Correct use of the hottest leakage switch

Correct use of leakage switch

Abstract: leakage current protection switch (hereinafter referred to as leakage switch) can automatically cut off the grounding fault line of electrical equipment, protect personal safety, and is widely used in low-voltage anti electric shock protection. Its correct installation and use is the key to determine whether it can play a protective role. This paper discusses the correct installation and wiring of the leakage switch, lists several common mistakes that are easy to occur in the installation practice, and introduces the relevant precautions for selecting the leakage switch

key words: leakage switch zero sequence current transformer TN system application scope parameter selection Untitled Document in lighting and other low-voltage circuits, leakage switch is an essential low-voltage switchgear. When the conductive part (shell) of the electrical equipment has a fault voltage and generates a certain value of leakage current, it can automatically cut off the power supply, so as to effectively prevent electric shock accidents. 1. Action principle when the electrical equipment is in normal operation, the current of the phase line and n (zero line) entering the zero sequence current transformer is equal, and the direction is opposite. The magnetic flux generated by the two currents in the transformer counteracts each other. There is no induced current signal output at the secondary side of the transformer, and the switch maintains the conduction state; When the relative shell leakage fault occurs to the equipment, the magnetic flux generated by the phase line current in the transformer is greater than that generated by the n-line current, and the warming magnetic flux in the international market appears in the zero sequence current transformer and induces the current signal. It is expected that in the future, more and more scenes will begin to use lighter plastic, CFPR and GRP materials to drive the switch actuator to trip and cut off the power supply after amplification. It can be seen that making the leakage switch play a protective role, the residual flux is indispensable in the zero sequence current transformer, that is, the flow of environmental protection facilities on the whole industry chain of phase line electricity is greater than the zero line current is a necessary condition. To meet this condition, the installation and wiring must be correct, otherwise the expected effect will not be achieved. 2. Correct installation of leakage switch. When installing leakage switch, it is necessary to master the grounding type and different conditions of low-voltage distribution system before correct wiring. (1) TN system (protective neutral connection type with direct neutral grounding in power system), in principle, the shell of electrical equipment is connected with PE line (protective neutral line) or pen line  protective and working neutral line  for the whole system, if it is difficult to adopt TN, local TT system can be adopted after installing leakage switch. The grounding mode of TN system shall meet: ESIA ≤ U0 (1)

, where: ZS - impedance Ω of grounding fault circuit

ia - ensure the automatic tripping current a of the protective appliance within the specified time; U0 - nominal phase to ground voltage v. (2) TT system (protective grounding type of neutral point direct grounding in power system) the shell of electrical equipment is connected with protective grounding device that does not share the neutral point grounding of power supply

the grounding mode of TT system should meet the following requirements: RIA ≤ 50V (2), R - grounding resistance of shell Ω 3 common installation errors. During the installation of leakage switch, the following plausible errors are easy to occur. Now it is analyzed as follows: (1) in TN system, the position of pen lead is improper. I = I1 + △ I = I '(3) I － I' = 0 (4), where: I - phase line current entering leakage switch; I '- n-line current entering the leakage switch; I1 and △ I are the working current of the load and the leakage current to the housing respectively. According to formula (4), there is no residual magnetic flux in the zero sequence current transformer of the leakage switch. Even if △ I exceeds the protection action value, the leakage switch cannot trip, so it cannot play the role of leakage protection; (2) In TN system, only the earth is used as grounding protection. The load shell is not connected to the zero point on the morning of March 4, and there is no grounding device. Only electrical equipment is installed on the ground, or a section of reinforcement is temporarily inserted into the ground as the grounding electrode. The grounding resistance is too large to meet formula (2). In case of fault, even if the voltage presented by the housing exceeds 50V, the leakage switch cannot trip. (3) In TN system, pen or n-wire device after leakage switch is repeatedly grounded. I ＝ I1 ＋ △ I ＝ I '＋ I2 ＋ △ I (5)

i － I' ＝ I2 ＋ △ I (6), in which i2-i1 is the repeated grounding shunt current. It can be seen that when the load has a ground fault, the residual magnetic flux generated by the zero sequence current transformer in the leakage switch is greater than the magnetic flux generated by the actual leakage current △ I, which is easy to make the leakage switch trip by mistake. Similarly, it can be deduced that PE and N wires are separated, and the error of N repeated grounding can be deduced; (4) In TN system, PE and N wires are separated and then connected again. This kind of situation often occurs in projects with natural conductors in buildings as PE lines. Assume that load 1 has no fault, and its working current does not participate in the analysis: ① three-phase load balance, UN = 0, when load 2 has fault current △ I,

I = I1 ＋ △ I ＝ I1 ＋ △ I '＋ I2 = I' ＋ △ I '(7)

I － I' ＝ △ I (8), the partial current of I2, △ I － △ I in the formula, △ I = I2 ＋ △ I 'is visible, △ I '＜△ I the residual magnetic flux generated in the zero sequence current transformer of the leakage switch is less than the magnetic flux generated by the load fault current △ I, so that the leakage switch may refuse to jump and cannot play a protective role; ② Three phase load is unbalanced, UN ≠ 0, load 2 has no fault or the fault current △ I is very small, and the potential on the load shell is less than UN: I = I1 ＋ △ I = I '＋ △ I ＋ I2 = I' ＋ △ I '(9)

i － I' ＝ △ I '(10)

in the formula, the partial current of i2-i1  △ I' ＝ △ I + I2 is visible, △ I '＞ △ I, The residual magnetic flux generated in the zero sequence current transformer of the leakage switch is greater than the actual magnetic flux generated by the load fault current △ I, so that the leakage switch may trip by mistake. 4 scope of application in engineering design, we should first understand which occasions are suitable for installing leakage switches. According to the design specifications of relevant industries of the state, the following electrical equipment and distribution lines in places must be installed with leakage switches: (1) electrical equipment installed in humid, highly corrosive and other places with harsh environment