect when current flows through the body is called the perception threshold. For AC currents between 15 and 100 Hz, this value is 0.5 mA. The maximum current that a person can withstand while holding an electrode is known as the let-go current; for alternating currents in the range of 15–100 Hz, this value is 10 mA. As the current flowing through the human body continues to increase, the relationship between the body current IB and the duration of time t during which the current flows is shown in Figure 7. Figure 7 shows the effects along the path of current flowing through the human body, from the left hand to the feet. When the current is 500mA and the time is 100ms, the probability of ventricular fibrillation occurring is 14%. Region I in Figure 7 generally shows no reactive effect ； Zone II generally has no harmful physiological effects ； Zone III usually shows no organic damage, but muscle contractions and breathing difficulties may occur. There are reversible disturbances in the generation of excitation waves and their conduction in the heart, including atrial fibrillation and brief cardiac arrest ； In zone IV, ventricular fibrillation begins to appear; by curve c1, the probability is 5% ； By curve c2, the probability is 50% ； Outside curve c3, the probability exceeds 50%. As the current and time increase, cardiac arrest, respiratory failure, and severe burns may occur. 　The current in Figure 7 is the current flowing along the path \"from the left hand to the feet\"; for other paths, it is calculated using the following formula: IB = Iref/F　　　　　　　　　(2) Where: IB – the current flowing through the human body along other paths, in mA; Iref – the current flowing along the path \"from the left hand to the feet\", in mA; F – the cardiac current coefficient, as shown in Table 2. The sensory threshold, liberation threshold, and ventricular fibrillation threshold shown in Figure 7 all apply to alternating current frequencies of 15–100 Hz. 　　In industrial enterprises and civil buildings, many electrical devices are used at frequencies exceeding 100Hz; for example, some power tools and welding machines can operate at 450Hz ； Most electrotherapy devices use 4000–5000Hz ； Devices powered by switching mode operate at 20kHz to 1MHz ； Microwave and radio equipment also use higher frequencies. For these alternating current frequencies above 100 Hz, the impedance of human skin, at contact voltages in the tens of volts range, is roughly inversely proportional to the frequency; for example, at 500 Hz the skin impedance is only about 1/10 of that at 50 Hz. In many cases, the skin impedance can be disregarded. However, because it is a high-frequency current, its effect on the human body and on the heart is less than that of alternating currents below 100 Hz. To compare with the threshold at 50Hz, the frequency coefficient Ff is commonly used as a measure; it is the ratio of the threshold current required to produce a corresponding physiological effect at frequency f to the threshold current at 50Hz. For frequencies from above 100 Hz up to 1000 Hz, the frequency coefficients of the sensory threshold and the release threshold are shown in Figure 8 ； When the electric shock lasts longer than the cardiac cycle and a longitudinal current flows through the human torso, the frequency coefficient of the ventricular fibrillation threshold is shown in Figure 9. There is no experimental data available when the duration of electric shock is less than the cardiac cycle. The frequency coefficients for the threshold of perception and the threshold for escape of alternating current at frequencies above 1000 Hz up to 10000 Hz are shown in Figure 10 ； The frequency coefficient for the ventricular fibrillation threshold is still under consideration by the IEC. When the frequency is between 10 kHz and 100 Hz, the threshold rises roughly from 10 mA to 100 mA (RMS). At frequencies above 100 kHz and with current strengths in the range of several hundred milliamps, there is a pricking sensation at lower frequencies, while at higher frequencies a warming sensation occurs. At frequencies above 100 kHz, there are neither experimental data showing a departure from the threshold nor any accident reports in this regard. Burns can occur when the frequency is above 100 kHz and the current is in the ampere range; the severity of the burns depends on the duration for which current flows. 　　(2) Electric shock effect of direct current: The effects of electric current on the human body, such as stimulating nerves and muscles and causing atrial or ventricular fibrillation, are related to changes in the magnitude of the current, especially when the current is turned on or off. To produce the same effect, a direct current with a constant amplitude requires a much larger value than an alternating current. Holding a DC electrical device makes it easier to get free in case of an accident ； When the electrical shock lasts longer than the cardiac cycle, the threshold for ventricular fibrillation is much higher than that for alternating current. The direct current that flows from the hands to the feet, passing through the human torso, is called a longitudinal current ； The current that flows from hand to hand through the human torso is called a lateral current ； With both feet as the positive poles, the current flowing through the body is an upward current ； With the feet as the negative pole, the current flowing through the body is a downward current. The ratio of direct current to the equivalent alternating current (r.m.s.) with the same probability of inducing ventricular fibrillation is called the DC/AC equivalence factor. 　The relationship between the duration of the direct current and its amplitude is shown in Figure 11. Region I in the figure usually shows no reactive effect ； Zone II generally has no harmful physiological effects ； In Zone III, no organ damage is generally expected; the severity increases with the amplitude and duration of the current, and reversible disturbances in the formation and conduction of excitation waves in the heart may occur ； Ventricular fibrillation may occur in zone IV; as the amplitude and duration of the current increase, in addition to the effects seen in zone III, pathological and physiological effects such as severe burns are expected to occur. Regarding ventricular fibrillation, the diagram shows the current flowing from the left hand to both feet, which is the effect of an upward current. If it is a downward current, the current should be multiplied by a factor of 2 for conversion. When current flows from hand to hand, ventricular fibrillation is unlikely to occur. In this graph, when the duration for which current flows is less than 500 ms, there is no data regarding the boundary line between regions II and III. 　The perceptual threshold for direct current depends on factors such as the contact area, the condition of the contact (dryness/wetness, pressure, temperature), the duration for which the current flows, and individual physiological characteristics. Unlike alternating current, when a current flows through the human body at a strength corresponding to the perceptual threshold, sensation is only experienced when the current is turned on and off; there is no sensation at other times. Under conditions equivalent to those for measuring the perceptual threshold of alternating current, the perceptual threshold for direct current is approximately 2 mA. 　　The threshold for escaping from direct current differs from that of alternating current; for direct currents below about 300 mA, there is no definite threshold for escape, and painful, spastic muscle contractions occur only when the current is turned on or off. When the current exceeds 300mA, it may be impossible to get free, or freedom might only become possible after the electric shock lasts for a few seconds or minutes. 　　