This standard specifies the definition, principle, equipment and instruments, method steps, determination and written procedures to be followed during inspection and identification. This standard is applicable to the investigation of the cause of electrical fires. Under the condition that the short circuit melting mark and lightning melting mark cannot be found at the fire point at the fire scene, the generation of short circuit and lightning can be determined based on the residual magnetic data, and the relationship with the cause of the fire can be further analyzed. GB 16840.2-1997 Technical identification method for the cause of electrical fires Part 2: Residual magnetic method GB16840.2-1997 Standard download decompression password: www.bzxz.net

GB16840.2—1997
The series of standards of "Technical identification methods for electrical fire causes" consists of 4 parts: Part 1 Macroscopic method; Part 2 Residual magnetism method; Part 3 Component analysis method; Part 4 Metallographic method. This standard is Part 2 of the series of standards of "Technical identification methods for electrical fire causes": Residual magnetism method. The residual magnetism method is to determine the short circuit of the wire and the intrusion of lightning under the condition that there are no short-circuit melting marks and lightning traces in the fire scene. This standard is proposed by the National Fire Protection Standardization Technical Committee. This standard is under the jurisdiction of the Sixth Subcommittee of the National Fire Protection Standardization Technical Committee. The drafting unit of this standard: Shenyang Fire Science Research Institute of the Ministry of Public Security. The drafters of this standard: Han Baoyu, Wang Xiqing, Di Man, Gao Wei. 69
1 Scope
National Standard of the People's Republic of China
Technical determination methods for electrical fire cause
Part 2: Recidual magnetic method
Technical determination methods for electrical fire cause Part 2: Recidual magnetic method GB 16840.2
This standard specifies the definition, principle, equipment and instruments, method steps, determination and written procedures to be followed during inspection and identification. This standard is applicable to the determination of short circuit and lightning generation based on residual magnetism data when investigating the cause of electrical fire, when no short circuit melting marks and lightning melting marks can be found at the fire point at the fire scene, and further analysis of the relationship with the cause of the fire. 2 Definitions
This standard adopts the following definitions:
2. 1 Residual magnetism data data of residual magnetism The magnetic value that remains after a ferromagnetic body is magnetized by the magnetic field formed by the short circuit current of the conductor and the lightning current. The unit is millitesla (mT). 2.2 melted mark induced by lightning Melted mark formed on the surface of metal by the high temperature of lightning. 2.3 residual magnetism in conducting wire short circuit caused by fire burning Copper and aluminum wires are charged, and short circuit occurs under the action of flame and high temperature to form a magnetic field. The magnetism of ferromagnetic material is retained after being magnetized. 3 Principle
Due to the magnetic effect of electric current, a magnetic field is generated in the space around the electric current. The ferromagnetic material in the magnetic field is magnetized. When the magnetic field escapes, the ferromagnetic material still retains a certain magnetism.
The magnetism retained by the ferromagnetic material in the magnetic field is related to the strength of the current and the magnetic field. Usually, the current in the wire will also generate a magnetic field under normal conditions, but its strength is small, and the residual magnetism left on the ferromagnetic material is also limited. When a short circuit occurs in the line or lightning passes through, an abnormally large current will be generated, resulting in a magnetic field with considerable strength. The ferromagnetic material will also be strongly magnetized and retain a large magnetism.
When it is suspected that the fire was caused by a short circuit or lightning at the fire scene and there is no melting mark to serve as a basis, the residual magnetism of the ferromagnetic body around the wire and lightning is detected. According to the presence and size of the residual magnetism, it is determined whether there was a short circuit or lightning phenomenon in the fire scene, and the relationship with the cause of the fire is further analyzed.
4 Equipment and Instruments
4.1 Tesla meter
For laboratory use or on-site portability, the range is 0~100mT, the accuracy is ±2.5%, and the operating temperature is +5~+40℃. 4.2 Instruments
Sampling tools, paper bags for samples, brushes, alcohol, acetone and other solvents. Approved by the State Administration of Technical Supervision on June 3, 1997, 70
Implemented on May 1, 1998
5 Methods and Steps
5.1 Types of Samples
Iron nails, iron wires;
Iron casing for threading wires;
GB16840.2-1997
Ferromagnetic materials on incandescent lamps and fluorescent lamps; ferromagnetic materials on switchboards;
Rebars and iron nails on herringbone frames (with wires); equipment components and other stray metals, but small in size is preferred. 5.2 Sample Extraction
5.2.1 Location
Samples for testing should be taken from the confirmed fire point or the surrounding wires of the fire point at the scene. The distance between the sample and the wire should not exceed 20mm, but for sites where lightning is possible, it can be extracted according to the situation without being restricted by the location. 5.2.2 Photography
Before extracting the sample, the on-site photography should be carried out. The photography is divided into two parts: sample orientation and sample close-up. 5.2.3 Extraction
A pair of samples fixed on the wall or other objects should not be bent, knocked or dropped during extraction; it is advisable to extract samples with lower fire temperatures. A pair of samples located near magnetic materials should not be extracted; if it is confirmed that the circuit has been short-circuited in the past, it should not be extracted; if it is inconvenient to extract, the sample can be tested at its original location. 5.3 Storage
The extracted samples should be properly stored in sampling bags, and the sample name and extraction location should be marked. They should not be mixed with magnetic materials or other objects.
