An experimental method for evaluating ground fault ignition capability

Abstract

The application discloses an experimental method for evaluating the ignition ability of a grounding fault, installs N grounding stakes which are vertically inserted into the ground and are in parallel, touches the fire wire and the external conductor in different contact modes to generate a fault, and places external combustibles of different materials at the contact points; test data is measured during the touching process, and the ignition of the combustibles is observed; the ignition ability value is calculated according to the test data, and an ignition ability matrix is obtained; the ignition probability of the grounding fault for various combustibles is calculated according to the ignition ability matrix; and the ignition ability of the combustibles is classified according to the ignition probability of the combustibles. The application realizes the simulation and evaluation of the ignition ability of different combustibles under different conditions, records the corresponding ignition conditions, realizes the quantitative evaluation of the fire hazard of the grounding fault, provides a reference for the prevention and treatment of the fire hazard of the grounding fault, and reduces the life and property safety loss caused by the grounding fault fire.

Description

Technical Field

[0001] This invention relates to the field of electrical fire prevention technology, specifically to an experimental method for evaluating the ignition capability of grounding faults. Background Technology

[0002] Common electrical faults that cause electrical fires include short circuit faults, grounding (leakage) faults, poor contact, overcurrent, and over / undervoltage. Among these, grounding (leakage) faults are a significant cause of electrical fires and a challenging issue in fire investigation and prevention. When a grounding fault occurs, determining which combustibles can be ignited, under what conditions and how, and the voltage, current, and protective device operation during ignition are crucial questions for fire departments in determining whether a fire is caused by a grounding fault, and are also key references for grounding fault prevention and control.

[0003] Through experimental research and theoretical analysis, combined with field investigations, this study investigates the ignition characteristics of grounding fault ignition sources on various combustible materials such as conductor insulation, plastics, fabrics, wood, and paper. It summarizes and categorizes the occurrence conditions, ignition conditions, experimental phenomena, and products, as well as the physical characteristics of ignition such as current, voltage, energy, and temperature. This establishes the criteria for identifying the causes of grounding faults and provides a technical basis for electrical fire investigations. However, my country currently lacks simulation experimental platforms and methods for grounding fault ignition capabilities. Summary of the Invention

[0004] To address the aforementioned problems, the purpose of this invention is to provide an experiment for evaluating the ignition capability of grounding faults. This experiment simulates and evaluates the ignition capability of grounding faults on different combustibles under various conditions, enabling a quantitative assessment of the fire hazard of grounding faults. This provides a reference for the prevention and control of grounding fault fires, thereby reducing the loss of life and property caused by grounding fault fires. The technical solution is as follows:

[0005] An experimental method for evaluating the ignition capability of a ground fault includes the following steps:

[0006] Step 1: Lead the power supply system out from the power distribution room, install the grounding grid, and install N grounding stakes within M meters of the power distribution room. The grounding stakes are vertically inserted into the ground.

[0007] Step 2: Select n grounding stakes from the N stakes and connect them in parallel, then connect them to an external conductor.

[0008] Step 3: Induce a fault by bringing the live wire into contact with the external conductor in different ways, and place external flammable materials of different materials at the contact points; during the contact process, measure the temperature T at two points on the outer surface of the live wire insulation layer at different distances from the contact points. 远 and T 近 and the temperature T at the contact point 触; and observe the ignition of combustibles, including external combustibles and wire insulation;

[0009] Step 4: For each type of combustible material, select a value of n and conduct a set of experiments. For each set of experiments, use different materials of ignition wire and different contact methods to conduct one experiment, and record the experimental data for each experiment.

[0010] Step 5: Select n values ​​from 1 to N sequentially, and conduct N experiments. The results are based on the ignition of the combustible material and the temperature T. 远 Temperature T 近 and temperature T 触 The measured values ​​are used to determine the weight of each temperature, and then the ignition capability value under the selected n value is calculated, and the ignition capability matrix is ​​obtained. The final ignition capability value is obtained based on the ignition capability matrix.

[0011] Step 6: Calculate the grading index for evaluating ignition capability by calculating the average ignition capability value of cotton and insulation layer;

[0012] Step 7: Classify the ignition ability of a specific combustible material according to the grading index for evaluating ignition ability.

