The “cabin ” concept has been use d widely for fire safety in big public transport terminals not fully protected by sprinkler nor smoke exhaust system in the Far East for the past 20 years . There, only the areas with a higher fire risk, such as retail shop s, are protected. The big terminal s are used to be crowded, an d so there are concerns on whether the cabin design can provide adequate fire protection in such big halls, and whether the design will give problems in firefighting. Consequently, additional fire safety provisions such as long -throw sprinkler had been installed in areas with higher fire risk. Smoke exhaust system have to be added to the large hall . In the new edition of fire safety provisions requir ed for railway systems , the open type of cabin is specified. No in -depth study supported by systematic experiments on how the fire can be limited to 2 MW reported in the literature. On the other hand, p reliminary experimental studies on cabin with sprinkler and smoke exhaust carried out indicated that there are many concerns . In this paper, five tests on cabin fire carried out in a big burning hall at the State Key Laboratory of Fire Science, University of Science and Technol ogy of China (USTC) will be taken out to demonstrate the problems associated with such open type cabins . Problems in big halls without fully covered by smoke exhaust system had been demonstrated clearly in an airport terminal fire in 1998. It took a long t ime for firefighters to locate the fire source in that small fire at the airport terminal. The fire safety m ust be upgraded by appropriate hardware engineering systems and software safety management to reduce the chance of having big fire, and facilitate f irefighting. 3 1. Introduction In the new edition of fire safety provisions required for railway systems r eleased in 2013 [1], the open type of cabin is specified with concerns raised while implementing performance -based design [2,3]. In fact, t he “cabin ” concept has been used widely for fire safety in big public transport terminals without fully protected by sprinkler and smoke exhaust systems in the Far East for about 20 years [4,5]. Only the areas with a higher fire risk, such as retail shop s, are protected and, the big terminals are used to be crowded . Any inciden ts might lead to uncontrolled evacuation as experienced in Lan Kwai Fong , Hong Kong [6], Kwai Yuan in Beijing [7], and others [8] . The smoke filling problem associated with fire in big hall h ad been clearly demonstrated in a small fire in the Hong Kong airport terminal before opening [9]. In fact, it took a long time for firefighters to locate the fire source. There are also concerns [10-22] on whether the cabin design can protect fire s within 2 MW in such big halls not fully covered by sprinkler nor smoke control. Consequently, long -throw sprinkler had been in stalled in some areas [23] in some existing airport terminal, and smoke exhaust systems are specified to be installed in big halls for new development . Very few experimental data are reported in the literature [4,5,15,24, 25] on how the fire can be limited 2 MW. Preliminary studies [26-36] indicated that there are many problems in such cabin design. Some preliminary works were reported i n Chinese literature without in -depth analysis. In this paper, five tests on cabin fire carried out in a big burning hall at the State Key Laboratory of Fire Science, University of Science and Technology of China (USTC) will be taken out to demonstrate the problems associated with such open type cabins . 4 It is necessary to carry out systematic experimental stud ies [22] on cabin with sprinkler and smoke exhaust operating as expected, say by limiting the fire load stored in each cabin, installing more effect ive fire extinguishing system, providing additional protection in the big halls such as long-throw sprinkler and smoke exhaust system. Appropriate fire safety management and tight fire inspection scheme must be implemented to reduce the chance of having bi g fires. 