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In accordance with the requirements of the Code for Design of Low-Voltage Distribution Systems GB50054-2011, low-voltage distribution systems have three grounding types: IT system, TT system, and TN system. This post covers the calculation formulas for ground fault current for these three types of grounding systems, the methods for calculating ground fault current, and the selection of appropriate protective devices. Ground fault current yunrun.com.cn/tech/3455.html 1. Calculation and analysis of ground fault current in TN grounding systems. The equivalent circuit diagram of a TN system when a ground fault occurs at the load side (hereinafter referred to as a fault) is shown in Figure 1. http://yunrun.com.cn/upload/202011/20/202011202009115241.png Figure 1: Equivalent circuit diagram when the TN system is faulty. Note: Rph represents the resistance of the phase conductor, in mΩ ； Xph is the phase conductor reactance, in mΩ ； RPE is the resistance of the PE conductor, in mΩ ； XPE is the reactance of the PE conductor, in mΩ ； RB is the grounding resistance of the transformer neutral point, in mΩ ； Rs is the internal resistance on the low-voltage side of the transformer, in mΩ ； Xs is the internal reactance on the low-voltage side of the transformer, in mΩ ； Id is the ground fault current (RMS value in AC), hereinafter referred to as fault current, in kA. The ground fault current (Id) in a TN system is calculated as follows: http://yunrun.com.cn/upload/202011/20/202011201819324067.png .............................Formula 1. In Formula 1, Unom represents the nominal voltage of the phase conductor with respect to the ground, in volts ； Rph•p is the phase protection circuit resistance, in mΩ ； Xph•p is the reactance of the phase protection circuit, in mΩ ； It represents the resistance of the faulty circuit, as well as the reactance of that circuit; this includes the impedance of the high-voltage side system (converted to the low-voltage side), plus the sum of the phase resistances and reactances of the transformer, low-voltage busbars, and low-voltage lines, in mΩ. When the fault point in a TN system is located at a considerable distance from the power source (transformer), and when both the PE conductor and the phase conductors are within the same cable or run through the same conduit or trunking, it can be assumed that Xph•p is much smaller than Rph•p; therefore, Xph•p can be ignored, as well as the internal resistance on the low-voltage side of the transformer. The formula for calculating the grounding fault current in a TN system can then be simplified to Formula 2: ..........................Formula 2 It should be noted that: ① When the cross-sectional area of the conductors in a TN system is large (for example, when the cross-sectional area of the phase conductors is 120 mm2 or more), their resistance values are low, and ignoring the reactance (Xph•p) will result in significant errors. When the cross-sectional area of the phase conductor is 120 mm2 (70 mm2 for the PE conductor), ignoring reactance increases Id by 3%-4% ； When the cross-sectional area of the phase conductor is 150 mm2, Id increases by 5%-6% ； When the cross-sectional area of the phase conductor is 185 mm2, Id increases by 7%-8%. To this end, Chinese electrical expert Ren Yuanhui recommends that for phase conductors with a cross-sectional area of 120 mm2 or more, the 1d value calculated using the formula should be multiplied by a correction factor of 0.96–0.92. ②When the fault point in the TN system is very close to the transformer (with a low probability of occurrence), the transformer has a small capacity and the wire cross-sectional area is large, ignoring the transformer impedance will result in significant errors. Simplified formulas cannot be used for calculations; instead, the proper formulas must be applied, taking the transformer impedance into account. When the fault location is close to the transformer (e.g., a few dozen meters), it can be calculated using a formula, but the necessary correction factors must be applied. ③The magnitude of the fault current Id in the TN system is crucial for selecting the parameters of protective devices. The value of Id depends on factors such as wire length, cross-sectional area, and installation method; it can range from several hundred to several thousand amperes, or even higher. What is more concerning is that when the length of the circuit is large and as a result the Id value is small, this will prevent the overcurrent protection devices from operating within the specified time frame. Calculations are necessary; articles such as \"Maximum Allowable Length of Copper-Cored Cables When Using Circuit Breakers for Fault Protection\" outline the requirements for fault protection as well as simple calculation methods. 2. Calculation and analysis of grounding fault current in TT grounding system. The equivalent circuit diagram when a fault occurs in the TT system is shown in Figure 2. http://yunrun.com.cn/upload/202011/20/202011201915522094.png Figure 2: Equivalent circuit diagram in the event of a fault in the TT system (RA represents the protective grounding resistance of Class I electrical equipment and the exposed conductive parts of electrical devices, in ohms). The grounding fault current (Id) in a TT system is calculated using Equation 3 (ignoring the impedance of the power supply and the reactance of the wiring): ...................... Equation 3. Usually, (RA+RB) in a TT system is much larger than (Rph+RPE), so Equation 3 can be simplified to Equation 4: .............................................. Equation 4. When applied to TT systems, RA and RB are expressed in ohms (Ω), while the fault current Id is expressed in amperes (A). Due to the limitations of the resistance values of RA and RB, the fault current Id in a TT system is generally only a few amps to several dozen amps, which is much lower than the fault current in a TN system. In such cases, overcurrent protection devices are not sufficient to cut off the power supply; therefore, a Residual Current Device (RCD) should be used in a TT system for fault protection. 3. Calculation and analysis of grounding fault current in IT grounding systems. The equivalent circuit diagram for the first fault in an IT system is shown in Figure 3. http://yunrun.com.cn/upload/202011/20/202011201909334138.png Figure 3: Equivalent circuit diagram during the first fault in an IT system. In IT systems where the power supply neutral point is not grounded, during the first fault, the fault current can only form a circuit through the lines of the non-faulty phases and the capacitive reactance to ground. Due to the high capacitive reactance, this fault current is only in the milliampere range. Given that the fault current in IT systems is very low, these systems can continue to operate despite being faulty, without the need to interrupt the power supply ； Since the fault current is very small, the contact voltage of the fault point to ground is also very low, so there is no risk of electric shock. It is not normal for a system to continue operating in a faulty state; therefore, IT systems should be equipped with insulation monitors that issue alarms to alert operators and maintenance personnel to take necessary actions, thereby preventing further failures resulting from operation under faulty conditions. If a second heterophase grounding fault occurs during this period, it will lead to a short circuit between phases (or between a phase and the neutral conductor), resulting in fault currents similar to those in a TN or TT system; in such cases, shutting down the power supply is inevitable. IT systems should not be equipped with N conductors, that is, a three-phase three-wire system should be used. When an N conductor is provided, if a ground fault occurs in that N conductor, the IT system will turn into a TT system, thereby losing the advantages of the IT system. Ren Yuanhui, an electrical expert in our country, pointed out that even after the use of detection and alarm devices for N-conductor ground faults, three-wire or four-wire systems can still be employed; this facilitates the provision of a relative ground voltage (220V in our country) for lighting, socket circuits, and small electrical appliances. This article provides a brief introduction to the calculation of grounding short-circuit current in low-voltage distribution systems. Mastering this knowledge lays the foundation for selecting protective devices in electrical design; moreover, high-performance electrical meters are also essential for such design. You are welcome to visit Changhui Instrument Network to learn more about these products.