When the current passing through the human body is around 30mA, a warm sensation is felt in the limbs. When a transverse current of 300 mA or less flows through the body for several minutes, reversible cardiac rhythm disorders may occur as time and current levels increase. Electric current burns, scalds, dizziness, and sometimes loss of sensation; when the current exceeds 300mA, loss of sensation often occurs. 　　(3) Shock effects of currents with special waveforms (3) Shock effects of currents with special waveforms. There are several types of currents with special waveforms used in electrical equipment in industrial enterprises and residential buildings; the shock effects on the human body for each type are described as follows: ① Effects of alternating current with a DC component. The waveforms of standard alternating current and direct current are shown in Figures 12(a) and (b), while the waveform of alternating current with a DC component is shown in Figure 12(c). The waveforms of common half-wave rectification and full-wave rectification are shown in Figures 13(a) and (b). After rectification, the perception threshold and escape threshold of the AC waveform shown in Figure 13 depend on the contact area between the human body and the electrodes, the conditions of contact (humidity, pressure, temperature), as well as individual physiological characteristics; these thresholds are still under consideration by the IEC. 　When discussing the ventricular fibrillation threshold, it is necessary to distinguish between the following current values: Irms is the effective value of the current in the composite waveform ； Ip is the peak value of the synthesized waveform current ； Ipp is the peak-to-peak value of the synthesized waveform current ； Iev represents the effective value of the sine current that generates a risk equivalent to that of the waveform involved in ventricular fibrillation; this value is used to replace the human body current IB shown in Figures 7 and 11 in order to estimate the risk of ventricular fibrillation. 　When the duration of the electric shock is greater than 1.5 times the cardiac cycle, Iev = Ipp/√2; when the duration is less than 0.75 times the cardiac cycle, Iev = Ip/√2. The above relationship becomes less applicable as the ratio of AC to DC decreases. For direct current shocks with a duration of less than 0.1 s, the threshold is equal to the corresponding current value in Figure 11. 　　When the duration of the electric shock is between 0.75 and 1.5 times the cardiac cycle, the value parameter changes from the peak value to the inter-peak value; IEC believes that further research is needed for this transition process. 　　As shown in the waveforms of half-wave and full-wave rectification in Figure 13, since the current peak value is equal to its peak-to-peak value, when the duration of the electric shock is more than 1.5 times the cardiac cycle and less than 0.75 times the cardiac cycle, Iev is respectively Ipp/(2√2) = Ip/(2√2) and Ipp/√2 = Ip/√2. As shown in Figure 13, for half-wave rectification, Irms = Ip/2, while for full-wave rectification it is Ip/√2. Therefore, for half-wave rectification, the Iev values are Irms/√2 and √2Irms respectively ； During full-wave rectification, the Iev values are Irms/2 and Irms respectively. 　　② Effects of AC current with phase control: Generally, the waveforms of AC current with corresponding control are divided into symmetric control and asymmetric control, as shown in (a) and (b) of Figure 14 respectively. 　　The effects of this waveform current in producing sensations and preventing escape are roughly the same as those of a pure alternating current with the same Ip. When the phase control angle is above 120°, the peak value increases as the duration of current flow decreases. 　　For symmetric control: when the duration of the electric shock is greater than 1.5 times the cardiac cycle. Iev has the same effective value as the corresponding waveform current involved ； When the duration of the electric shock is less than 0.75 times the cardiac cycle, Iev represents the effective value with the same peak current as that of the corresponding waveform current; if the phase control angle is above 120°, the threshold for ventricular fibrillation increases ； When the shock duration is between 0.75 and 1.5 times the cardiac cycle, Iev changes from its peak value to its effective value; IEC believes that this transition process requires further research. 　　For asymmetric control, the current it generates may also have a DC component. When the duration of the electric shock is more than 1.5 times the cardiac cycle, IEC is still considering this issue ； When the duration of the electric shock is less than 0.75 times the cardiac cycle, Iev is the effective value having the same peak current as the corresponding waveform current involved. When the phase control angle is above 120°, the threshold for ventricular fibrillation increases. 　③Effects of alternating current with multi-period control. The waveform of the alternating current with multi-period control is shown in Figure 15. ts is the conduction time. tp is the non-conduction time, and ts+tp is the operating cycle. p = ts/(ts+tp) is the degree of power control. I1rms is the effective value of the current during conduction, that is, Ip/√2 ； I2rms is the effective value of the current over one operating cycle, that is, I1rms√p. 　　The sensory threshold and release threshold are still under consideration by the IEC. 　　Ventricular fibrillation threshold: IEC conducted experiments on piglets, and the results are shown in Figure 16; these can serve as a reference for humans. When the duration of the electric shock is greater than 1.5 times the cardiac cycle, the threshold depends on p. When p approaches 1, Iev has the same effective value as that of a sinusoidal alternating current with the same duration. When p is close to 0.1, I1rms is equal to the threshold for alternating currents with a duration of less than 0.75 times the cardiac cycle. When p takes values between 1 and 0.1, as shown in Figure 16, the current flowing through the human body gradually increases, until the effective value of fiber I1rms becomes equal to that of a sinusoidal alternating current with the same duration. 　　④ Effects of short-duration unidirectional single-pulse currents: Rectangular or sine-shaped pulses can be generated when the insulation of electrical appliances containing electronic components is damaged or when there is direct contact with their live parts, as shown in Figures 17(a) and (b) ； The short-duration unidirectional pulse resulting from capacitor discharge is shown in Figure 17(c). When their duration is 10 ms or more, the effects of these pulses on the human body are the same as those shown in Figure 7 ； For pulses with a duration of 0.1 ms to 10 ms, their effect is characterized by the following energy rate. 　　Ventricular fibrillation energy rate Fe: Under given conditions such as the current path and cardiac phase (the relationship between the amplitude of heartbeats and time), it represents the minimum I2t value of a short-duration unidirectional pulse that can cause ventricular fibrillation with a certain probability; expressed in integral form as Fe = ∫0^t i² dt. Multiplying Fe by the human body’s resistance yields the energy dissipated in the body during the pulse. 　　