CB/T 1408 Insulating materials - Insulating materials - Test methods for electric strength includes the following three parts:
——Part 1: Tests at power frequencies;
——Part 2: Additional requirements for tests using direct voltage;
——Part 3: Additional requirements for 1.2/50 μs impulse tests.
This is Part 1 of GB/T 1408.
This part replaces GB/T 1408.1-2006 Insulating materials - Test methods for electric strength - Part 1: Tests at power frequencies. The following main technical changes have been made with respect to GB/T 1408.1-2006:
——the “normative references” are modified (see Clause 2; Clause 2 of 2006 Edition);
——the test requirements of sphere and plate electrodes are added (see Sub-clause 5.2.1.3);
——the test requirements of elastomers are added (see Sub-clause 5.2.6.2.4);
——the “Tests in solid materials” is added (see Sub-clause 7.4);
——the schematic diagram “sphere and plate electrodes” is added [see Figure 1c)];
——Annex B is deleted (see Annex B of Edition 2006).
By translation, this part is identical with IEC 60243-1: 2013 Insulating materials - Test methods for electric strength - Part 1: Tests at power frequencies (Edition 3).
The Chinese documents identical to the normative international documents given in this part are as follows:
——GB/T 1981.2-2009 Varnishes used for electrical insulation - Part2: Methods of test (IEC 60464-1: 2001, IDT)
——GB 2536-2011 Fluids for electrotechnical applications - Unused mineral insulating oils for transformers and switchgear (IEC 60296:2003, MOD)
——GB/T 5471-2008 Plastics - Compression moulding of test specimens of thermoplastic materials (ISO 295: 2004, IDT)
——GB/T 7113.2-2014 Flexible insulating sleeving - Part 2: Methods of test (IEC 60684-2: 2003, MOD)
——GB/T 9352-2008 Plastics - Compression moulding of test specimens of thermoplastic materials (ISO 293: 2004, IDT)
——GB/T 10580-2015 Standard conditions for use prior to and during the testing of solid electrical insulating materials (IEC 60212: 2010, IDT)
——GB/T 15022.2-2007 Resin based reactive compounds used for electrical insulation - Part2: Methods of test (IEC 60455-1: 1998, IDT)
——GB/T 17037.3-2003 Plastics - Injection moulding of test specimens of thermoplastic materials - Part 3: Small plates (ISO 294-3: 2002, IDT)
——GB/T 21218-2007 Specification for unused silicone insulating liquids for electrotechnical purposes (IEC 60836: 2005, IDT)
This part was proposed by China Electrical Equipment Industry Association.
This part is under the jurisdiction of National Technical Committee on Insulating Materials of Standardization Administration of China (SAC/TC 301).
The previous editions of this standard are as follows:
——GB/T 1408-1978, GB/T 1408-1989, GB/T 1408.1-1999, GB/T 1408.1-2006.
Insulating materials - Test methods for electric strength - Part 1: Test at power frequencies
1 Scope
This part of GB/T 1408 provides test methods for the determination of short-time electric strength of solid insulating materials at power frequencies between 48 Hz and 62 Hz.
This standard does not cover the testing of liquids and gases, although these are specified and used as impregnates or surrounding media for the solid insulating materials being tested.
Note: Methods for the determination of breakdown voltages along the surfaces of solid insulating materials are included.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 293 Plastics - Compression moulding of test specimens of thermoplastic materials
ISO 294-1 Plastics - Injection moulding of test specimens of thermoplastic materials - Part 1: General principles, and moulding of multipurpose and bar test specimens
ISO 294-3 Plastics - Injection moulding of test specimens of thermoplastic materials - Part 3: Small plates
ISO 295 Plastics - Compression moulding of test specimens of thermosetting materials
ISO 10724 Plastics - Injection moulding of test specimens of thermosetting powder moulding compounds
IEC 60212 Standard conditions for use prior to and during the testing of solid electrical insulating materials
IEC 60296 Fluids for electrotechnical applications - Unused mineral insulating oils for transformers and switchgear
IEC 60455-2 Specification for solvent-less polymerizable resinous compounds used for electrical insulation - Part 2: Methods of lest
IEC 60464-2 Varnishes used for electrical insulation - Part 2: Methods of test
IEC 60684-2 Flexible insulating sleeving - Part 2: Methods of test
IEC 60836 Specifications for unused silicone insulating liquids for electrotechnical purposes
IEC 61099 Insulating liquids - Specifications for unused synthetic organic esters for electrical purposes
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
electric breakdown
severe loss of the insulating properties of test specimens while exposed to electric stress, which causes the current in the test circuit to operate an appropriate circuit-breaker
Note: Breakdown is often caused by partial discharges in the gas or liquid medium surrounding the test specimen and the electrodes which puncture the specimen beyond the periphery of the smaller electrode (or of both electrodes, if of equal diameter).
