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What is the Glow Wire Test, What is the purpose of the glow wire test?
Historically, there have been a variety of methods to evaluate the flammability and fire resistance of materials, including direct and indirect flame-burning test methods.
The UL-94 specification is an example of the direct flame ignition test method used. This flame test method, which has long been used, involves the direct application of flame to a vertically or horizontally mounted sample under controlled conditions. The opposite is the indirect flame combustion method, which is the application of a heat source to the sample. The glow wire test method is one of the indirect flame-burning methods. The International Electrotechnical Commission (IEC) developed the glow wire test method in 2001 and the test standard called the glow wire test IEC 60695. The test results obtained using the glow wire test method can be used to compare the flame resistance of the material, automatic extinguishing (if ignition occurs), and the possibility that the droplets formed by burning do not cause flame spread. This kind of flame test method is applicable to electrical and electronic equipment and its components, and parts, but also for solid electrical insulation materials or other solid combustible materials.
The test principle of The glow wire test
The glow wire test, or glow wire flammability test, is the use of simulation technology to assess the risk of fire caused by thermal stress in a short period of time due to scorching elements or overload resistance and other heat sources. Usually， the specified material (￠ 4 Ni80 / Cr20) and the shape of the electric heating wire with high current heating to the test temperature ( 550 ℃ ~ 960 ℃ ) 1min， Scorch the sample vertically for the 30s with the specified pressure (1.0N)， observe whether the test product and the liner wrapping paper starts to burn or continues to burn for a period of time, to determine the risk of fire of finished electrical and electronic equipment, or the determination of solid insulation materials and other solid combustible materials, the ignition, the glow wire ignition temperature, etc.
What are the GWT， GWIT， and GWFI in the Glow Wire Test?
GWT (Glow Wire Test) or end product burning wire test, used to test the flame resistance of finished relays (test temperature must meet 750℃).
GWIT (Glow Wire Ignition Temperature) is used to test the ignition of the material (the test temperature should meet 775℃).
GWFI (Glow Wire Flammability Index) is used to test the ability of a material to extinguish after being ignited (the test temperature must meet 850℃).
As long as the operating current of unattended appliances is greater than 0.2A, it has to pass the IEC 60335 test( a kind of glow wire test standard), of which GWFI is the material-selection test and is the first step; after passing it, it needs to do one of the GWIT or GWT tests again. So two options are available: GWFI+GWIT and GWFI+GWT.
More details about the GWT， GWIT， GWFI
GWT (Glow Wire Test) – in accordance with standard IEC60695-2-11
end product burning wire test, using the finished relay to do the test, the test point is located around the contact several surfaces, qualified requirements are at 750 ℃ conditions, three consecutive tests, the finished relay does not burn, or in the removal of the burning wire within the 30S, its burning or flame extinguished, and the wrapping paper laid under the sample did not start to burn.
GWIT (Glow Wire Ignition Temperature) – in accordance with standard IEC60695-2-13
Glow Wire Ignition Temperature test, usually done with plastic samples to measure the ignition of the material, tests 3 samples, qualified requirements are that the sample does not ignite throughout the test course, or the flame time does not exceed 5s and the sample does not burn out. After the test results qualified, the glow wire temperature plus 25 ℃ as the material GWIT index, which is 775 ℃.
GWFI (Glow Wire Flammability Index)–in accordance with standard IEC60695-2-12
The Glow Wire Flammability Index test, usually done with plastic samples, measures the ability of the material to extinguish in the event of ignition. Qualified requirements are the three samples, at 850 ℃, the sample in the removal of the burning wire within the 30S, its burning or flame extinguished, and the wrapping paper laid under the sample did not start to ignite, the sample did not burn out.
The Glow Wire Tester
The glow wire tester, also known as the glow wire test apparatus, of course, is used to do the glow wire flammability test. It is suitable for the research, production, and quality inspection departments of electric and electronic products and their component parts, such as lighting equipment, low-voltage electrical appliances, household appliances, machine tools and appliances, motors, power tools, electronic instruments, electrical instruments, information technology equipment, electrical equipment, electrical connections, and accessories, etc. It is also suitable for insulating materials, engineering plastics, or other solid combustible materials industries.
The structure of The Glow Wire Tester （take TESTEX TF346 as an example）
(7)Internal Main Structure
(11)Thick Hinge Assembly
Description of Internal Main Structure of TESTEX glow wire tester:
(2)the inhibiting device of a glowing filament
(3)the adjusting screw of the inhibiting device of a glowing filament
(5)the temperature sensor of a glowing filament
The glow wire test operation procedures
Preface: Since different use environments require different flame resistance properties for electrical insulation materials, the temperature required for the glow wire test can be selected according to the actual situation (not only limited to 750℃ or 850℃, etc), the following is the recommendation for the test temperature selection, which can be used for reference.
Test Sample Preparation For The Glow Wire Test
The test sample shall be a plane part with a fixed size and large enough, and the plane part shall be in a vertical position during the test.
The test sample can be made in the following ways:
- transfer molding
- the casting
- Machined parts with large enough surface
The test sample’s flat surface shall be:
The length is greater than or equal to 60mm
Width (inside fixture) greater than or equal to 60mm
Thickness (3.0 + / – 0.2 mm)
The evaluation of flammability usually requires a set of 3 test samples(or 10samples).
Note: flammability usually changes with the thickness of the tested material. In addition to the specified standard thickness, the actual test sample with a thickness of 0.8mm/1.6mm/6.0mm is often encountered, and the test results obtained are also useful.
If possible, the test sample shall be complete equipment, components, or components. The selection of the test sample shall ensure that there is no significant difference between the test conditions and those that occur during normal use, such as the effect of shape, ventilation, thermal stress, and the effect that the flame or burning particles or hot particles that may occur in the test sample fall near the test sample. If the test cannot be carried out on a complete equipment, component, or component, one of the following methods may be used:
A: cut a piece of the part to be checked.
