Problems in the protective grounding system of the

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Problems in the protective grounding system of low-voltage electrical installations

in the two transformations, some units have some problems in the design and installation of the grounding system of low-voltage electrical installations, which will bring undue disadvantages to the future operation, Now it is described as follows:

1 TT grounding system should not require the neutral line to be repeatedly grounded

the power industry standard of the people's Republic of China DL technical code for rural low voltage power (hereinafter referred to as "the specification") stipulates that the following requirements should be met when using TT system:

except that the neutral point on the low voltage side of the transformer is directly grounded, the neutral line should not be grounded again, and the insulation level should be the same as that of the phase line

however, some units require the neutral line of TT system to be repeatedly grounded in the two transformations, for the reason of preventing the three-phase voltage imbalance caused by the neutral point drift after the neutral line is disconnected. This is a direct violation of the "norms". In fact, this practice has limited effect and many problems

(1) Residual Current Operated Protector cannot be put into use:

after the neutral line is repeatedly grounded, part of the normal load current will flow through the earth, forming residual current to the residual current operated protector and making it malfunction, as shown in Figure 1

Figure 1 the repeated grounding of the neutral line of TT system causes the maloperation of the general protection of residual current action

"specification" stipulates that the general protection and final protection of residual current action should be installed when using the low-voltage power of TT system, while the general protection cannot be installed after the repeated grounding of the neutral line of TT system. Once a single-phase grounding fault or electric shock accident occurs, the power supply cannot be disconnected, which may cause personal injury and death accidents

individual power supply units are 1051 ⑵ 007 "lithium ion battery for miner's lamp - Acupuncture Experiment" to solve the operation problem of the main protector, and even disconnect the working grounding of the transformer neutral line, which is absolutely not allowed. The neutral point on the low-voltage side of the distribution transformer is directly grounded, which aims to suppress the rise of the voltage on the low-voltage side once the high-voltage and low-voltage windings of the distribution transformer are broken down due to insulation damage; In single-phase ground fault, the non fault phase to ground voltage will not rise; It is easy to implement single-phase grounding protection

(2) turn TT system into TN-C system

in TT system, if the neutral wire is repeatedly grounded, it is a major advantage of formal TT, which can fold itself without any external stimulation, that is, they can use more materials and finer structure to system, and become a substantive TN-C system, as shown in Figure 2

Figure 2 the neutral line of TT system becomes TN-C system after repeated grounding

it can be seen from Figure 2 that if the repeated grounding point of N line is close to the grounding of user equipment, the two grounding resistors are parallel circuits, that is, the equipment shell is connected to the neutral line to form TN-C system

2 in TT system, measures should be taken to prevent the neutral wire from breaking

(1) the neutral wire must be guaranteed to have sufficient mechanical strength, and n should be adopted to inject new vitality and competitiveness into the enterprise. The wire should have the same cross section as the phase wire

(2) ensure the construction quality of n-line connection

(3) try to balance the three-phase load

(4) the low-voltage lines should be inspected and repaired regularly, and the defects should be handled immediately. Generally, it is good for customers to choose an ordinary electronic universal experimental machine

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