This study uses a metal stud partition fireproof drywall measuring 83 mm in thickness as a test specimen to explore the impact of an embedded junction box on the firefighting performance of the wall through one time of standard fire test on a 300 cm × 300 cm area and five times of standard fire test on a 120 cm × 120 cm area. The results show that the quality of calcium silicate board plays a big role in the fireproof effectiveness. The embedded junction box located on the backside of the fire can reduce the effectiveness of the wall, especially the area above the socket. The thickness of rock wool may increase the performance, but in a limited rate. External junction box may not impact the fireproofing performance of the wall but it still possesses some safety risks. An embedded junction box measuring 101 × 55 mm could already damage the fire compartment, and in reality there may be more complicated situations that should be noted and improved.
The walls installed in fire prevention areas should be possessed of flame retardant effectiveness. For the trend of architectural engineering is towards increasing in dimension and high-rise, the conventional heavy building materials and high labor intensive methods are descending. Take panel closure walls, for example; the metal frame light panel closure system is well received for the characteristics of fixed construction method, shortened period, various techniques, light materials, and the stable material quality compared to concrete. Currently there are many studies on the performance issues of metal stud drywall partitioning system. Chuang et al. [
This study is different from the previously published studies in that it does not inform the manufacturers of the fire tests to be conducted and instead directly purchases the commercially available boards to be used as the test samples. The previously published studies all focus on the thermal conductivity of board material [
This study uses two sets of test equipment that both can conduct material testing horizontally or vertically. The first furnace measures 300 cm in width, 300 cm in height, and 240 cm in depth. The second one measures 120 cm in width, 120 cm in height, and 120 cm in depth. Both equipment sets use electronic ignition and the control systems are computerized PID temperature controllers. The furnaces are manufactured by Kuo Ming Refractory Industrial Co., Ltd. The full-size furnace has 8 burners, of which only 4 are switched on for wall test. Two temperature control thermocouples are inside, controlling the operation of 2 burners each on the left and right sides. The remaining 7 thermocouples measure the furnace temperature and they are all inserted from the top of the test furnace (see Figure
Full-size high-temperature furnace (inner size: 300 cm in width, 300 cm in height, and 240 cm in depth).
Small-size high-temperature furnace (inner size: 120 cm in width, 120 cm in height, and 120 cm in depth).
This study uses 9 mm calcium silicate boards that are commercially available (calcium silicate boards of Test 1: flexural strength: 125 kgf/cm2, thermal conductivity: 0.14 w/mk, bulk specific gravity: 0.81 g/cm3; calcium silicate boards of Tests 2~6: flexural strength: 124 kgf/cm2, thermal conductivity: 0.13 w/mk, bulk specific gravity: 0.81 g/cm3). It uses vertical closure boards and self-tapping screws to stabilize them. The screws are 3.5 mm in diameter, 25.4 mm in length, and 250 mm in spacing. The columns are the CH channel iron measuring 65 × 35 × 0.6 mm, the upper and lower slots are the C channel iron measuring 67 × 25 × 0.6 mm, and the spacing within the column is 406 mm. Rock wool used is measured 50 mm in thickness and 60 kg/m3 and 100 kg/m3 in density, respectively. For the embedded sockets, the external part is a switch panel measuring 120 mm × 70 mm and the internal part is a junction box measuring 101 × 55 × 36 mm. For the external sockets, the external part is a 120 mm × 70 mm switch panel and the internal part is a 120 × 70 × 47 mm junction box. The external switch panels are all made of ABS (Acrylonitrile Butadiene Styrene) and the inside is a galvanized iron box.
ISO 834-1 [
Test specimen standard.
Exposed surface | Unexposed surface | Density of the fireproof cotton | Size of the metal stud partition fireproof drywall | |
---|---|---|---|---|
Test 1 | None | None | 60 kg/m3 | 3.0 m × 3.0 m |
Test 2 | None | None | 60 kg/m3 | 1.2 m × 1.2 m |
Test 3 | None | Embedded internal socket | 60 kg/m3 | 1.2 m × 1.2 m |
Test 4 | None | Embedded internal socket | 100 kg/m3 | 1.2 m × 1.2 m |
Test 5 | None | Installed external socket | 60 kg/m3 | 1.2 m × 1.2 m |
Test 6 | Embedded internal socket | None | 60 kg/m3 | 1.2 m × 1.2 m |
Test 1: the exposed surface and unexposed surface of specimen and geometry of the thermocouple (the arrangement of boards and measuring points of specimen).
Test 1 follows the specifications of ISO 834-1 [
From Tests 2 to 6, the heating temperature follows the standard heating curve in the ISO 834-1 [
In Test 1, 8 thermocouples are placed on the surface of the test specimen away from the fire as shown in Figure
Test 2~Test 6: exposed surface and unexposed surface of specimen and geometry of the thermocouple (the arrangement of boards and measuring points of specimens for Test 2~Test 6).