Ventricular fibrillation charge rate Fq: Under given current paths and equal heart conditions, it represents the minimum It value of a short-duration unidirectional pulse that can induce ventricular fibrillation with a certain probability; expressed in integral form as Fq = ∫0tiidt. Take capacitor discharge as an example. The time interval from the start of discharge until the discharge current drops to 5% of its peak value is the duration of the electrical shock caused by capacitor discharge, denoted as t1. The time required for it to decay exponentially to 1/e = 0.3679 times the initial amplitude is the time constant T. When ti = 3T, almost all pulse energy is exhausted. 　　The sensory threshold and pain threshold for capacitor discharge depend on the shape of the electrodes, the charge of the pulse, and its current peak. Figure 18 shows the sensory threshold and pain threshold for a person holding a large electrode with dry hands as the discharge target. The pain threshold is the level at which a person feels pain similar to that caused by a bee sting or a cigarette burn. The pain threshold expressed in terms of energy rate Fe is on the order of (50–100)×10-6 A2s for current paths passing through the hands and feet as well as those with a large contact area (in Figure 18, if the right side of the figure is considered east, then capacitance C is measured along the diagonal pointing northeast, while energy W is measured along the diagonal pointing northwest). If the charging voltage is known to be 100V and the capacitance is 100nF, then at the intersection point K of these two lines, it can be determined that the charge of the pulse is 10μC and its energy is 0.5mJ. 　　The ventricular fibrillation threshold depends on the shape, duration, and amplitude of the pulsed current, the cardiac phase at the start of the pulse, the path of the current through the body, and the physiological characteristics of the individual. 　　 The IEC conducted experiments on animals, and the results showed that for pulses of short duration, ventricular fibrillation generally occurs only when the pulse falls during a vulnerable period of the cardiac cycle ； For unidirectional pulses with a shock duration of less than 10 ms, the occurrence of ventricular fibrillation is determined by Fq or Fe. Figure 19 shows the threshold for ventricular fibrillation; for a 50% probability of fibrillation, Fq is 0.005As, while Fe increases from 0.01A2s at a pulse duration of t1 = 4ms to 0.02A2s at t1 = 1ms. This curve shows the probability of ventricular fibrillation risk for currents flowing from the left hand to both feet along a given path. For other current paths, the cardiac current coefficient F from Table 2 is multiplied. Below the c1 curve in the graph, there is no fibrous movement ； From above curve c1 to below curve c2, the risk of ventricular fibrillation is low, with a probability as low as 5% ； From above the C2 curve to below the C3 curve, there is a moderate risk of fibrillary tremor, with a probability of up to 50% ； Above the c3 curve, there is a high risk of fibrillary tremor, with a probability of over 50%. 　　The energy rate Fe of fibrillation for various types of pulses can be calculated using the following formulas: For rectangular pulses: Fe = IDC²ti. For sine waves: Fe = (IAC(p)²/2)ti = IAC(rms)²ti. For capacitor discharge with a time constant of T: Fe = IC(p)²(T/2) = IC(rms)²ti. The current parameters in these formulas can be determined from Figure 17: IDC represents the magnitude of the current in rectangular pulses, IAC(p) is the peak value of the current in sine waves, IAC(rms) is the effective value of the current in sine waves, IC(p) is the peak value of the current during capacitor discharge, and IC(rms) is the effective value of the current during capacitor discharge over a duration of 3T. Rectangular pulses, sine waves, and capacitor discharges with the same ventricular fibrillation energy rate and the same shock duration are shown in Figure 20. 　　Defined by Fe, the value of Fe1 for capacitive discharge is given by Fe1 = IC(p)²∫₀^∞e^-2t/T dt = IC(p)²(T/2). For rectangular and sine pulses, the values of Fe2 and Fe3 are respectively Fe2 = IDC²/3T and Fe3 = IC(rms)²/3T. Since Fe1 = Fe2 = Fe3, it follows that IC(p)²(T/2) = IDC²/3T = IC(rms)²/3T; hence IC(p)/(1/√6) = IC(rms) = IDC. By using this relationship to convert IDC and IC(rms) into their corresponding IC(p)/(1/√6) values, the corresponding IC(p) values can be used to determine the ventricular fibrillation thresholds for rectangular and sine pulses in Figure 19. 3. Protection measures against direct electric shock. Protection against direct electric shock, also known as protection during normal operation or basic protection, is aimed at preventing direct contact with live parts; the following measures are generally taken. 　　(1) Insulate the live parts – Completely cover the live sections with insulation. The type of insulation must comply with the standards for the corresponding electrical equipment, and it can only be removed after mechanical damage occurs. The insulating capacity must meet the requirements of withstanding mechanical, chemical, electrical, and thermal stresses encountered during operation over the long term. Ordinary paints, varnishes, and spray paints do not meet the requirements. The insulation used during installation must also be tested, and it can only be used after it is proven to meet the requirements. 　　(2) Protection using remote barriers and external guards. The external guards are generally the enclosures of electrical equipment, and they are components that provide direct contact protection in all directions. The barrier provides direct contact protection only against any direction from which it is frequently approached. The protection requirements for both are as follows: ① Minimum protection requirements – In the electrical operation area, the protection level is IP2X, while it is IP4X at the top. In electrical operation areas, if there is no potential difference between the live parts that can be touched simultaneously, the protection rating can be IP1X. Protection may not be provided in enclosed electrical operation areas. 　　② Strength and finish quality: The barriers or protective covers should be firmly fixed in place; their material, size, and installation method must provide sufficient stability and durability, as well as the ability to withstand the stresses and strains that may occur during normal use. 　　③ To enable removal, a key or tool must be used, and an interlock device should be installed; that is, when the barrier or protective cover is opened or removed, the power supply to all live parts that might be accidentally touched is automatically cut off, and the power supply is not restored until the barrier or protective cover is returned to its original position. Where energy storage devices such as capacitors or cable systems are present in barriers or protective enclosures and may pose a danger, it is necessary not only to discharge the energy within the specified time frame, but also to use interlock devices that meet the same requirements as those mentioned above. An isolation mesh cover can also be inserted between the live parts and the fences or protective enclosures, so that the live parts are not touched when the fences or protective enclosures are opened or removed. The mesh cover can be fixed, or it can slide in automatically when the barriers and protective covers are removed. The mesh protection rating must be at least IP2X, and it can only be removed using a key and tools. If large holes are left in the enclosures and barriers due to the need to replace bulbs or fuses, appropriate measures must be taken to prevent people and animals from accidentally coming into contact with live parts. Additionally, clear signs must be installed to warn that touching the live parts through these holes is dangerous. 　　