3.2
flashover
loss of the insulating properties of the gas or liquid medium surrounding a test specimen and electrodes while exposed to electric stress, which causes the current in the test circuit to operate an appropriate circuit-breaker
Note: The presence of carbonized channels or punctures through the specimen distinguishes tests where breakdown occurred, from others where flashover occurred.
3.3
breakdown voltage
3.3.1
(tests with continuously rising voltage) voltage at which a specimen suffers breakdown under the prescribed test conditions
3.3.2
(step-by-step tests) highest voltage which a specimen withstands without breakdown for the duration of the time at that voltage level
3.4
electirc strength
quotient of the breakdown voltage and the distance between the electrodes between which the voltage is applied under the prescribed test conditions
Note: The distance between the test electrodes is determined as specified in Sub-clause 5.5, unless otherwise specified.
4 Significance of the test
Electric strength test results obtained in accordance with this standard are useful for detecting changes or deviations from normal characteristics resulting from processing variables, ageing conditions or other manufacturing or environmental situations. However, they are not intended for use in evaluating the behavior of insulating materials in an actual application.
Measured values of the electric strength of a material may be affected by many factors, including:
a) Condition of test specimens:
1) the thickness and homogeneity of the specimen and the presence of mechanical strain;
2) previous conditioning of the specimens, in particular drying and impregnation procedures;
3) the presence of gaseous inclusions, moisture or other contamination.
b) Test conditions:
1) the frequency, waveform and rate of rise or time of application of the voltage;
2) the ambient temperature, pressure and humidity;
3) the configuration, the dimensions, and thermal conductivity of the test electrodes;
4) the electrical and thermal characteristics of the surrounding medium.
The effects of all these factors shall be considered when investigating materials for which no experience exists. This standard defines particular conditions which give rapid discrimination between materials and which can be used for quality control and similar purposes.
The results given by different methods are not directly comparable but each may provide information on relative electric strengths of materials. The electric strength of most materials decreases as the thickness of the specimen between the electrodes increases and as the time of voltage application increases.
The measured electric strength of most materials is significantly affected by the intensity and the duration of surface discharges prior to breakdown. For designs which are free from partial discharges up to the test voltage, it is very important to know the electric strength without discharges prior to breakdown. However, the methods in this standard are generally not suitable for providing this information.
Materials with high electric strength will not necessarily resist long-term degradation processes such as heat, erosion or chemical deterioration by partial discharges, or electrochemical deterioration in the presence of moisture, all of which may cause failure in service at much lower stress.
5 Electrodes and specimens
5.1 General
The metal electrodes shall be maintained smooth, clean and free from defects at all times. Electrode arrangements for tests on boards and sheets perpendicular to the surface are shown in Figure 1.
Note: This maintenance becomes more important when thin specimens are being tested. Stainless steel electrodes e.g. minimize electrode damage at breakdown.
The leads to the electrodes shall not tilt or otherwise move the electrodes, nor affect the pressure on the specimen, nor appreciably affect the electric field configuration in the neighborhood of the specimen.
When very thin films (for example < 5 µm thick) are to be tested, the standards for those materials shall specify the electrodes and special procedures for handling and specimen preparation.