The time for Test 1 lasts 60 minutes. Seven minutes after the test started, the gap between top right corners of the unexposed surface away from the right frame starts showing a bit odorous white smoke. The temperature at all the detection points also shows a significant uptrend and keeps rising till the 11th minute when it shows a downtrend till the 27th minute and then goes up again all the way till the test ends. At the 27th minute, the highest temperature is at the upper left center at 73.9°C. At this point, a horizontal crack appears on the surface not facing the fire on the left panel and the center. At the 37th minute, the horizontal crack on the left keeps extending toward the center. At the 60th minute when the test ends, the highest temperature at the upper left center is 97.6°C and the highest average temperature is at 89.5°C (see Figure
Time-temperature chart for the specimen in Test 1.
Test 2 lasts 40.5 minutes. Six minutes into the test there seems to be an explosion. The temperature inside the rock wool center also shows a clear uptrend at this time, indicating that the calcium silicate board facing the fire is damaged due to the rising temperature. At the 8th minute, the cross-shaped gap not facing the fire starts generating smoke. At the 12th minute, the temperature inside the rock wool center continues going up, indicating that the rock wool continues touching the higher temperature. At the 39th minute, the temperature in the middle heats up to 180°C (see Figure
Time-temperature chart for the specimen in Test 2.
Test 3 lasts 40 minutes. Six minutes into the test there seems to be an explosion. The temperature inside the rock wool center also shows a clear uptrend, indicating that the calcium silicate board facing the fire is damaged due to the rising furnace temperature. At the 15th minute when the furnace temperature is at 750°C, the temperature at the detection point is already above 180°C and afterward it quickly approaches the furnace temperature, indicating that the rock wool center is totally on fire. The calcium silicate board facing the fire and part of the rock wool are also burned, resulting in the continuously higher temperature measured from the surface not facing the fire. At the 19th minute, the junction box panel has begun to melt and the heated gas begins to spring from the gap between the box and the board, leading to the significant increase in the temperature of the upper junction box measured by the thermocouple. At the 31st minute, the detection point goes over 180°C (see Figure
Time-temperature chart for the specimen in Test 3.
Test 4 lasts 43.8 minutes. Six minutes after the test started there seems to be an explosion. The temperature inside the fireproof cotton center also shows a clear uptrend, indicating that the calcium silicate board facing the fire may have been damaged due to the rising furnace temperature. At the 17th minute, the temperature inside the rock wool center is already more than 180°C and at 20th minute it quickly approaches the furnace temperature, indicating that the rock wool center is totally on fire. The calcium silicate board facing the fire and part of the rock wool are also burned. At the 25th minute, the junction box panel has begun to melt. At the 34th minute, the temperature at the upper junction box goes over 180°C (see Figure
Time-temperature chart for the specimen in Test 4.
Test 5 lasts 39 minutes. Six minutes into the test there seems to be an explosion. The temperature inside the rock wool center also shows a clear uptrend after the 7th minute, indicating that the calcium silicate board facing the fire is damaged due to the rising temperature. After the 7th minute, the cross-shaped gap not facing the fire starts generating smoke. At the 25th minute, the junction box has begun to melt due to heat. At the 29th minute, the part connected to the screw is totally melted and then falls off. At this point, the temperature at the junction box is 53.9°C because the box has already fallen off and away from the furnace (see Figure
The result of the unexposed surface of the sample in Test 5.
Time-temperature chart for the specimen in Test 5.
Test 6 lasts 37.6 minutes. Six minutes into the test there seems to be an explosion. The temperature inside the rock wool center also shows a clear uptrend, indicating that the calcium silicate board facing the fire is damaged due to the rising temperature. At the 9th minute, the cross-shaped gap not facing the fire starts generating smoke. At the 12th minute, the temperature inside the rock wool center continues going up, indicating that the rock wool continues touching higher temperature. At the 36.8th minute, the temperature in the middle heats up to 180°C (see Figure
Time-temperature chart for the specimen in Test 6.
The board used in Test 1 is provided by the supplier. These board materials are known as the laboratory grade. Although there are some cracks on the surface facing the fire during the experiment, the surface does not explode, and the integrity is good by visual inspection (see Figure
The result of the specimen after 60 min standard fire test in Test 1.
Tests 2 to 6 use the commercially available calcium silicate boards. These boards are claimed to have passed the fire ratings inspection, but every test finds that at the 6th minute the surface facing the fire explodes. Without the protection from the calcium silicate board, the fire in the furnace can directly damage the rock wool. The rock wool may have some strength and tension due to the glue added in during the production, but it starts to have pores after the glue is damaged [
The result of the exposed surface of the specimen in Test 5.