(3) Protection with barriers: Barriers can only prevent accidental contact with live parts, but not intentional contact by people. For example, protective barriers, railings, or partitions can prevent people from unintentionally approaching live parts. Similarly, mesh covers or the protective handles of fuses can prevent accidental contact with live parts while operating electrical equipment. The barrier can be removed without a key or tools, but it must be secured to prevent it from being moved unintentionally. 　　(4) Place outside the reach range. The reach range is shown in Figure 21. Placing the live parts outside the reach of the arm can prevent accidental contact. Parts that can be touched simultaneously at different potentials must not be placed within the reach of the extended arm. If the two parts are less than 2.5m apart, they are considered to be reachable simultaneously. When people’s normal range of movement S is restricted by a barrier with a protection level lower than IP2X (such as a railing), the specified distance shall be measured from that barrier. In areas where a large or long conductive object must be held during normal operation, the dimensions of that object must be taken into account when calculating the distance. 　　(5) Use an RCD (Residual Current Device, also known as a leakage circuit breaker) as additional protection. An RCD cannot be used as the sole protection against direct electric shock; it can only serve as additional protection, that is, as an extra safeguard against electric shock in cases where other protections fail or due to user negligence. The setting value for residual current operation is generally 30mA. 　 4. Protective measures against indirect electric shock. Protection against indirect electric shock is also known as protection against electric shock under fault conditions, or as additional protection. The following measures are generally adopted: (1) Automatic power disconnection. In the event of a fault, when the duration of the maximum electric shock current exceeds the allowable limit, the power supply is automatically disconnected (except in the case of the first fault in an IT system), thereby preventing the electric shock current from causing harmful physiological effects. The prerequisite for using this method is that the exposed conductive parts of electrical equipment must be connected to the protective wire in accordance with the system’s grounding scheme; moreover, main equipotential bonding is also advisable. The automatic power cut method makes the most of existing overcurrent protection devices; it is simple to implement and requires minimal investment, making it a commonly used approach. 　　(2) Use Class II equipment or adopt protection with equivalent insulation. Class II equipment has both basic insulation as well as double insulation or reinforced insulation ； Without considering the protective grounding method ； The conductive parts inside the equipment must not be connected to the protective wire. The insulating covering of such equipment must be able to withstand possible mechanical, electrical, or thermal stresses; coatings made of ordinary paint, varnish, and similar materials do not meet the requirements. Bolts made of any non-insulating material are strictly prohibited on the insulating outer cover to prevent damage to its insulation. 　　(3) Use in non-conductive areas: In non-conductive areas, protective wires are strictly prohibited, and grounding measures are not taken either; therefore, Class 0 equipment can be used (such equipment has only basic insulation and no means of protective grounding). Non-conductive areas should have insulated floors and walls (for equipment with a nominal voltage not exceeding 500V, the insulation resistance should be no less than 50kΩ) ； If the nominal voltage exceeds 500V (in which case it is 100kΩ), the protective measures are as follows: ① The distance between exposed conductive parts, as well as between exposed conductive parts and external conductive parts, must be no less than 2m ； If it is outside the reach range, it is 1.25m. 　　② If the above distance cannot be achieved, an insulating barrier shall be placed between the two conductive parts, ensuring that the distance across the barrier is not less than 2 m. 　　③ Insulate the external conductive parts; the insulating material must have sufficient mechanical strength and be able to withstand a voltage of 2000V, with a leakage current of no more than 1mA under normal conditions. 　　The aforementioned arrangement must be permanent; even when using portable or mobile devices, the above requirements must still be met ； Additionally, measures should be taken to ensure that walls and floors do not lose their original resistance value due to moisture, and that external conductive parts do not allow electrical potential to be introduced from the outside. 　　(4) Ungrounded local equipotential bonding: For all exposed conductive parts and external conductive parts that can be touched simultaneously, an equipotential bonding system that is not connected to the ground is used to bring their potentials close to each other, thereby preventing electric shock. Local equipotential bonding systems must not come into contact with the ground through exposed or external conductive parts; if this condition cannot be met, measures for automatic power disconnection must be employed. To prevent people entering equipotential areas from being exposed to dangerous potential differences, measures must be taken to reduce the potential difference at the point where the earth-insulated conductive floor meets the ungrounded equipotential bonding system. 　　(5) Electrical isolation: Electrical isolation of a circuit is carried out to prevent electric shock currents from occurring when touching exposed conductive parts with damaged insulation. The following measures are generally taken: ① The circuit must be powered by an isolation transformer or a generator with multiple equivalent isolation windings, and the power supply equipment must be of Class II or have insulation equivalent to that level. If such a power supply device powers several electrical devices, the exposed conductive parts of these devices must not be connected to the metal casing of the power supply device. 　　② The voltage of this circuit must not exceed 500V; its live parts must not be connected to other circuits or to the ground, and care must be taken to maintain insulation from the ground. Such electrical isolation is also required between the live parts of electrical devices such as relays, contactors, and auxiliary switches and any part of other circuits. 　　③ Different circuits should be wired separately; if separation is not possible, multi-core cables without a metal sheath must be used, or insulated wires should be installed in insulated conduits or trunking. The rated voltage of these cables or wires is not lower than the highest voltage that may occur, and each circuit is equipped with overcurrent protection. 　　④ The exposed conductive parts of the isolated circuit must be connected to an insulated, ungrounded equipotential connection; such a connection cable must not be connected to the protective wires or exposed conductive parts of other circuits, nor to any external conductive parts. The socket must have protective terminals that are connected to the equipotential bonding system. Flexible cables must also have a protective core wire for equipotential bonding (except for cables supplying Class II equipment). 　　⑤ In the event of a fault that affects two exposed conductive parts connected to wires of different phases, there must be a protective device capable of automatically cutting off the power supply. 