5.2 Tests perpendicular to the surface of non-laminated materials and normal to laminate of laminated materials
5.2.1 Boards and sheet materials, including pressboards, papers, fabrics and films
5.2.1.1 Unequal electrodes
The electrodes shall consist of two metal cylinders with the edges rounded to give a radius of (3 ± 0.2) mm. One electrode shall be (25 ± 1) mm in diameter and approximately 25 mm high. The other electrode shall be (75 ± 1) mm in diameter and approximately 15 mm high. These two electrodes shall be arranged coaxially within 2 mm as in Figure 1a).
Note: Radii for surface not in contact with the electrode are not critical with respect to test results but should avoid partial discharges in the surrounding medium.
5.2.1.2 Equal diameter electrodes
If a fixture is employed, which accurately aligns upper and lower electrodes within 1.0 mm, the diameter of the lower electrode may be reduced to (25 ± 1) mm, the diameters of the two electrodes differing by no more than 0.2 mm. The results obtained will not necessarily be the same as those obtained with the unequal electrodes of 5.2.1.1.
5.2.1.3 Sphere and plate electrodes
The electrodes shall consist of a metal sphere and a metal plate. The upper electrode shall be a sphere of (20 ± 1) mm in diameter and the lower one is a metal plate of (25 ± 1) mm in diameter with the edge rounded to give a radius of 2.5 mm. The discrepancy of the central axes between upper and lower electrodes shall be within 1 mm (see Figure 1c).
5.2.1.4 Tests on thick sample
When specified, boards and sheets over 3 mm thick shall be reduced by machining on one side to (3 ± 0.2) mm and then tested with the high-potential electrode on the non-machined surface.
When it is necessary in order to avoid flashover or because of limitations of available equipment, specimens may be prepared by machining to smaller thicknesses as needed.
5.2.2 Tapes, films and narrow strips
The electrodes shall consist of two metal rods, each (6 ± 0.1) mm in diameter, mounted vertically one above the other in a jig so that the specimen is held between the faces of the ends of the rods.
The upper and lower electrodes shall be coaxial within 0.1 mm. The ends of the electrodes shall form planes at right angles to their axes, with edge radii of (1 ± 0.2) mm. The upper electrode shall have a mass of (50 ± 2) g and shall move freely in the vertical direction in the jig.
Figure 2 shows an appropriate arrangement. If specimens are to be tested while extended, they shall be clamped in a frame holding them in the required position relative to the assembly shown in Figure 2. Wrapping one end of the specimen around a rotatable rod is one convenient way of achieving the required extension.
To prevent flashover around the edges of narrow tapes, the test specimen may be clamped using strips of film or other thin dielectric material overlapping the edges of the tape. Alternatively, gaskets that surround the electrodes may be used, provided that there is an annular space between electrode and gasket of 1 mm to 2 mm. The distance between the bottom electrode and the specimen (before the top electrode comes in contact with the specimen) shall be less than 0.1 mm.
Note: For testing films see IEC 60674-2.
5.2.3 Flexible tubing and sleeving
To be tested according to IEC 60684-2.
5.2.4 Rigid tubes (having an internal diameter up to and including 100 mm)
The outer electrode shall consist of a band of metal foil (25 ± 1) mm wide. The inner electrode is a closely fitting internal conductor, e.g. rod, tube, metal foil or a packing of metal spheres 0.75 mm to 2 mm in diameter, making good contact with the inner surface. In each case, the ends of the inner electrode shall extend for at least 25 mm beyond the ends of the outer electrode.
Where no adverse effect will result, petroleum jelly may be used for attaching the foil to the inner and outer surfaces.
5.2.5 Tubes and hollow cylinders (having an internal diameter greater than 100 mm)
The outer electrode shall be a band of metal foil (75 ± 1) mm wide and the inner electrode, a disk of metal foil (25 ± 1) mm in diameter, flexible enough to conform to the curvature of the cylinder. The arrangement is shown in Figure 3.
Foreword i
1 Scope
2 Normative references
3 Terms and definitions
4 Significance of the test
5 Electrodes and specimens
6 Conditioning before tests
7 Surrounding medium
8 Electrical apparatus
9 Test procedures
10 Mode of increase of voltage
11 Criterion of breakdown
12 Number of tests
13 Test report
Annex A (Informative) Treatment of experimental data
Bibliography
CB/T 1408 Insulating materials - Insulating materials - Test methods for electric strength includes the following three parts:
——Part 1: Tests at power frequencies;
——Part 2: Additional requirements for tests using direct voltage;
——Part 3: Additional requirements for 1.2/50 μs impulse tests.