[0013] Furthermore, each grounding stake is 1 to 1.5 meters long and spaced 2.5 to 5 meters apart, and at least 500 ml of saturated NaCl solution with a temperature exceeding 80°C is injected into the gap between each grounding stake and the soil.

[0014] Furthermore, after installing the grounding stakes in step 1, connect all the grounding stakes in parallel and measure the open-circuit voltage U between one phase of the power supply and the grounding stake. LN And measure the short-circuit current I when the live wire touches an external conductor under this open-circuit voltage. LN Calculate the grounding resistance R based on this. E If R E If the value is less than or equal to r, where r is a preset resistance threshold, it indicates that the number of grounding stakes N meets the experimental conditions. Otherwise, increase the number of grounding stakes and remeasure until R is reached. E <= r.

[0015] Furthermore, the external conductor is selected from smooth angle steel or a V-file with no coating; the way the live wire contacts the external conductor includes: the live wire in close contact with the angle steel, the live wire touching the angle steel at a certain frequency, and the live wire tracing across the file.

[0016] Furthermore, for a specific combustible material, the steps for calculating the ignition capability of a ground fault are as follows:

[0017] Step 5.1: Calculate the weight of each ignition probability.

[0018] (1) Calculate temperature T 远 and temperature T近 weight w 远 and w 近

[0019] Regarding the outer surface temperature T of the live wire insulation layer 远 and temperature T 近 For each test group, the weights of the probability that the combustible material will be ignited are:

[0020]

[0021] Where t0 is a random number, T 测 T represents the temperature of the outer surface of the insulation layer observed during the experiment. 远 and T 近 Two types of data, the calculated w 绝缘层 They correspond to w respectively 远 and w 近 ;T min and T max These are the preset minimum and maximum temperature thresholds, respectively; t is the intermediate value representing the exponent.

[0022] (2) Calculate temperature T 触 weight w 触

[0023]

[0024] Among them, T 分解 T is the initial thermal decomposition temperature of the combustible material, measured by a thermogravimetric analyzer. 燃点 The ignition point of combustible materials;

[0025] (3) Calculate the total weight w when the combustible material is not ignited. 未 :

[0026]

[0027] Where, a = max{w 近 w 远 w 触};

[0028] Step 5.2: Calculate the ignition capacity value

[0029] The ignition values ​​for each test group are obtained based on the ignition status: P = p1, p2, p3, ..., p X The value is 1 when ignited and w when not ignited. 未 Let X be the total number of trials; then the ignition capacity of a specific combustible material at the selected value of n is:

[0030]

[0031] This yields the ignition capability vector q for m experiments using different materials of fuses and different contact methods. n1 ,q n2 ,…q nm ;

[0032] The ignition capability matrix Q of the combustible material obtained by repeating experiments with n values ​​ranging from 1 to N is:

[0033]

[0034] The calculated final ignition capacity value Y of the combustible material:

[0035]

[0036] Furthermore, the specific grading index for calculating and evaluating ignition capability is as follows:

[0037] Step 6.1: Select cotton as the combustible material, and repeat the test H times according to 5.1 to 5.2 to obtain the final ignition capacity value Y1 of H cotton samples. 棉 , Then calculate the average ignition capacity value Y for cotton. 棉 :

[0038]

[0039] Step 6.2: Select the combustible material as the insulation layer, and repeat the test H times according to 5.1 to 5.2 to obtain the final ignition capacity value Y1 of H insulation layers. 绝缘 , Then calculate the average ignition capability value Y of the insulation layer. 绝缘 :

[0040]

[0041] Step 6.3: Calculate the grading index for evaluating ignition capability.

[0042]

[0043] Where x takes the values ​​0, 1, 2, 3, we get e0, e1, e2, e3.