5. Measures to prevent both direct and indirect electric shock To provide protection against both direct and indirect electric shock, that is, protection against such shocks under normal operating conditions as well as in case of faults, the following measures can be taken. 　　(1) The nominal voltage used for safe voltage shall not exceed 50V; if a neutral conductor is provided, the insulation of the neutral wire shall be the same as that of the phase wires. 　　(2) Powered by a safe power supply. There are several types of safe power supplies: ① Safe isolation transformers, in which the primary and secondary windings should preferably be isolated by a grounded shield. 　　② Electrochemical power sources, such as batteries. 　　③ Other power sources that are not related to higher-voltage circuits, such as diesel generators. 　　④ Electronic devices manufactured to standard specifications ensure that in the event of an internal fault, the terminal voltage does not exceed 50V; or although the terminal voltage may exceed 50V, the amount of electrical energy is so small that the voltage drops below 50V as soon as a person comes into contact with the terminals. 　　(3) Circuit configuration ① The live parts at safe voltage must not be connected to the ground, the live parts of other circuits, or the protective wire. 　　② The wires of the safe voltage circuit are isolated from those of other circuits, with this isolation having a strength not lower than that of the insulation between the input and output coils of the safety transformer. If isolation is not possible, the wires of safety voltage circuits must be equipped with a sealed non-metallic sheath in addition to the basic insulation; wires from circuits with different voltages must be separated by a grounded metal shield or metal sheath. If the wires of the safe voltage circuit are in the same cable or combined conductor as those of other voltage circuits, the wires of the safe voltage circuit must be insulated separately or collectively according to the highest voltage. 　　③ Plugs for safe voltage must not be inserted into sockets designed for other voltages, and sockets for safe voltage must not be plugged in by plugs from other power sources; moreover, protective contacts are required. 　　④ When the standard voltage exceeds 25V, a properly functioning shock protection system must use IP2X-rated guards or enclosures, or be covered with insulation that can withstand a voltage of 500V for 1 minute without breaking down. 　　 6. Grounding methods to prevent electric shock These involve establishing a good metallic connection between the metal parts of electrical equipment that are not charged under normal conditions and the grounding electrode, in order to protect human safety. 　　As can be seen from Figure 22, when the insulation at a certain point of an electrical device is damaged, the enclosure becomes charged. Due to the grounding of the power supply neutral point, even if the equipment is not grounded, the presence of capacitance between the wires and the ground, or poor insulation somewhere along the wires, can cause current to flow through the human body if it comes into contact with the outer shell of such an electrically faulty device. This results in the risk of electric shock. 　　Figure 23 shows an electrical device with a grounding device. When the insulation is damaged and the enclosure becomes charged, the grounding current Id will flow through both the grounding electrode and the human body. The current flowing through each path will be inversely proportional to the value of its resistance, with the currents being Id and IB respectively. That is, IB/Id’ = Rd/RB　　　　　　　(Equation 3) Where: Id’ – the current flowing through the grounding electrode; IB – the current flowing through the human body; RB – the resistance of the human body; Rd – the grounding resistance of the grounding electrode. It can be seen from Equation 3 that the lower the resistance of the grounding electrode, the lower the current flowing through the human body. Typically, the resistance of the human body is hundreds of times greater than that of the grounding electrode, so the current flowing through the human body is also hundreds of times smaller than the current flowing through the grounding electrode. When the ground resistance is extremely low, the current flowing through the human body is almost zero; that is, IB ≈ 0, and Id’ ≈ Id. Thus, the human body can avoid the risk of electric shock. 　　Therefore, whether during construction or operation, in any season of the year, it is necessary to ensure that the grounding resistance does not exceed the value specified in the design or regulations, in order to avoid the risk of electric shock. (II) Ensuring the normal operation of the electrical system The grounding of power systems is generally neutral point grounding. The grounding resistance of the neutral point is very low, so the potential between the neutral point and ground is close to zero. When the phase wire comes into contact with the casing or ground, the voltages of the other two phases with respect to ground increase to √3 times the phase voltage in a system with an insulated neutral point ； In a system with a grounded neutral point, the voltage is close to the phase voltage, which facilitates stable operation of the system and prevents oscillations. Moreover, since the insulation levels of electrical equipment and circuits in such a system need to be designed based on the phase voltage, this reduces the manufacturing costs of electrical equipment as well as the construction costs of the circuits. Thanks to the grounding wire at the neutral point, the reliability of relay protection can also be ensured. 　　Communication systems generally use positive grounding to prevent noise from intruding and ensure the proper operation of communication equipment. 　　Electronic circuits require a stable reference point to function properly, and therefore must also be grounded. (III) Preventing the hazards of lightning strikes and static electricity: Lightning strikes generate static and electromagnetic induction, while static electricity arising from friction during the production and transportation of materials can all pose risks of electric shock or fire. 　　The harm caused by direct lightning strikes is greater than that of induced lightning, and such incidents occur more frequently as well. Therefore, to prevent direct lightning strikes, lightning protection devices must be installed. 　　For all lightning protection devices and measures to prevent static electricity hazards, the primary method is to install grounding systems. Its functions are outlined as follows: 1. Direct lightning strikes. During hot weather, there are often many thunderclouds in the sky. For example, when a thundercloud carrying a positive charge approaches the ground, negative charges are induced in the surrounding ground, especially on tall buildings that protrude above the ground. When the charge density accumulated on the ground and buildings is very high, and the thundercloud is very close to the ground or buildings, intense discharge phenomena occur. This is what is commonly referred to as lightning strike. The destructive power of lightning strikes is very great. It not only kills humans and animals, burns or uproots trees, but also damages buildings and can even cause fires and explosions. 　　