GB/T 16840.2-2021Technical determination methods for electrical fire evidence—Part 2: Residual magnetism method (English PDF)

3.1 Residual magnetism

Magnetism retained by a ferromagnet after it is magnetized by the magnetic field formed by the short-circuit current of a wire or the lightning current.

3.2 Melted mark induced by lightning

Melted mark formed by the high temperature of the lightning current on a metal.

6.1 Types of test materials

6.1.5 Equipment components and other magnetic metal materials, preferably small in size.

6.2 Selection of test materials

6.2.1 Technical requirements for selection of test materials

6.2.1.1 Test materials shall be selected from representative parts.

6.2.1.2 The distance between the test material and the wire should be less than or equal to 20 mm.

6.2.1.3 For sites where lightning strikes may occur, testing shall be carried out according to actual conditions, without being restricted by the part.

6.2.1.4 Before testing, the location, state and morphological characteristics of the test material shall be recorded by taking photos.

6.2.2 Precautions

6.2.2.1 Test materials fixed to walls or other objects shall not be bent, knocked or dropped during extraction.

6.2.2.2 Test materials that are less affected by the temperature of the fire scene should be tested.

6.2.2.3 Test materials located near magnetic materials shall not be tested.

6.2.2.4 If it is confirmed that a short circuit has occurred in the line near where the test material is extracted, testing shall not be carried out.

7.1 Preparation

7.1.1 Use cleaning reagent to clean the surface of the test material before measurement.

7.1.2 Clear the testing equipment to zero before measurement.

7.2 Measurement operation

7.2.1 Select measurement points according to the test material. For example, measure two ends of iron nails, iron pipes, steel bars, etc.; measure corners of iron plates; measure edges and corners of stray iron parts.

7.2.2 Place the probe (Hall element) flat on the test material, slowly change the position and angle of the probe to perform search measurement until the maximum value when the measurement data is stable.

7.2.3 The probe can be in contact with the test material, and no force shall be pressed.

8.1 Data determination

8.1.1 When the test material is iron nails and iron wires, etc.

8.1.1.1 If the measured residual magnetic data is less than 0.5 mT, it is not used as a criterion for the occurrence of short circuit or lightning strike.

8.1.1.2 If the measured residual magnetic data is greater than 0.5 mT and less than 1.0 mT, it is used as a reference for determining whether a short circuit or lightning strike has occurred.

8.1.1.3 If the measured residual magnetic data is greater than 1.0 mT, it is used as a criterion for the occurrence of short circuit or lightning strike.

8.1.2 When the test material is iron pipes and steel bars, etc.

8.1.2.1 If the measured residual magnetic data is less than 1.0 mT, it is not used as a criterion for the occurrence of short circuit or lightning strike.

8.1.2.2 If the measured residual magnetic data is greater than 1.0 mT and less than 1.5 mT, it is used as a reference for determining whether a short circuit or lightning strike has occurred.

8.1.2.3 If the measured residual magnetic data is greater than 1.5 mT, it is used as a criterion for the occurrence of short circuit or lightning strike.

8.1.3 When the test material is stray iron pieces 8.1.3.1 Stray iron pieces include iron bars, angle irons, magnetic metals, etc. near the wire.

8.1.3.2 If the measured residual magnetic data is greater than 1.0 mT, it is used as a criterion for the occurrence of short circuit or lightning strike.

8.2 Comparative determination

When electrical lines pass through the same facilities at different locations on site, measure the residual magnetic data of metal components on the facilities near the lines.

By measuring the size of the residual magnetic data, determine whether the wire passing through the facilities with residual magnetic data has ever experienced a short circuit or lightning strike.