B: open a small hole on the complete test equipment, components, or components to make them contact the hot wire;
C: remove the parts to be checked from the complete equipment and conduct separate tests.
Those parts should be clearly specified in the relevant glow wire test standards so that they can be removed in order to touch the glowing filament. The holes may be too small to affect the test results due to the ignition of the surrounding objects, the decrease in the temperature of the hot wire, or the limitation of the oxygen utilization rate. However, if the holes are too large, more oxygen will be obtained than normal. During the glow wire test, if any part of the sample jig is supported, it will ignite due to the heat of the hot wire, thus affecting the heating condition of the test sample, such test shall be invalid.
Testing shall ensure that: under certain conditions, the electric heating to the temperature stipulated in the relevant subjects equipment, the glow wire light-off doesn’t make the parts, or under certain conditions, make parts with the heat of the electric heating wire light-off, the combustion duration is limited, so don’t because of test samples of flame combustion particles or test sample sets or hot particles and spread.
Install the sample and start the glow wire tester
Prepare the sample and fix the sample on the test clamp, and adjust the sample insertion size to meet the test requirements. (Note: After the sample is installed, be sure to close the observation window door, otherwise the temperature does not meet the test requirements.) If you need to install a layer of packing silk paper, just lay it under the sample as required.
Correctly plug in the power cord, then confirm that the incoming power supply is well grounded and that the power supply voltage is AC 220V.
Turn on the power switch on the operation panel and check if the meter is normal.
Start the control panel and display the interface home page.
Click the “Heating” button.
At this time the glow wire flammability test machine is in the heating state, please do not operate the LCD screen, otherwise, the heating will be interrupted; if you accidentally click the LCD screen, you need to press the emergency stop button to restart the machine. Before testing, you need to press the “reset” button to ensure that the glow wire test machine maintains the initial state.
Adjust the current, until it reaches the test-needed temperature value.
Turn the “current adjustment” knob(shown as 3) until the temperature on the temperature control table reaches the experimental requirements(Shown as 2), at this time, the current meter shows 120V ~ 150V(Shown as 1), (Note: while adjusting the current, “test temperature” temperature controller is also in real-time display of the temperature of the glow wire head). Observe the “test current” and the “Test temperature ” data display.
(1)Test current： glowing filament work currently
(2)Test temperature： glowing filament temperature
(3)The rotary knob of the Test Current
(4)The display screen
(5)Emergency stop button
Start the “TEST” button to begin the glow wire test.
After the “Test temperature” (i.e. the temperature value of the glow wire head) on the machine reaches the required temperature value (e.g. 850℃) and remains constant for the 60s, press the “TEST” button on the control panel, the trolley carrying the sample will automatically move to the right and approach the glow wire head, “Ignition time Ti” is automatic timing.
When the sample is ignited, press the button on the first line(Triangle icon) to stop the “Ignition time Ti”; at this time, the sample starts to burn and the “Flame extinction time Te” starts timing automatically.
Observe the burning situation and stop timing when the flame is extinguished.
Carefully observe the burning of the sample, when the flame on the sample is extinguished, press the button (Triangle icon in the first line) again to make the “Flame extinction time Te” also stop timing. At the same time, the machine defaults to the end of this test, and the test current will automatically stop running (adjust the current to the minimum value after the test). Note: In case of unexpected accidents during the test, press the red emergency stop button on the right immediately.
End of test.
After the test, open the “fan” and “light” switches on the operation panel’s right side to observe the sample and remove the waste fumes.
Open the observation window door, take off the sample, record the test results, clean the dirt inside the box and turn off the power.
Judgment of test results (GWFI test as an example)
The test sample is considered to have survived the test if it does not ignite or if the following two conditions are met.
1) If the flame or scorch line of the test sample is extinguished within the 30s after the removal of the scorching wire.
2) the packaging silk paper did not ignite.
If the above two conditions or one of the two conditions are not met, then select a lower test temperature and repeat the test with a new test sample. If both of the above conditions are met, a higher test temperature is selected and the test is repeated with a new test sample.
GWT and GWIT tests are subject to the same rules as above.
Record the test results and plot the graph (usually presented as a correlation between the glow wire temperature and the heating current).
Note: The above glow wire test procedures are based on the functions of TESTEX TF346 Glow Wire Tester. In practice, the test should be performed in conjunction with the test standards and the specific functions of different manufacturers’ instruments.
Cautions of the glow wire test
1. The user of this glow wire flammability tester is recommended to use a regulated power supply, otherwise the temperature will be affected by excessive voltage fluctuations. This glow wire test equipment must be placed in a dry indoor environment without direct sunlight. During the test, this glow wire test equipment must be grounded to avoid electrostatic reactions.
2. During the test, the door of the test chamber must be closed so that the temperature of the glow wire head will reach the test requirement.
3. After the test, please do not use your hands to touch the scorching wire to avoid high-temperature burns. After the test, you can use the exhaust fan to discharge the exhaust gas generated during combustion, but do not open the fan during the test, so as not to affect the glow wire test results. Clean up the glow wire test equipment in time after use, clean the combustion chamber in the case of power failure, and pay attention not to collide with the thermocouple and scorching wire, keep the equipment dry and clean.
4. Regularly inject lubricant into the transmission parts of the glow wire test equipment to avoid affecting the smooth conduct of the test. If you need to repair, especially the replacement of the glow wire should pay attention to the dialed screws to re-tighten, otherwise poor contact affects the current circuit and generate a high-temperature interface. Users should tighten all screws after replacing parts.
How do you test a glow wire? Do you have better solutions? Welcome to share with us, thanks.