To prevent direct lightning strikes, lightning rods or lightning conductors are often installed on the top of buildings. Both lightning rods and lightning conductors are connected to the grounding device via down conductors, ensuring a good connection to the ground. In this way, when thunderclouds appear in the area above a building, the opposite charges generated on the ground flow through the grounding devices, down conductors, and lightning rods or lightning strips into the atmosphere, where they neutralize the charges in the thunderclouds, thereby preventing large-scale, intense discharges from occurring. This prevents lightning strikes from occurring. 　　Experience with lightning protection devices shows that they must be properly designed and installed correctly. Otherwise, not only will it fail to provide protection against lightning, but it may even make the structure more susceptible to lightning damage. This is because a backstroke can be generated during a lightning strike. A counterattack can cause damage to the insulation of electrical equipment, burn through metal pipes, and even lead to fires, explosions, and injuries. 　　The high potential generated by the lightning protection grounding system may discharge onto metal pipes and cables underground in buildings, resulting in electrical backflow. Therefore, a certain distance must be maintained between the lightning protection grounding system and such underground metal pipes and cables, or they must be equipotentially connected. 　　2. Electrostatic induction lightning: When a metal roof or other conductor is placed in the electric field formed between a thundercloud and the ground, a large amount of charge with an opposite polarity to that of the thundercloud is induced on the roof or conductor. After a thundercloud discharge, the electric field between the cloud and the ground disappears. The charges on conductors and rooftops do not have time to dissipate immediately, which results in very high electrostatic induction overvoltages relative to the ground, and this can lead to fires or explosions. 　　To prevent the hazards caused by electrostatic induction overvoltage, the metal roofs of buildings, as well as all large metal objects inside them such as steel roof trusses, reinforced concrete columns, metal pipes, and water tanks, must be properly grounded. This ensures that the static charges generated by induction are quickly conducted into the ground, preventing them from accumulating. This prevents the generation of electrostatic induction overvoltage. 　　3. Electromagnetic induction lightning: Since lightning strikes generate lightning currents with very large amplitudes and steep gradients, a strong alternating electromagnetic field is created in the space surrounding them. A conductor placed in this electromagnetic field will induce a very high potential. If a conductor happens to form a closed loop with a small gap, spark discharge will occur at that gap. 　　The phenomenon of electromagnetic induction can also induce an induced current in metal objects that form a closed circuit. If the conductors between circuits are in poor contact, local heating will occur. This is very dangerous for buildings that store flammable or explosive substances. To prevent the adverse effects caused by electromagnetic induction, all metal objects that are close to each other, such as metal equipment, pipes, and metal structures, should be properly connected using metal wires, and they must also be well connected to the grounding system. 4. Static electricity Physics has taught us that when two different materials come into contact and rub against each other, static charges can be generated. This phenomenon of triboelectricity is also commonly encountered in daily life. For example, on very dry days, when using a plastic comb to brush one’s hair, the hair tends to float away and not stay attached to the comb; this is because friction between the comb and the hair generates electricity. 　　In production, this phenomenon of triboelectricity is even more common. For example, in industrial enterprises, when belt drives or rubber conveyor devices made of non-conductive rubber are in use ； When stirring substances in various mixers ； When a material is processed using rollers or rolling mills ； When shaking a liquid or transferring it from one container to another ； When liquid flows at high speeds in pipes..., static electricity often arises due to friction. These charges accumulate not only on pipes, containers, and tanks but also on processing equipment, resulting in very high potentials that pose a risk to human safety as well as to the equipment and buildings. 　　To prevent the risk of spark discharge caused by the accumulation of static electricity, there are many protective measures that can be taken. But the simplest and most reliable measure is to ground the equipment, pipes, containers, etc., that may generate or accumulate static electricity, so that any static charge generated is conducted into the ground, thereby eliminating the possibility of its accumulation. 　　5. Electromagnetic interference Shielding is one of the effective measures for suppressing interference in the radio industry. 　　In radio industry production, the debugging of radio equipment is carried out in a shielded room to prevent external interference. This is because the electric field generated by any external interference source has its power lines terminating perpendicularly on the metal shielding layer, and cannot penetrate into the shielded room. The function of this shielding is to protect the radio equipment or conductors inside the shielded room from external interference sources. 　　On the other hand, it is also possible to prevent radio interference sources from affecting any radio receiving devices or live conductors outside the shielded room. At this time, the shielded room needs to have a good electrical connection to the ground or the enclosure of the noise source. For example, if conductor A, which carries radio interference, is placed inside a closed metal enclosure, and if conductor A is charged positively, the inner surface of the metal enclosure will become negatively charged as a result of induction; this negative charge will then attract the positive charge on conductor A. The outer surface of the metal enclosure, on the other hand, will acquire an equal amount of positive charge due to induction. The distribution of the power lines is shown in Figure 24(a); the power lines outside the metal enclosure will interfere with other conductors. If the metal enclosure is grounded or connected to the enclosure of the interference source, the positive charges outside the enclosure are neutralized by the negative charges coming from the ground or the enclosure of the interference source, and the electric fields outside the enclosure disappear as well, as shown in Figure 24(b). In this way, conductor A carrying radio interference inside the enclosure has no effect outside the enclosure. 　　Therefore, the function of electrostatic shielding grounding is to confine the electric field generated by the interference source within the metal shield, and to conduct the charges induced on the surface of the metal shield into the ground, thereby protecting the outside environment from the influence of the interference source inside the metal shield. III. Classification of Grounding --------------------------------------------------------------------------------　　　　　　　　　(1) Classification by function of grounding: It is generally divided into protective grounding and functional grounding ； 1. Protective grounding (1) Shock protection grounding: To prevent electric shock caused by the energization of exposed conductive parts that are normally not charged, in the event of insulation failure or leakage current in electrical equipment, these exposed conductive parts are grounded, a practice known as shock protection grounding. This grounding can also limit line inrush currents, as well as the high voltages that can occur in low-voltage lines and equipment due to the intrusion of high voltage ； When an electrical fault occurs, it facilitates the operation of the overcurrent protection device to cut off the power supply. This type of grounding is also the narrow sense of “protective grounding”. 　　