5 2. Earlier E xperimental Study To evaluate the performance of cabin fires with sprinkler and smoke ex haust systems in operation , a cabin model was constructed in an atrium of height 27 m. The cabin is of length 3 m, width 4 m and h eight 3 m as shown in Figs. 1a to c. A door of height up to 1.4 m was designed. The height can be varied to adjust the ventilation factor . A centrifugal water - cooling high temperature exhaust fan with adjustable flow rate was installed . A sprinkler system was installed with normal sprinkler head commonly used [37] to protect shops in an atrium . The nozzles and spray pattern discharged are shown in Fig. 2, with operating parameters listed in Table 1. The sprinkler operating curve is shown in Fig. 3. Air temperature inside the cabin w as measured by thermocouples inserted in the hole s drilled on the northern cabin wall as shown in Figs. 4a and b . Nine holes (labeled as TC1 to TC9 in Fig. 4a) spaced uniformly were provided along the vertical line in tw o parts. As water discharged from the sprinkler nozzle acting on the thermocouples would affect the temperature measurement , thermocouples were shielded by steel plates as shown in Fig. 4c. Temperatures at the outside hall wer e measured by thermal resist ors. Carbon monoxide ( CO) concentration inside the cabi n was measured by a gas analyzer inserted in the center of the fan duct connected to the cabin. When the smoke exhaust fan operated, gas inside the cabin was extracted into the duct. A maximum reading of 5000 ppm was adjusted . Photograph of the CO analy zer is shown in Fig. 5 . 6 The wood crib fires were set up as in Fig. 6a with key parameters listed in Table 2. The crib was ignited over a pool of size 0.25 m by 0.25 m with 0.2 kg of oil. Heat re lease rate of one wood crib of size 0.6 m by 0.6 m by 0.4 m was measured separately in an oxygen consumption calorimeter as shown in Fig. 6b. The wood crib broke down at 650 s with the heat release rate decreased. A steady heat release rate of 650 kW was observed at about 250 s . Four wo od cribs were tested and labeled as C1 to C4 as in Fig. 1. Heat release rate of burning four cribs would be about 2.6 MW, when their separation distance was longer than 0.3 m. Five full-scale burning tests [30-36] with smoke exhaust rates and sprinkler operating conditions shown in Table 3 were taken out for illustrating the fire hazards .  Test T1: The smoke exhaust rate was 8000 m3/h, and the sprinkler pressure was 0.2 MPa. Four wood cribs were placed at the center.  Test T2: The smoke exhaust rate was 1582 m3/h, and the sprinkler pressure was 0.2 MPa. Four wood cribs were placed at the center.  Test T3: The smoke exhaust rate was 8000 m3/h, and the sprinkler pressure was 0.1 MPa. Four wood cribs were placed at the cente r.  Test T4: The smoke exhaust rate was 1582 m3/h, and the sprinkler pressure was 0.1 MPa. Four wood cribs were placed at the center.  Test T5: Without operating the smoke exhaust and sprinkler system s, all four wood crib s were placed at the center, known as a ‘bare cabin’ [17]. 7 3. Observation s The following were observed in the five tests [30-36]:  Test T1 After ignition, smoke spilled out from the cabin rapidly. Smoke exhaust fan was operated at 30 s, the smoke layer height rose with little smoke spill ing out. The flame height of the two wood crib s near the fan is higher than that of the other two cribs. The fire grew rapidly, and spread to the edge of the wood crib at about 160 s. Sprinkler was operated at 160 s and the flame he ight descended . The flame became not so bright , but the fire did not reduce in size . More smoke spilled out from the cabin . The smoke layer height fell to about 0.65 m with high s moke concentration . The cribs in the cabin were observed to be burning . Wood crib C4 broke down and C2 extinguished at 625 s. Wood crib C1 extinguish ed at 735 s and broke down at 790 s. All wood cribs extinguish ed at 840 s. Pictures inside the cabin after doing t est T1 is shown in Fig. 7a. Wood crib C2 did not break into s maller parts but almost bur ned out . W ood crib C3 burned out without break ing down.  Test T2 The burning process was similar to Test T1 though the smoke exhaust fan was turned on upon ignition . As the exhaust rate was low, smoke spilled out from the cabi n. Sprinkler 8 was operated at 206 s and more smoke spill ed out. The bur ning process in the cabin could not be observed at 235 s as the smoke layer fell to about 0.25 m. Smoke concentration was the highest among the five tests. The hall was also filled up rapidly with smoke. Smoke exhaust fan of the hall was operated at 580 s. The picture inside the cabin a fter test T2 is shown in Fig. 7b . A ll wood cribs did not break down, and most of them did not burn out.  Test T3 The burning process was similar to Test T1 after ignition . Smoke exhaust fan was operated at 20 s. After that, smoke did not spill out of the cabin . The flame height of the two wood cribs near the fan is higher than that of the other two. Sprinkler was operated at 160 s and some smoke spilled out from the cabin. Smoke concentration was the lowest among the five tests. The burning process in the cabin could be observed clearly . The smoke layer height was about 0.9 m. Wood crib C1 broke down at 490 s and extinguish ed at 690 s. Wood crib C2 broke down at 580 s and extinguish ed at 750 s. All wood cribs extinguish ed at 860 s. The photograph inside the cabin after t est T3 is shown in Fig. 7c. Wood crib C4 did not break down but almost burned out . Wood crib C3 burned out but did not break down.  