Fire hazard testing for electric and electronic products — Part 10: Glowing/hot-wire based test methods — Glow-wire apparatus and common test procedure
1 Scope
This part of GB/T 5169 specifies the glow-wire apparatus and common test procedure to simulate the effects of thermal stresses which may be produced by heat sources such as glowing elements or overloaded resistors, for short periods, in order to assess the fire hazard by a simulation technique.
The test procedure described in this part is a common test procedure intended for the small-scale tests in which a standardized electrically heated wire is used as a source of ignition.
It is a common part of the test procedures applied to end products and to solid electrical insulating materials or other solid combustible materials.
A detailed description of each particular test procedure is given in the respective standards IEC 60695-2-11, IEC 60695-2-12 and IEC 60695-2-13.
This part is intended for use by technical committees in the preparation of standards in accordance with the principles laid down in IEC Guide 104 and ISO/IEC Guide 51:1999.
One of the responsibilities of a technical committee is, wherever applicable, to make use of the series standards in the preparation of its standards. The requirements, test methods or test conditions of this part will not apply unless specifically referred to or included in the relevant standards.
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 documents (including any amendments) applies.
ISO 4046-4:2002 Paper, board, pulps and related terms — Vocabulary — Part 4: Paper and board grades and converted products
ISO 13943:2008 Fire safety — Vocabulary
IEC 60584-1 Thermocouples — Part 1: Reference tables
IEC 60584-2 Thermocouples — Part 2: Tolerances
IEC 60695-2-11 Fire hazard testing — Part 2-11: Glowing/hot-wire based test methods — Glow-wire flammability test method for end-products (GWEPT)
IEC 60695-2-12 Fire hazard testing — Part 2-12: Glowing/hot-wire based test methods — Glow-wire flammability index (GWFI) test method for materials
IEC 60695-2-13 Fire hazard testing — Part 2-13: Glowing/hot-wire based test methods — Glow-wire ignition temperature (GWIT) test method for materials
IEC Guide 104:2010 The preparation of safety publications and the use of basic safety publications and group safety publications
ISO/IEC Guide 51:1999 Safety aspects — Guidelines for their inclusion in standards
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13943:2008, as well as the following apply. For the convenience of application, some terms and definitions specified in ISO 13943:2008 are listed below.
3.1
combustible (adjective)
capable of being ignited and burned
[ISO 13943:2008, definition 4.43]
3.2
draught-free environment
space in which the results of experiments are not significantly affected by the local air speed
Note: A qualitative example is a space in which a wax candle flame remains essentially undisturbed. Quantitative examples are small-scale fire tests in which a maximum air speed of 0.1 m/s or 0.2 m/s is sometimes specified.
[ISO 13943:2008, definition 4.70]
3.3
fire hazard
physical object or condition with a potential for an undesirable consequence from fire
[ISO 13943:2008, definition 4.112]
3.4
fire test
test that measures behaviour of a fire or exposes an item to the effects of a fire
Note: The results of a fire test may be used to quantify fire severity or determine the fire resistance or reaction to fire of the test specimen.
[ISO 13943:2008, definition 4.132]
3.5
flame (noun)
rapid, self-sustaining, sub-sonic propagation of combustion in a gaseous medium, usually with emission of light
[ISO 13943:2008, definition 4.133]
3.6
flammability
ability of a material or product to burn with a flame under specified conditions
[ISO 13943:2008, definition 4.151]
3.7
ignition
Deprecated: sustained ignition
〈general〉 initiation of combustion
[ISO 13943:2008, definition 4.187]
3.8
test temperature
temperature to which the tip of the glow-wire is heated and stabilized prior to any contact with the test specimen
4 Description of the test apparatus
4.1 Glow-wire
The glow-wire is formed from nickel/chromium (> 77% Ni/20 ± 1% Cr) wire, having an overall diameter of 4.00 mm ± 0.07 mm (before bending). The dimensions of the glow wire loop are as detailed in Figure 1.
A new glow-wire shall be annealed for a total of at least 10 h by being subjected to a current of at least 120 A before being used for a test run. The total annealing time may be achieved cumulatively. To avoid damage, the thermocouple shall not be installed during annealing. At the end of annealing, the depth of the thermocouple pocket hole shall be verified.
Note 1: The temperature of a new glow-wire which has not been annealed gradually lowers during the first few hours when subjected to a flow of current. After a period of time the temperature then reaches equilibrium.
The test apparatus shall be designed so that the glow-wire is kept in a horizontal plane and applies a force of 0.95 N ± 0.10 N to the test specimen during the application of the glow-wire. The force shall be maintained at this value when the glow-wire or the test specimen is moved horizontally one towards the other. The penetration of the tip of the glow-wire into and through the test specimen shall be limited to 7.0 mm ± 0.5 mm.
The test apparatus shall be so designed that burning or glowing particles falling from the test specimen are able to fall without obstruction onto the layer as specified in 4.4.
Two typical examples of the test apparatus are shown in Figures 3a) and 3b).