This is Part 1 of GB/T 1408.
This part replaces GB/T 1408.1-2006 Insulating materials - Test methods for electric strength - Part 1: Tests at power frequencies. The following main technical changes have been made with respect to GB/T 1408.1-2006:
——the “normative references” are modified (see Clause 2; Clause 2 of 2006 Edition);
——the test requirements of sphere and plate electrodes are added (see Sub-clause 5.2.1.3);
——the test requirements of elastomers are added (see Sub-clause 5.2.6.2.4);
——the “Tests in solid materials” is added (see Sub-clause 7.4);
——the schematic diagram “sphere and plate electrodes” is added [see Figure 1c)];
——Annex B is deleted (see Annex B of Edition 2006).
By translation, this part is identical with IEC 60243-1: 2013 Insulating materials - Test methods for electric strength - Part 1: Tests at power frequencies (Edition 3).
The Chinese documents identical to the normative international documents given in this part are as follows:
——GB/T 1981.2-2009 Varnishes used for electrical insulation - Part2: Methods of test (IEC 60464-1: 2001, IDT)
——GB 2536-2011 Fluids for electrotechnical applications - Unused mineral insulating oils for transformers and switchgear (IEC 60296:2003, MOD)
——GB/T 5471-2008 Plastics - Compression moulding of test specimens of thermoplastic materials (ISO 295: 2004, IDT)
——GB/T 7113.2-2014 Flexible insulating sleeving - Part 2: Methods of test (IEC 60684-2: 2003, MOD)
——GB/T 9352-2008 Plastics - Compression moulding of test specimens of thermoplastic materials (ISO 293: 2004, IDT)
——GB/T 10580-2015 Standard conditions for use prior to and during the testing of solid electrical insulating materials (IEC 60212: 2010, IDT)
——GB/T 15022.2-2007 Resin based reactive compounds used for electrical insulation - Part2: Methods of test (IEC 60455-1: 1998, IDT)
——GB/T 17037.3-2003 Plastics - Injection moulding of test specimens of thermoplastic materials - Part 3: Small plates (ISO 294-3: 2002, IDT)
——GB/T 21218-2007 Specification for unused silicone insulating liquids for electrotechnical purposes (IEC 60836: 2005, IDT)
This part was proposed by China Electrical Equipment Industry Association.
This part is under the jurisdiction of National Technical Committee on Insulating Materials of Standardization Administration of China (SAC/TC 301).
The previous editions of this standard are as follows:
——GB/T 1408-1978, GB/T 1408-1989, GB/T 1408.1-1999, GB/T 1408.1-2006.
Insulating materials - Test methods for electric strength - Part 1: Test at power frequencies
1 Scope
This part of GB/T 1408 provides test methods for the determination of short-time electric strength of solid insulating materials at power frequencies between 48 Hz and 62 Hz.
This standard does not cover the testing of liquids and gases, although these are specified and used as impregnates or surrounding media for the solid insulating materials being tested.
Note: Methods for the determination of breakdown voltages along the surfaces of solid insulating materials are included.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 293 Plastics - Compression moulding of test specimens of thermoplastic materials
ISO 294-1 Plastics - Injection moulding of test specimens of thermoplastic materials - Part 1: General principles, and moulding of multipurpose and bar test specimens
ISO 294-3 Plastics - Injection moulding of test specimens of thermoplastic materials - Part 3: Small plates
ISO 295 Plastics - Compression moulding of test specimens of thermosetting materials
ISO 10724 Plastics - Injection moulding of test specimens of thermosetting powder moulding compounds
IEC 60212 Standard conditions for use prior to and during the testing of solid electrical insulating materials
IEC 60296 Fluids for electrotechnical applications - Unused mineral insulating oils for transformers and switchgear
IEC 60455-2 Specification for solvent-less polymerizable resinous compounds used for electrical insulation - Part 2: Methods of lest
IEC 60464-2 Varnishes used for electrical insulation - Part 2: Methods of test
IEC 60684-2 Flexible insulating sleeving - Part 2: Methods of test
IEC 60836 Specifications for unused silicone insulating liquids for electrotechnical purposes
IEC 61099 Insulating liquids - Specifications for unused synthetic organic esters for electrical purposes
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
electric breakdown
severe loss of the insulating properties of test specimens while exposed to electric stress, which causes the current in the test circuit to operate an appropriate circuit-breaker
Note: Breakdown is often caused by partial discharges in the gas or liquid medium surrounding the test specimen and the electrodes which puncture the specimen beyond the periphery of the smaller electrode (or of both electrodes, if of equal diameter).