(2) Lightning protection grounding: Conducts lightning current into the ground to prevent electric shock to people or damage to property caused by lightning currents. 　　(3) Anti-static grounding: Directs static electricity into the ground to prevent harm to humans and equipment caused by the accumulation of static charge. In particular, integrated circuits are widely used in electronic devices today, and these circuits are prone to failure due to static electricity; grounding can prevent damage to them. 　　(4) Corrosion protection grounding: Metal objects buried underground are used as sacrificial anodes or cathodes to prevent cables, metal pipes, etc. from suffering from electrocorrosion. 　　2. Functional grounding 　　(1) Operating ground: To ensure the proper operation of the power system and prevent system oscillations, as well as to guarantee the reliability of relay protection, grounding is carried out at appropriate locations in AC and DC power systems. In AC systems, this is usually at the neutral point, while in DC systems it is at the midpoint. In electronic equipment systems, the grounding other than that of the electronic equipment itself is referred to as power ground. 　　(2) Logical ground: To ensure a stable reference potential, an appropriate metal component in the electronic device is used as the “logical ground,” with a metal base plate typically being employed for this purpose. Logical grounding and the grounding of other analog signal systems are often collectively referred to as DC ground. 　　(3) Shield grounding: Directs electrical interference sources into the ground, thereby suppressing the impact of external electromagnetic interference on electronic devices; it also reduces the interference generated by electronic devices from affecting other electronic devices. 　　(4) Signal grounding: A type of grounding implemented to ensure that signals have a stable reference potential; for example, it is used for grounding in the detection of leakage current, as well as for grounding in the measurement of electrical parameters such as impedance bridges and corona discharge losses. (II) Classification by grounding type Grounding electrodes can be divided into external grounding electrodes and loop-type grounding electrodes based on their arrangement method. Based on their shape, there are several basic forms: tubular, strip-shaped, and ring-shaped. Based on their structure, they can be divided into natural grounding electrodes and artificial grounding electrodes. Those used as the geographic poles in nature include metal pipes for water supply and drainage ； Metal structures of buildings and structures that have a reliable connection to the ground ； The metal sheaths of cables laid underground in a quantity of not less than two, as well as various metal pipes laid underground. Except for pipelines of flammable liquids and flammable or explosive gases. Steel materials such as steel pipes, angle steel, flat steel, and round steel are used as artificial grounding electrodes. In chemically corrosive soil, the aforementioned galvanized steels or copper counter electrodes should be used. The schematic diagram of the grounding device is shown in Figure 25. 　　Of course, electrical equipment is much safer after having a grounding installation in place compared to when no such installation is available. However, if the grounding arrangement consists of a single grounding electrode or an external grounding electrode, then due to the uneven distribution of potential, there is still a risk of electric shock to humans. Furthermore, the reliability of single grounding electrodes or externally connected grounding electrodes is also relatively poor. As can be seen from Figure 25, the external grounding electrode is connected to the indoor grounding main line relying on only two main lines. If these two main lines are damaged, the entire grounding main line becomes disconnected from the grounding electrode. Of course, it is relatively rare for damage to occur to both main lines at the same time. 　　To eliminate the disadvantages of single grounding electrodes or externally led grounding electrodes, we can lay loop-type grounding electrodes, as shown in Figure 26(a). The potential distribution of a loop-type grounding electrode is very uniform. The contact voltage Ut and step voltage Uk in the human body are relatively low. However, the potential distribution outside the grounding electrode remains uneven, and its step voltage is still very high, as shown in Figure 26(b). To avoid this drawback, some flat steel strips that are not connected to the grounding electrode can be laid outside the loop-type grounding electrode. As a result, the potential distribution outside the grounding electrode drops smoothly as shown in Figure 26(c). Therefore, in all cases, priority should be given to the use of loop grounding electrodes. External grounding electrodes are used only when it is difficult or costly to employ loop-type grounding electrodes. IV. Scope of grounding --------------------------------------------------------------------------------　　　　　　　　(1) DC systems 　　1. Two-wire DC systems 　　DC two-wire distribution systems shall be grounded. However, grounding is not required in the following cases: systems equipped with a ground detection device and that supply power only to industrial equipment within a confined area ； Systems with an inter-wire voltage equal to or below 50V, or above 300V, and using ground insulation ； A DC system powered by rectifier equipment supplied by a grounded AC system ； DC fire alarm signal circuits with a maximum current of 0.03A or less. 　　2. Three-wire DC system: The neutral wire in a three-wire DC power supply system should be directly grounded. (II) AC systems 1. AC circuits with voltages below 50V are generally not grounded, but those meeting any of the following conditions should be grounded ； 　　(1) Powered by a transformer, and the voltage of the transformer’s power supply system with respect to ground exceeds 150V ； 　　(2) Powered by a transformer, whose power supply system is ungrounded ； 　　(3) In the case of isolation transformers, they should not be grounded, but the core must be grounded ； 　　(4) Overhead lines installed outside buildings. 　　2. AC systems of 50–1000 V may be exempt from grounding if they meet the following conditions: (1) Electrical systems dedicated to powering industrial electric furnaces for melting, refining, heating, or similar purposes ； 　　(2) Separate drive system for the rectifier designed to power industrial speed control drive systems ； 　　(3) A separate drive system powered by a transformer, a dedicated control system whose primary side rated voltage is below 1000V ； Its control power supply ensures continuity of power supply; the control system is equipped with a ground detection device, and it is ensured that only trained personnel can monitor and maintain it. 　　3. AC systems of 1–10 kV: Arc suppression coils or resistive grounding can be used as needed. However, the 1–10 kV AC systems for mobile devices should be grounded. (III) Mobile and vehicle-mounted generators 1. Mobile generators The frame of a mobile generator does not need to be grounded under the following conditions: it can serve as the grounding point for the generator’s power supply system, provided that the generator supplies power only to devices mounted on it and/or to devices connected via flexible wires and plugs in sockets on the generator, with the exposed conductive parts of those devices and the grounding terminals on the sockets being connected to the generator frame. 