A calcium silicate board mainly consists of inorganic silicate and lime. Manufacturers all use different formula, and some may mix in a certain proportion of coal ash to replace cement to reduce the production cost. Also, the board is made by high-pressure steam curing, so if the material ratio varies, the poor control of high-pressure steam environment may cause the strength variance of the calcium silicate boards, further impacting the heat resistance during the fire test. The impact can be observed from Test 1 and the other tests. Before taking the possible cut-corners of the suppliers or poor quality into consideration, this is just to show what the circumstances may be if the calcium silicate boards are in poor quality. This can really happen in Taiwan and other places, so extra attention is needed for this issue. The commercially available board materials need to put sample inspection or other controlling methods to prevent the inconsistency of quality between the ones in the market and the ones sent for tests.
This study is to understand the actual firefighting performance of the walls in daily life. For example, Tests 1 and 2 show that the products presumably made by the same company but in reality containing different materials can have almost 20 minutes of difference in their fire ratings. Tests 3 to 6 show the impact of socket and junction box on firewalls. Reviewing the firewall fire ratings tests conducted worldwide, there is not yet any test conducted with installed socket and junction box. Embedding socket and junction box into the drywalls requires breaking the wall body, and it is almost unavoidable to mount them on the wall. The installed amount may be more than just one and there are more varieties (such as for Internet or telephone lines), so these combined issues really need to be resolved. When an unqualified board is installed with socket and junction box, the actual fire performance can make people worry.
Comparing the results from Tests 3 and 4 with Test 2, we can see that the embedded junction box impacts significantly on the fire performance of the wall body. Fire performance is determined together by the calcium silicate boards on the two sides and the fireproof cotton in between. When the calcium silicate board on the side not facing the fire is damaged, a weak spot is produced. Hot air can come out from this spot. The metal junction box (fastened to the frame through screws and metal bars) is installed in after cutting a hole on the board not facing the fire, and there should be some gaps in between the metal box and the calcium silicate board. The frame can also deform after being heated, making the gap even bigger, and the surrounding edges and the place above can be affected by the heat. Although panels and sockets may be installed outside of the junction box, they are not noncombustible materials and therefore will still be melted by the hot air or burned (see Figures
The result of the unexposed surface of the specimen after 40 min standard fire test in Test 3.
The result of the unexposed surface of the specimen after 43.8 min standard fire test in Test 4.
The junction box panel in Test 3 starts to smoke at the 8th minute, and it starts to melt at the 19th minute and totally melts to make the panel fall to the ground at the 27th minute, and at the 31st minute the surface temperature not facing the fire exceeds the limitation in ISO 834-1 [
The above analysis revealed the following: When surfaces are flamed and fell, the flame retardant effectiveness is decreased for 20 mins (the flame retardant effectiveness is 40 mins) (without junction box inserted in). When surfaces with inserted junction box are flamed and fell, the flame retardant effectiveness is further decreased for 9 mins (the flame retardant effectiveness is 31 mins). When surfaces with inserted junction box are flamed and fell and the density of mineral wool is increased from 60 kg/m3 to 100 kg/m3, the flame retardant effectiveness is increased for 3 mins at most (the flame retardant effectiveness is 34 mins). When junction box fixed on the surfaces is not affected by flame, the flame retardant effectiveness is 37 mins. When junction box inserted on the surfaces is not affected by flame and the flamed surfaces fall, the flame retardant effectiveness is approximately 36.8 mins.
Following the above analysis we can see that the commercially available boards have significantly weaker fire performance, and embedding junction box into the side away from the fire would not only reduce the fire ratings even more but also concentrate the weak spot in the upper junction box. Adding rock wool density may help improve the fire ratings but the effectiveness is not so significant. The junction box used in this study measures 101 × 55 mm and is close to the 100 × 57 mm specified in National Electrical Code [
Installing an embedded junction box to the drywall can pose a certain level of risk. A box measuring 101 × 55 mm can already damage the fire compartment. In reality, there are a lot of more boxes installed on the wall, so this requires more attention and improvement. The conclusions are as follows: When surfaces are flamed and fell, the flame retardant effectiveness is decreased for 20 mins (the flame retardant effectiveness is 40 mins) (without junction box inserted in). When surfaces with inserted junction box are flamed and fell, the flame retardant effectiveness is further decreased for 9 mins (the flame retardant effectiveness is 31 mins). When surfaces with inserted junction box are flamed and fell and the density of mineral wool is increased from 60 kg/m3 to 100 kg/m3, the flame retardant effectiveness is increased for 3 mins at most (the flame retardant effectiveness is 34 mins). When junction box fixed on the surfaces is not affected by flame, the flame retardant effectiveness is 37 mins. When junction box inserted on the surfaces is not affected by flame and the flamed surfaces fall, the flame retardant effectiveness is approximately 36.8 mins.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank TFPT Laboratory for technically supporting this research.