[0044] The beneficial effects of this invention are: This invention can solve the problem that my country currently lacks a simulation experimental platform and method for the ignition capability of ground faults, thereby realizing the simulation and evaluation of the ignition capability of ground faults on different combustibles under different conditions, and recording the corresponding ignition conditions, realizing the quantitative evaluation of the fire hazard of ground faults, providing a reference for the prevention and control of fire hazards caused by ground faults, and reducing the loss of life and property safety caused by ground fault fires. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the grounding stake installation and wiring for the experimental method of evaluating the ignition capability of grounding faults according to the present invention. Detailed Implementation

[0046] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0047] I. Establishing a Ground Fault Simulation Experiment Platform

[0048] Step 1: Draw out the TN-S power supply system from the power distribution room as the test power supply, and select various models of air switches with and without leakage protection as the air switches for testing;

[0049] Step 2: Install a grounding grid. Within M meters (20-40 meters) of the power distribution room, install N copper-plated steel core grounding stakes. Each grounding stake is 1-1.5 meters long. Insert the grounding stakes vertically into the ground (the lower end should be at least 1 meter deep) and extend the grounding lead. The grounding stakes should be spaced 2.5-5 meters apart. Inject at least 500 ml of a saturated NaCl solution with a temperature exceeding 80℃ into the gap between each grounding stake and the soil. A schematic diagram of the grounding stake installation and wiring is shown below. Figure 1 As shown.

[0050] Step 3: Connect all grounding stakes in parallel, that is, connect the upper ends of the grounding stakes together with a grounding wire. Measure the open-circuit voltage between one phase of the power supply and the grounding stakes to obtain U. LN ; Measure the short-circuit current I when this voltage touches all the grounding stakes. LN Calculate the grounding resistance R E If R E If the value is less than or equal to r (r is generally taken as 4Ω, and 10Ω is taken for small power supply systems with a total capacity not exceeding 100kVA), it indicates that the number of grounding stakes N meets the experimental conditions. Otherwise, return to step 2, add more grounding stakes, and measure again. A clamp multimeter is used to measure the voltage and current values.

[0051] II. Experimental System and Methods for Ignition Capability Evaluation

[0052] Step 4: Select n (values ​​from 1 to N, i.e., the number of grounding stakes used ranges from 1, 2, or 3, up to N, and repeat this process once) grounding stakes and connect them in parallel. After parallel connection, connect them to an external conductor, such as... Figure 1 As shown. The outer conductor is made of a smooth angle steel with an uncoated surface, and a V-file is used.

[0053] Step 5: Initiate a fault by touching the live wire to an external conductor. The live wire uses insulated copper or aluminum wire. The contact method is as follows: the live wire (using 1.5-4 square millimeter single-core copper or aluminum wire, both insulated, with 3-5 cm of the wire exposed at the contact point) touches the external conductor (parallel contact). This is done by: close contact with the angle steel; contact with the angle steel at a certain frequency (2-5 times per second on average); and rubbing it across the surface with a file (the file should be a fine-V size to better approximate the texture of the rough angle steel surface). During the contact process, the following data is measured: the outer surface temperature of the live wire insulation layer (two temperature points are taken, one at a distance of 30-50 cm from the contact point, temperature T). 远 One location is the insulation layer near the contact point, with a temperature T. 近 ), contact point temperature (T) 触 ), and observe the ignition of the combustibles, such as the location. Figure 1 As shown. Combustible materials are placed in two categories: one is external combustible materials (such as cotton, dry sawdust, fibers, etc.), placed at the contact point; the other is the wire insulation layer (the section of the live wire closest to the contact point, which is directly observed). A thermal imager is used to measure the temperature of the fault point and the outer surface of the wire insulation layer.

[0054] Step 6: Repeatedly test for each type of combustible material. For each selected value of n, conduct tests for m different scenarios, and test X sets of data for each scenario (X is not less than 10). (There are 2 options for the live wire and 3 ways to contact it, so there are a total of m = 2 * 3 = 6 scenarios.) Step 7: For each test set, determine the corresponding ignition capacity value (e.g., the nth grounding stake corresponds to a certain combustible material, the live wire is copper wire, and the contact method is close contact with the angle steel).

[0055] Step 7.1: Calculate the weight of the ignition probability for each step.

[0056] For each test, the weight is 1 when the combustible material is ignited, and w when it is not ignited. 未 Values ​​are determined according to the following rules:

[0057] (1) Calculate temperature T 远 and temperature T 近 weight w 远 and w 近

[0058] Regarding the outer surface temperature T of the live wire insulation layer 远 and temperature T 近 For each test group, the weights of the probability that the combustible material will be ignited are:

[0059]

[0060] Where t0 is a random number, T 测T represents the temperature of the outer surface of the insulation layer observed during the experiment. 远 and T 近 Two types of data, the calculated w 绝缘层 They correspond to w respectively 远 and w 近 T min and T max These represent the preset minimum and maximum temperature thresholds, respectively; t is the intermediate value representing the exponent. The weighting calculation results will differ depending on the location of the experimental observation.