5.4 Measurement
5.4.1 Dirt Removal
Before measurement, use water and solvent to remove carbon ash and dirt on the surface of the sample. 5.4.2 Measurement Preparation
According to the instrument instructions, turn on the power of the instrument, and prepare it after calibration and preheating. 5.4.3 Operation
Select measurement points according to different samples, such as iron nails, iron pipes, both ends of steel bars, corners of iron plates, corners and tips of stray iron parts; place the probe (Hall element) flat on the sample, slowly change the position and angle of the probe to perform search measurement until the instrument displays a stable maximum value;
The probe can be in contact with the sample, and no force should be pressed; after measurement, make records according to the sample. 6 Judgment
6.1 Data Judgment
6.1.1 Iron nails and iron wires
In the short-circuit state, due to the size of the short-circuit current and the distance from the short-circuit point, it is generally 0.2~1.5mT, and the larger one is above 2mT. Because the lower limit of the residual magnetic data overlaps with the residual magnetic data of the normal current, 0.5mT or less is not used as a criterion, 0.5~1.0mT or less can be used as a reference value for determining short circuit, and 1.0mT or more can be used as residual magnetic data for determining short circuit. The larger the residual magnetism data, the more accurate the qualitative analysis is. However, it cannot be judged based on individual data only. Only when there are more data can a judgment be made. 71
6.1.2 Iron pipes and steel bars
GB16840.2-1997
No judgment is made if the value is below 1.0mT. 1.0~1.5mT is used as a reference value, and 1.5mT or more is used as the data for judging short circuit. 6.1.3 Stray iron parts
Iron bars, angle irons, metal frames, tools, etc. near the conductor are generally large in size and are not obviously magnetized. They should be judged if the value is above 1.0mT.
6.1.4 Lightning residual magnetism
When a current of 20kA flows through the lightning conductor, the residual magnetism data of the embedded brackets and U-shaped clips on the lightning conductor is 2.0~3.0mT. The lightning current passes vertically through a 1×2m iron plate, and the residual magnetism at the four corners of the iron plate is 2.0~3.0mT. The residual magnetism at the tip of the lightning rod is not large, at 0.6~1.0mT. The residual magnetism data of stray iron parts, nails, steel bars, and metal pipes in the electric channel are all between 1.5 and 10mT. The above data are obtained from experiments and on-site lightning detection, and can be used as a reference for judgment. 6.2 Comparative judgment
Make a judgment after comparison on site. If there are lines passing through the same two facilities, but one has residual magnetism and the other has no residual magnetism, it proves that the wire on the side with residual magnetism has had a short circuit. 6.3 Magnetization law judgment
The strength of the magnetism of a ferromagnet is related to its distance from the wire (short circuit). The closer it is to the wire, the stronger its magnetism. If you can find a law from strong to weak during measurement, combined with the measured data, you can further determine whether the wire has had a short circuit. 6.4 Determination of residual magnetism of short-circuited wires on firebZxz.net
When a short circuit occurs in a wire on fire, a magnetic field will also be generated, which will cause the ferromagnetic material to maintain its magnetism. To determine whether the short circuit was caused before the fire or during the fire, the source of the fire should be found out and a judgment should be made based on the actual situation on site. 7 Written procedures to be followed when submitting for inspection and appraisal 7.1 When submitting for inspection, the unit submitting the inspection shall first fill out the application form for technical appraisal of the cause of the electrical fire, which shall include the name, address, and contact person of the unit applying for the appraisal, the name of the unit on fire, the name and quantity of the sample, the sampling location, the sampler, and the purpose of the appraisal. 7.2 After accepting the appraisal task, the appraisal unit shall fill out the sample receipt form, task form, reception record, and original record. 7.3 After the appraisal is completed, the appraisal conclusion shall be filled in the appraisal report approval form, signed by the person in charge of the laboratory, and submitted to the leader for approval after the quality review is correct.
7.4 The original of the approved appraisal report shall be handed over to the unit submitting the inspection, and a copy shall be kept for record. 72
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