GB 16840.2-1997 English PDF (GB16840.2-1997)

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GB 16840.2-1997: [GB/T 16840.2-1997] Technical determination method for electrical fire cause. Part 2: Recidual magnetic method
GB/T 16840.2-1997
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 130.220
C 82
GB 16840.2-1997
Technical Determination Methods for Electrical Fire
Cause - Part 2: Residual Magnetic Method
ISSUED ON: JUNE 03, 1997
IMPLEMENTED ON: MAY 01, 1998
Issued by: State Bureau of Technical Supervision
According to the Standardization Administration of China Announcement
No.7 of 2017, it is converted into recommended standard
Table of Contents
Foreword ... 3
1 Scope ... 4
2 Definitions ... 4
3 Principle ... 4
4 Apparatus ... 5
5 Method Procedures... 5
6 Judgment ... 7
7 Written Procedures to be Followed during Inspection and Appraisal ... 8
Technical Determination Methods for Electrical Fire
Cause – Part 2: Residual Magnetic Method
1 Scope
This Standard specifies definitions, principles, apparatus, method procedures,
judgments, and written procedures to be performed during inspection and appraisal.
This Standard is applicable to determine the occurrence of short-circuit and lightning
based on data of residual magnetism under the condition that melted mark induced by
short-circuit and lightning cannot be found at the fire site of the fire point when
investigating the cause of electrical fires; and further analyze relationship with the
cause of the fire.
2 Definitions
For the purpose of this Standard, the following definitions apply.
2.1 Data of residual magnetism
The magnetic value that the ferromagnetic body retains after being magnetized by the
magnetic field formed by the short-circuit current of the wire and the lightning current.
The unit is millitesla (mT).
2.2 Melted mark induced by lightning
The melted marks that are 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
The magnetism of the ferromagnetic body that is maintained after being magnetized
by the magnetic field that is formed by the short-circuit of copper-aluminum wire that
is charged under high-temperature of fire.
3 Principle
Due to the magnetic effect of the current, a magnetic field is generated in the space
around the current; and the ferromagnetic body in the magnetic field is magnetized.
When the magnetic field escapes, the ferromagnetic body still maintains a certain
degree of magnetism.
5.2.1 Location
As the test specimen, it shall be taken from the fire point or around the wire of the fire
part that has been confirmed on the site. The distance between the specimen and the
wire should not exceed 20mm; but for the site with the possibility of thunder and
lightning, it may be extracted according to the actual situation; and it is not restricted
by the location.
5.2.2 Taking photos
Before extracting the samples, take photos on the site. The method of taking photos
may be divided into two items, namely, the orientation of the specimen and the close-
up of the specimen.
5.2.3 Extraction
--- Specimens that are fixed on walls or other objects shall not be bent, beaten, or
dropped during extraction;
--- Specimens with lower fire temperature should be extracted;
--- Specimens that are located near the magnetic material shall not be extracted;
--- When it is confirmed that the line has been short-circuited in the past, it shall not
be extracted;
--- If it is inconvenient to extract, it may be tested at the original location of the
specimen.
5.3 Storage
The extracted specimens should be put in a sampling bag for safekeeping; and the
specimen name and extraction location shall be indicated; and shall not be mixed with
magnetic materials or other objects.
5.4 Measurement
5.4.1 Dirt removal
Use water and solvent to remove carbon dust and dirt on the specimen surface before
measurement.
5.4.2 Preparation for measurement
According to the instructions of the instrument, turn on the power of the instrument,
and get ready after calibration and preheating.
5.4.3 Operation
--- Select the measurement points depending on the specimens, such as the two
ends for nails, iron pipes, steel bars; the corners for iron plates; the corners and
the tip for stray iron parts;
--- Place the probe (Hall element) flat on the specimen; slowly change the position
and angle of the probe to perform search-style measurement until the instrument
displays a stable maximum value;
--- The probe is only required to be in contact with the specimen, no pressure is
applied;
--- Make records according to the specimens after measurement.
6 Judgment
6.1 Data judgment
6.1.1 Nails and wires
In the short-circuit state, due to the difference in the magnitude 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 2mT above. Because the lower limit of the data of residual magnetism
overlaps with the data of residual magnetism for the normal current, the value below
0.5mT is not used as a criterion; the value 0.5~1.0mT below may be used as a
reference value for short-circuit determination; and the value 1.0mT above may be
used as the data of residual magnetism for short-circuit determination. The larger the
data of residual magnetism, the more accurate the qualitative; but it cannot be judged
only based on individual data; It may be judged only when there are more data.
6.1.2 Iron pipes and rebars
1.0mT below is not used as a criterion; 1.0~1.5mT is used as a reference value; and
1.5mT above is used as a short-circuit judgment data.
6.1.3 Stray iron parts
The iron rods, angle irons, metal frames, tools, etc. near the wires are generally large
in volume; and the magnetization is not obvious; therefore, 1.0mT and above shall be
used as the criterion.
6.1.4 Lightning residual magnetism
When 20kA current flows through the lightning protection cable, the data of residual
magnetism on the embedded support and U-shaped clip of the lightning protection
cable is 2.0~3.0mT. When the lightning current passes through the 1×2m iron plate
vertically, and the residual magnetism on the four corners of the iron plate is 2.0~3.0mT.