[0061] Note: This formula reflects the safe temperature of the insulation layer. During electrical testing, the minimum temperature for PVC is 70°C, for rubber-insulated wires it is 65°C, and the allowable temperature for terminals is 60-70°C. In this text, T... MIN Take 65, T MAX Take 120 (120 is the thermal decomposition temperature of the insulation layer in the experiment, when smoke is produced, which is the beginning of the danger).

[0062] (2) Calculate temperature T 触 weight w 触

[0063]

[0064] Among them, T 分解 T is the initial thermal decomposition temperature of the combustible material, measured by a thermogravimetric analyzer. 燃点 The ignition point of combustible materials;

[0065] When not ignited, examine the temperature values ​​T at three points. 远 T 近 and T 触 Finally, a total weight is obtained through calculation.

[0066] (3) Calculate the total weight w when the combustible material is not ignited. 未 :

[0067]

[0068] Where, a = max{w 近 w 远 w 触}

[0069] Step 7.2: Calculate the ignition capacity value

[0070] Thus, the ignition values ​​for each test group under this condition are obtained: P = p1, p2, p3, ..., p X The value is 1 when ignited and w when not ignited. 未 We obtain a vector consisting of X values, where X is the total number of trials; then, for a specific combustible material, the ignition ability value at the selected value of n is:

[0071]

[0072] Then, using different materials of ignition wire and different contact methods, m experiments were conducted to obtain the ignition capability vector (for a specific combustible material at a selected value of n): q n1 ,q n2 ,…q Nm .

[0073] Choose a new value for n (n ranges from 1 to N) and repeat the above experiment to obtain the ignition capability matrix Q for a specific combustible material:

[0074]

[0075] The calculated final ignition capacity value Y of the combustible material:

[0076]

[0077] Step 8: Calculate the grading index for evaluating ignition capability by calculating the average ignition capability value of cotton and insulation layer.

[0078] Step 8.1: Select cotton as the combustible material, and repeat the test H times according to 5.1 to 5.2 to obtain the average ignition capacity value Y1 of H cotton samples. 棉 , Then calculate the average ignition capacity value Y for cotton. 棉 :

[0079]

[0080] Step 8.2: Select the combustible material as the insulation layer, and repeat the test H times according to 5.1 to 5.2 to obtain the average ignition capacity value Y1 of H insulation layers. 绝缘 , Then calculate the average ignition capability value Y of the insulation layer. 绝缘 :

[0081]

[0082] Step 8.3: Calculate the grading index for evaluating ignition capability.

[0083]

[0084] Where x takes the values ​​0, 1, 2, 3, we get e0, e1, e2, e3.

[0085] Step 9: Classify the ignition ability of a specific combustible material according to the grading index for evaluating ignition ability.

[0086] Table 1 Ignition Capacity Classification Table

[0087]

[0088] Therefore, for a certain combustible material, the final ignition capability value Y of a grounding fault can be evaluated according to Table 1.

Claims

1. An experimental method for evaluating the ignition capability of a ground fault, characterized in that, Includes the following steps: Step 1: Lead the power supply system out from the power distribution room, install the grounding grid, and install N grounding stakes within M meters of the power distribution room. The grounding stakes are vertically inserted into the ground. Step 2: Select n grounding stakes from the N stakes and connect them in parallel, then connect them to an external conductor. Step 3: Create a fault by making the live wire touch the external conductor in different ways, and place external flammable materials of different materials at the contact points; During the contact process, the temperature T was measured at two points on the outer surface of the live wire insulation layer at different distances from the contact point. 远 and T 近 and the temperature T at the contact point 触 ; and observe the ignition of combustibles, including external combustibles and wire insulation; Step 4: For each type of combustible material, select a value of n and conduct a set of experiments. For each set of experiments, use different materials of ignition wire and different contact methods to conduct one experiment, and record the experimental data for each experiment. Step 5: Select n values ​​from 1 to N sequentially, and conduct N experiments. The results are based on the ignition of the combustible material and the temperature T. 远 Temperature T 近 and temperature T 触 The measured values ​​are used to determine the weight of each temperature, and then the ignition capability value under the selected n value is calculated, and the ignition capability matrix is ​​obtained. The final ignition capability value is obtained based on the ignition capability matrix. Step 6: Calculate the grading index for evaluating ignition capability by calculating the average ignition capability value of cotton and insulation layer; Step 7: Classify the ignition ability of a specific combustible material according to the grading index for evaluating ignition ability.