Note 2: The apparatus shown in Figure 3b) has been found useful when testing heavy and/or awkwardly shaped test specimens.
4.2 Test circuit and connections
The glow-wire is heated by a simple electric circuit as shown in Figure 2. There shall be no feedback mechanism or circuit to maintain the temperature. The power supply for the glow-wire test apparatus shall be a stabilized voltage source (± 2 % rms). The test circuit shall contain a current measuring device which indicates a true rms value with a maximum error of 1.0 %.
Due to the high currents involved, it is essential that all electrical connections for the glow-wire are capable of carrying the current without affecting the performance or long-term stability of the circuit. For the glow-wire to stud connection, a sufficient contact area (typically at least 60 mm2 at each end) is necessary for the stable and loss-less current necessary for the test. The glow-wire to stud connection shall be tightly screwed, soldered, or brazed between the glow-wire and studs.
Note: The typical current necessary for heating the tip to a temperature of 960°C is between 120 A and 150 A.
4.3 Temperature measuring system
The temperature of the tip of the glow-wire shall be measured by a class 1 (see IEC 60584-2) mineral-insulated metal-sheathed fine-wire thermocouple with an insulated junction. It shall have an overall nominal diameter of 1.0 mm. The thermocouple wires shall be suitable for continuous operation at temperatures up to 960 °C (e.g., chromel/alumel (Type K) – see IEC 60584-1). The welded point shall be located inside the sheath as close to the tip as practicable. The sheath shall consist of a metal resistant to continuous operation at a temperature of at least 1050 °C.
Note: A sheath made from a nickel-based heat-resistant alloy satisfies the above requirements.
The glow-wire, with the thermocouple inserted, is shown in Figure 1, Detail Z.
The thermocouple is arranged in a pocket hole, drilled in behind the tip of the glow-wire, and maintained as a close fit as shown in detail Z of Figure 1. The pocket hole shall be the smallest diameter that can accommodate the inserted thermocouple in order to reduce the occurrence of contamination during testing. The thermal contact between the tip of the thermocouple and the end of the drilled hole shall be maintained. Care shall be taken to ensure that the thermocouple is able to follow the dimensional changes of the tip of the glow-wire caused by heating.
The instrument for measuring the thermocouple voltages may consist of any commercial digital thermometer with a built-in reference junction.
Other temperature measuring instrumentation may be used, but, in case of dispute, the thermocouple method must be used.
4.4 Specified layer
To evaluate the possible spread of fire, for example by burning or glowing particles falling from the test specimen, a specified layer is placed underneath the test specimen.
Unless otherwise specified, a single layer of wrapping tissue resting on, and in close contact with the upper surface of a piece of wooden board (flat and smooth and having a minimum thickness of 10 mm) is positioned at a distance of 200 mm ± 5 mm below the place where the glow-wire is applied to the test specimen. See Figures 3a) and 3b).
Wrapping tissue (as specified in ISO 4046-4: 2002 Clause 4.215) is a soft and strong lightweight wrapping tissue with a mass per unit area of between 12 g/m2 and 30 g/m2.
4.5 Test chamber
The apparatus shall be operated in a draught-free environment having sufficient volume such that:
——oxygen depletion during the test does not significantly affect the result; and
——the test specimen can be mounted at least 100 mm from any surface.
Note 1: A chamber having a volume of at least 0.5 m3 is considered to have sufficient volume for the test.
Within the test location, the ambient light falling on the test specimen, not counting that from the glow-wire, shall not exceed 20 lx. This is measured with a lux meter positioned in place of the test specimen facing towards the rear part of the location.
Note 2: Use of a dark background material will usually help in achieving the 20 lx limit.
After each test, the test location shall be vented to replace any depleted oxygen.
4.6 Timing device
The timing device shall have a resolution of 0.2 s or less.
Foreword i
Introduction I
1 Scope
2 Normative references
3 Terms and definitions
4 Description of the test apparatus
4.1 Glow-wire
4.2 Test circuit and connections
4.3 Temperature measuring system
4.4 Specified layer
4.5 Test chamber
4.6 Timing device
5 Verification of the apparatus
5.1 Verification of the glow-wire tip
5.2 Verification of the temperature measuring system
6 Conditioning
7 Common test procedure
7.1 Test specimen support
7.2 Glow-wire temperature
7.3 Application of the glow-wire
Annex A (Informative) Equipment manufacturers and suppliers
Annex B (Informative) Guidance on “ignition” and “flaming” observations
Annex C (Informative) Guidance on the verification procedure of the glow-wire temperature measuring system by the heating current
Bibliography
Figure 1 Glow-wire and position of thermocouple
Figure 2 Test circuit
Figure 3 Test apparatus examples
Figure 4 Test specimen support [example — see Figures 3a) and 3b)]
Figure B.1 Example of a brightly shining flame
Figure B.2 Example of a blue corona at the glow-wire tip
Figure B.3 Example of a blue corona near the glow-wire tip
Figure C. 1 Correlation curve between the heating current and the glow-wire temperature (example)