2. For on-vehicle generators, the frame of the vehicle on which the generator power supply system is installed can be used as the grounding electrode for that system, provided all of the following conditions are met. 　　(1) The frame grounding of the generator is connected to the vehicle’s frame ； 　　(2) The generator supplies power only to the equipment installed on the vehicle and/or to equipment connected via cords and plugs in sockets installed on the vehicle or the generator ； 　　(3) The exposed conductive parts of the equipment and the grounding terminals on the socket are connected to the generator frame. 3. Connection of the neutral wire: When the generator is part of a standalone system, the neutral wire should be connected to the generator frame. (IV) Electrical Equipment 1. The following exposed conductive parts of electrical equipment shall be grounded: (1) the metal bases and enclosures of motors, transformers, electrical appliances, as well as portable and mobile electrical devices ； 　　(2) Generator neutral point cabinet enclosure, generator outlet cabinet enclosure ； 　　(3) Electrical equipment transmission mechanism ； 　　(4) Secondary winding of the transformer ； 　　(5) The metal frames and bases of panels (cabinets, boxes), control and protection equipment, as well as operation consoles, and the metal enclosures of fully enclosed switchgear ； 　　(6) Metal frames and reinforced concrete frames of indoor and outdoor power distribution installations, as well as metal fences and metal doors located near live parts ； 　　(7) Metal enclosures of AC and DC power cable junction boxes, terminal boxes, and expanders, as well as the metal shielding of cables, accessible steel pipes for passing wires, metal conduit trays for laying cables, and cable trays ； 　　(8) Exposed conductive parts of metal lighting fixtures ； 　　(9) In residential areas with non-asphalt surfaces, metal towers and reinforced concrete towers of overhead power lines that are ungrounded, have arc-suppression coil grounding or resistive grounding systems, as well as towers of overhead lines equipped with lightning conductors ； 　　(10) The exposed conductive parts and supports of electrical devices such as switchgear and capacitors installed on power line poles and towers ； 　　(11) The metal shielding layer of armored control cables, as well as 1–2 core wires that are idle in unarmored or non-metallic sheathed cables ； 　　(12) Enclosed busbar metal enclosure ； 　　(13) The metal enclosure of the box-type substation. 　　2. The following exposed conductive parts of electrical equipment may not be grounded: (1) Electrical equipment located in non-conductive areas, such as those with poorly conductive floors made of wood, asphalt, etc., and insulated walls, provided that the requirements specified in item 2.(1)4.(3) regarding non-conductive areas are met ； 　　(2) The exposed conductive parts of electrical equipment or electrical installations in dry areas, with an AC rated voltage of 50 V or less and a DC rated voltage of 120 V or less, except in explosion-hazardous areas ； 　　(3) The enclosures of electrical measuring instruments, relays, and other low-voltage electrical devices installed on distribution panels, control panels, and electrical installations, as well as the metal bases of insulators that prevent the generation of dangerous voltages on supports in the event of insulation failure ； 　　(4) Equipment with good electrical contact, such as bushing bases, installed on grounded metal frames, except in areas with explosion hazards ； 　　(5) Supports in battery rooms with a rated voltage of 220V or less ； 　　(6) The exposed conductive parts of motors and electrical apparatus that have a reliable electrical connection to an earthed frame, except in areas with explosion hazards. 3. External conductive parts: Areas in the external conductive parts that pose a risk of electric shock should be grounded. The parts that generally need to be grounded are as follows: (l) Large metal frameworks inside or on buildings that may become charged and come into contact with people should be grounded to improve safety ； 　　(2) Tracks and trusses of electrically operated cranes ； 　　(3) Elevator frame equipped with wire ropes ； 　　(4) The metal lifting rope or cable of the elevator does not need to be grounded if it is already connected to the elevator body to form a conductive path ； 　　(5) Metal enclosures, metal fences, and other similar metal shielding structures surrounding electrical equipment with inter-wire voltages exceeding 750V outside the substation or transformer room ； 　　(6) The exposed metal parts in mobile homes or campers, including the metal structure of mobile homes and the metal frame of campers, shall be grounded. V. Classification of environments based on the level of electric shock risk -------------------------------------------------------------------------------- Due to differences in the environment, some can cause insulation failure, while others can reduce the skin impedance of humans; both of these factors can easily lead to electric shock risks. The environment is divided into the following three categories based on the level of risk associated with electric shocks, which helps in taking appropriate measures during grounding work. 1. Particularly hazardous environments An environment is considered particularly hazardous if one of the following conditions exists within a building: (1) High humidity, that is, when the relative humidity of the air in the building is close to 100%. At this point, the ceilings, walls, floors, and objects inside the building are completely covered in moisture. Such as bathrooms, swimming pools. 　　(2) The building contains chemically active or organic substances; that is, there are often or continuously corrosive vapors, gases, and liquids within the building, which form deposits or mold layers that can damage the insulating and conductive parts of electrical equipment and wiring. For example, chemical workshops. 　　(3) Locations within a building where two or more of the following relatively hazardous conditions exist simultaneously. 　　(4) Location where outdoor electrical equipment is installed. 2. More hazardous environments An environment is considered more hazardous if one of the following conditions exists within a building: (1) A very humid or dusty environment, that is, a place where the relative humidity of the air in the building exceeds 75% ； Or locations where the amount of process dust emitted during production can accumulate on electrical equipment and wires. 　　(2) Locations with conductive floors such as metal, soil, brick, or reinforced concrete. 　　(3) High-temperature environments, namely places where, under the effect of various forms of thermal radiation, the temperature inside buildings frequently exceeds 35°C over a cycle of one day and night. For example, buildings equipped with dryers, ovens, calcination furnaces, or boilers. 　　(4) Places where a person may come into contact with the grounded metal structures, process equipment, and metal pipes inside a building, while at the same time being in contact with the metal enclosures of electrical equipment. 　　3. Environment with little danger: A place within a building where such more dangerous or particularly dangerous environments do not exist or cannot be created is considered an environment with little danger.