Test T4 9 The bu rning process was similar to Test T2 after ignition . Smoke exhaust fan was operated at 12 s with lower smoke exhaust rate. Smoke spilled out from the cabin. Sprinkler was operated at 145 s and more smoke spilled out . The smoke layer height descended to about 0.6 m. Smoke concentration was lower than Test T1 and T2, but higher than Test T3. The burning process in the cabin could be observed. Picture of the cabin after test T4 is shown in Fig. 7d . Wood crib C1 broke down, wo od cribs C2 and C4 almost b urned out, but did not break down . W ood crib C3 mostly burned out, but did not break down.  Test T5 Smoke spilled out from the cabin rapidly upon ignition. The fire grew up rapidly and the smoke layer height was about 0.9 m. Flame spread out from the do or at 360 s. Observers could not stand in front of the door to observe the burning process due to strong heat radiation . It was not tenable to stay at even 5 m away from the door. Noise of cracking the flooring materials was he ard at about 400 s. All wo od cribs broke down at 560 s, and burned out at 700 s. The wood residue was still burning. 10 4. Results on Temperatures Air temperature measured inside the cabin and average smoke layer temperature are sho wn in Figs. 8 and 9 . For tests T1 and T2, air temperatures inside the cabin decreased suddenly upon operating the sprinkler system. Temperature in test T1 was quite steady . Air temperature in test T2 kept on decreasing u ntil extinguishing the fire. The average smoke layer temperature in test T1 was higher than that in test T2 throughout the whole burning process. In tests T3 and T4, air temperature decreased to a steady value but still quite high . The steady air tempe rature in T3 was about 330 C, higher than that in T4 of about 125 C. These indi cated that air temperature inside the cabin would be affected by the smoke exhaust systems. Operating the s moke exhaust fan would draw in air to sustain combustion. Air tempe rature inside the cabin increased with increasing smoke exhaust rate. Compar ing air temperature in T1 with T3 under the same exhaust rate of 8000 m3/h, and compar ing air temperature in T2 with T4 under the same flow rate of 1582 m3/h, increasing the sprinkler pressure would decrease the temperature inside the cabin . As both smoke exhaust and sprinkler systems were not operate d in test T5, air temperature s in T5 were the highest. The highest temperature was 860C found near to the ceiling . The average smoke layer temperature increased to 600 C at 325 s but flashover did not occur . When 11 the average smoke layer temperature increased to 666 C at 400 s, air temperature increased rapidly. Corresponding to the noise of cracking the flooring materials heard a t 400 s, flashover should have occur red. Therefore, fire suppression system suc h as sprinkler must be installed to protect the cabin. Before operating the sprinkler , temperatures in tests T1, T3 and T4 were higher than that in T5. This suggested that o perating the smoke exhaust fan would sustain combustion. Air temperatures insid e the cabin increased with increasing smoke exhaust rate. 12 5. Results on CO Concentration Concentration of CO measured inside the cabin for tests T1 to T4 are shown in Fig. 10. CO concentration s measured in tests T1 and T2 were over the pre-set maximum value of 5000 ppm of the analyzer . CO concentration in T2 was higher than that in T1 with the same sprinkler pressure 0.2 MPa over some time . Simi larly, CO concentration in T4 was higher than that in T3 with the same pressure 0.1 MPa. These indicate d that CO concentration inside the cabin would be affected by the smoke exhaust system. Different smoke exhaust rates would give different ventilation conditions . The burning process would be affected . CO concentration inside the cabin decrease d with increa sing smoke exhaust rate . In compar ing CO concentration in T1 with T3 under the same exhaust rate of 8000 m3/h, and compar ing CO concentration in T2 with T4 under the same exhaust rate 1582 m3/h, increasing the sprinkler pressure would increase the CO con centration inside the cabin . As observed, there were two peaks in the CO concentration curves in T1, T3 and T4. Only one peak was observed in T2. The first peak appear ed after the sprinkler was operate d, and the second peak appear ed at the burning decay stage. These might suggest different fire suppressing effect s. This point will be further explored and reported later. 