2. The experimental method for evaluating the ignition capability of grounding faults according to claim 1, characterized in that, The grounding stakes are 1 to 1.5 meters long and spaced 2.5 to 5 meters apart. At least 500 ml of saturated NaCl solution with a temperature exceeding 80°C is injected into the gap between each grounding stake and the soil.

3. The experimental method for evaluating the ignition capability of grounding faults according to claim 1, characterized in that, After installing the grounding stakes in step 1, connect all the grounding stakes in parallel and measure the open-circuit voltage U between one phase of the power supply and the grounding stake. LN And measure the short-circuit current I when the live wire touches an external conductor under this open-circuit voltage. LN Calculate the grounding resistance R based on this. E If R E If the value is less than or equal to r, where r is a preset resistance threshold, it indicates that the number of grounding stakes N meets the experimental conditions. Otherwise, increase the number of grounding stakes and remeasure until R is reached. E <= r.

4. The experimental method for evaluating the ignition capability of grounding faults according to claim 1, characterized in that, The outer conductor is selected from smooth angle steel or V-file with no coating on the surface; The ways in which the live wire comes into contact with an external conductor include: the live wire making close contact with the angle steel, the live wire touching the angle steel at a certain frequency, and the live wire tracing across a file.

5. The experimental method for evaluating the ignition capability of grounding faults according to claim 1, characterized in that, The steps for calculating the ignition capability of a ground fault for a certain combustible material are as follows: Step 5.1: Calculate the weight of each ignition probability. (1) Calculate temperature T 远 and temperature T 近 weight w 远 and w 近 Regarding the outer surface temperature T of the live wire insulation layer 远 and temperature T 近 For each test group, the weights of the probability that the combustible material will be ignited are: Where t0 is a random number, T 测 T represents the temperature of the outer surface of the insulation layer observed during the experiment. 远 and T 近 Two types of data, the calculated w 绝缘层 They correspond to w respectively 远 and w 近 ;T min and T max These are the preset minimum and maximum temperature thresholds, respectively; t is the intermediate value representing the exponent. (2) Calculate temperature T 触 weight w 触 Among them, T 分解 T is the initial thermal decomposition temperature of the combustible material, measured by a thermogravimetric analyzer. 燃点 The ignition point of combustible materials; (3) Calculate the total weight w when the combustible material is not ignited. 未 : Where, a = max{w 近 w 远 w 触 }; Step 5.2: Calculate the ignition capacity value The ignition values ​​for each test group are obtained based on the ignition status: P = p1, p2, p3, ..., p X The value is 1 when ignited and w when not ignited. 未 Let X be the total number of trials; then the ignition capacity of a specific combustible material at the selected value of n is: This yields the ignition capability vector q for m experiments using different materials of fuses and different contact methods. n1 ,q n2 ,…q nm ; The ignition capability matrix Q of the combustible material obtained by repeating experiments with n values ​​ranging from 1 to N is: The calculated final ignition capacity value Y of the combustible material:

6. The experimental method for evaluating the ignition capability of a ground fault according to claim 5, characterized in that, The specific grading indicators for calculating and evaluating ignition capability are as follows: Step 6.1: Select cotton as the combustible material, and repeat the test H times according to 5.1 to 5.2 to obtain the final ignition capacity value Y1 of H cotton samples. 棉 Y2 棉 ,…Y H 棉 Then calculate the average ignition capacity value Y for cotton. 棉 : Step 6.2: Select the combustible material as the insulation layer, and repeat the test H times according to 5.1 to 5.2 to obtain the final ignition capacity value Y1 of H insulation layers. 绝缘 Y2 绝缘 ,…Y H 绝缘 Then calculate the average ignition capability value Y of the insulation layer. 绝缘 : Step 6.3: Calculate the grading index for evaluating ignition capability. Where x takes the values ​​0, 1, 2, 3, we get e0, e1, e2, e3.

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