3.2
flashover
loss of the insulating properties of the gas or liquid medium surrounding a test specimen and electrodes while exposed to electric stress, which causes the current in the test circuit to operate an appropriate circuit-breaker
Note: The presence of carbonized channels or punctures through the specimen distinguishes tests where breakdown occurred, from others where flashover occurred.
3.3
breakdown voltage
3.3.1
(tests with continuously rising voltage) voltage at which a specimen suffers breakdown under the prescribed test conditions
3.3.2
(step-by-step tests) highest voltage which a specimen withstands without breakdown for the duration of the time at that voltage level
3.4
electirc strength
quotient of the breakdown voltage and the distance between the electrodes between which the voltage is applied under the prescribed test conditions
Note: The distance between the test electrodes is determined as specified in Sub-clause 5.5, unless otherwise specified.
4 Significance of the test
Electric strength test results obtained in accordance with this standard are useful for detecting changes or deviations from normal characteristics resulting from processing variables, ageing conditions or other manufacturing or environmental situations. However, they are not intended for use in evaluating the behavior of insulating materials in an actual application.
Measured values of the electric strength of a material may be affected by many factors, including:
a) Condition of test specimens:
1) the thickness and homogeneity of the specimen and the presence of mechanical strain;
2) previous conditioning of the specimens, in particular drying and impregnation procedures;
3) the presence of gaseous inclusions, moisture or other contamination.
b) Test conditions:
1) the frequency, waveform and rate of rise or time of application of the voltage;
2) the ambient temperature, pressure and humidity;
3) the configuration, the dimensions, and thermal conductivity of the test electrodes;
4) the electrical and thermal characteristics of the surrounding medium.
The effects of all these factors shall be considered when investigating materials for which no experience exists. This standard defines particular conditions which give rapid discrimination between materials and which can be used for quality control and similar purposes.
The results given by different methods are not directly comparable but each may provide information on relative electric strengths of materials. The electric strength of most materials decreases as the thickness of the specimen between the electrodes increases and as the time of voltage application increases.
The measured electric strength of most materials is significantly affected by the intensity and the duration of surface discharges prior to breakdown. For designs which are free from partial discharges up to the test voltage, it is very important to know the electric strength without discharges prior to breakdown. However, the methods in this standard are generally not suitable for providing this information.
Materials with high electric strength will not necessarily resist long-term degradation processes such as heat, erosion or chemical deterioration by partial discharges, or electrochemical deterioration in the presence of moisture, all of which may cause failure in service at much lower stress.
5 Electrodes and specimens
5.1 General
The metal electrodes shall be maintained smooth, clean and free from defects at all times. Electrode arrangements for tests on boards and sheets perpendicular to the surface are shown in Figure 1.
Note: This maintenance becomes more important when thin specimens are being tested. Stainless steel electrodes e.g. minimize electrode damage at breakdown.
The leads to the electrodes shall not tilt or otherwise move the electrodes, nor affect the pressure on the specimen, nor appreciably affect the electric field configuration in the neighborhood of the specimen.
When very thin films (for example < 5 µm thick) are to be tested, the standards for those materials shall specify the electrodes and special procedures for handling and specimen preparation.
5.2 Tests perpendicular to the surface of non-laminated materials and normal to laminate of laminated materials
5.2.1 Boards and sheet materials, including pressboards, papers, fabrics and films
5.2.1.1 Unequal electrodes
The electrodes shall consist of two metal cylinders with the edges rounded to give a radius of (3 ± 0.2) mm. One electrode shall be (25 ± 1) mm in diameter and approximately 25 mm high. The other electrode shall be (75 ± 1) mm in diameter and approximately 15 mm high. These two electrodes shall be arranged coaxially within 2 mm as in Figure 1a).
Note: Radii for surface not in contact with the electrode are not critical with respect to test results but should avoid partial discharges in the surrounding medium.
5.2.1.2 Equal diameter electrodes
If a fixture is employed, which accurately aligns upper and lower electrodes within 1.0 mm, the diameter of the lower electrode may be reduced to (25 ± 1) mm, the diameters of the two electrodes differing by no more than 0.2 mm. The results obtained will not necessarily be the same as those obtained with the unequal electrodes of 5.2.1.1.
5.2.1.3 Sphere and plate electrodes
The electrodes shall consist of a metal sphere and a metal plate. The upper electrode shall be a sphere of (20 ± 1) mm in diameter and the lower one is a metal plate of (25 ± 1) mm in diameter with the edge rounded to give a radius of 2.5 mm. The discrepancy of the central axes between upper and lower electrodes shall be within 1 mm (see Figure 1c).
5.2.1.4 Tests on thick sample
When specified, boards and sheets over 3 mm thick shall be reduced by machining on one side to (3 ± 0.2) mm and then tested with the high-potential electrode on the non-machined surface.
When it is necessary in order to avoid flashover or because of limitations of available equipment, specimens may be prepared by machining to smaller thicknesses as needed.
5.2.2 Tapes, films and narrow strips
The electrodes shall consist of two metal rods, each (6 ± 0.1) mm in diameter, mounted vertically one above the other in a jig so that the specimen is held between the faces of the ends of the rods.
The upper and lower electrodes shall be coaxial within 0.1 mm. The ends of the electrodes shall form planes at right angles to their axes, with edge radii of (1 ± 0.2) mm. The upper electrode shall have a mass of (50 ± 2) g and shall move freely in the vertical direction in the jig.
Figure 2 shows an appropriate arrangement. If specimens are to be tested while extended, they shall be clamped in a frame holding them in the required position relative to the assembly shown in Figure 2. Wrapping one end of the specimen around a rotatable rod is one convenient way of achieving the required extension.
To prevent flashover around the edges of narrow tapes, the test specimen may be clamped using strips of film or other thin dielectric material overlapping the edges of the tape. Alternatively, gaskets that surround the electrodes may be used, provided that there is an annular space between electrode and gasket of 1 mm to 2 mm. The distance between the bottom electrode and the specimen (before the top electrode comes in contact with the specimen) shall be less than 0.1 mm.
Note: For testing films see IEC 60674-2.
5.2.3 Flexible tubing and sleeving
To be tested according to IEC 60684-2.
5.2.4 Rigid tubes (having an internal diameter up to and including 100 mm)
The outer electrode shall consist of a band of metal foil (25 ± 1) mm wide. The inner electrode is a closely fitting internal conductor, e.g. rod, tube, metal foil or a packing of metal spheres 0.75 mm to 2 mm in diameter, making good contact with the inner surface. In each case, the ends of the inner electrode shall extend for at least 25 mm beyond the ends of the outer electrode.
Where no adverse effect will result, petroleum jelly may be used for attaching the foil to the inner and outer surfaces.
5.2.5 Tubes and hollow cylinders (having an internal diameter greater than 100 mm)
The outer electrode shall be a band of metal foil (75 ± 1) mm wide and the inner electrode, a disk of metal foil (25 ± 1) mm in diameter, flexible enough to conform to the curvature of the cylinder. The arrangement is shown in Figure 3.
Contents of GB/T 1408.1-2016
Foreword i
1 Scope
2 Normative references
3 Terms and definitions
4 Significance of the test
5 Electrodes and specimens
6 Conditioning before tests
7 Surrounding medium
8 Electrical apparatus
9 Test procedures
10 Mode of increase of voltage
11 Criterion of breakdown
12 Number of tests
13 Test report
Annex A (Informative) Treatment of experimental data
Bibliography