13 6. Effect to the Hall Smoke was observed to spill out from the cabin for all the five sets of tests. A s table layer was formed at the hall with lower smoke layer height for the cabin with sprinkler operating, due to the hot steam generated . Average smoke layer temperature s meas ured in the outside hall are shown in Fig. 11. The highest smoke temperature s for the five tests were 20.7C for T1, 20.7C for T2, 22C for T3, 24C for T4 and 33.5 C for T5. Temperatures in the hall are very low due to the relative ly small fire (2.6 MW ) in a big hall of size 22.4 m by 1 2 m by 27 m. Smoke layer height in the outside hall was measured with the N-percentage rule [38,39] with N taken to be 30 and v isual observations, shown in Fig. 12.  Test T1 The steady smoke layer height was about 4 m. The time for the smoke layer height to descend to 4 m after sprinkler operat ion was about 150 s. The rate of smoke filling up the hall was fast.  Test T2 Smoke layer height started to descend at about 130 s before operating the sprinkler. Smoke layer height descended to 11 m with low smoke concentration . After the sprinkler was operate d, the descending rate of smoke layer bec ame slow, but smoke concentration became 14 very high. The time of smoke layer height descending to 4 m after sprinkler operat ion was about 200 s. It took 350 s to fall to 3 m after operating the sprinkler . Exhaust fan of the hall was operated at 580 s.  Test T3 Similar to T1, smoke filled the hall after operating the sprinkle r. The smoke layer hei ght was about 5 m at about 250 s. Smoke concentration was very low .  Test T4 Similar to T2, smoke layer height started to descend at about 140 s before the sprinkler was operate d. The smoke layer fell to 22 m at the time when sprinkler was operate d. The smoke concentration was low . After operating the sprinkler , smoke concentration increase d but lower than that in tests T 1 and T2. The smo ke layer height was about 5 m. The time of smoke layer height descending to 5 m after operating the sprinkler was about 260 s. Smoke concentration was low .  Test T5 Smoke layer height started to descend at about 80 s. The steady smoke layer height was even higher at about 7 m at about 360 s for the bare cabin . 15 7. Conclusions The following are observed in the five p reliminary tests with open cabin:  It is obvious that smoke spread out to the hall from an open cabin in all five sets of tests.  Temper ature in side the bare cabin increased quickly upon burning the crib s. The ceiling temperature could be up to 860C by burning four wood cribs of heat release rate 2.6 MW. Flashover occurred at 400 s. The average smoke layer temperature was 666 C, higher than the flashover criterion of 600C in some standards . The cabin structure might be damaged by the high temperatur e. Flame and large quantity of hot smoke spilling out might be a threat to occupants staying at the outside hall.  The operating sprink ler pressure is a key parameter . There was a critical operating pressure for the sprinkler system to control a fire.  With the fire suppression system operated , temperature in the hall was low . Smoke concentration reduced when the smoke exhaust system i n the cabin operated . Operating normal sprinkler will give a longer burning duration. More smoke was liberated . Although t emperature in the hall was very low, the smoke concentration was very high. It is certainly a potential risk to life safety.  It is doubtful whether normal operation conditions of sprinkler and smoke exhaust system 16 can control fire s bigger than about 2 MW [24,25] in a cabin . Using a sprinkler pressure of 0.1 MPa and a smoke exhaust system of 40 ACH might not be effective in contr olling the fire to 2 MW . Note that terminals with cabin design are usually crowded with people . Passengers are allo wed to carry luggage up to 25 kg even for economy class tickets. Appropriate fire suppression system s should be installed to control flash over cabin fires . Sprinkler system is only one of the active engineering systems commonly provided to give suppression effects . Other workable fire suppression system s with high er efficiency should be considered to put out the fire rapidly . Again, in -depth experimental studies must be carried out as raised by the principal author [2,3]. 17 8. Additional Comments The paper was pub lished as a journal study [40]. With the practice of publisher, preprint i s allowed to upload at a website. The cabin design looks good because the huge terminals need not fully covered by sprinkler nor smoke extraction . In respons e to the challenges encountered in the past 2 0 year s, there are furth er studi es on this topic because of safety concerns after some users put lots of combustibles outside the cabins, even for catering. Further, some fires occu rred in such big terminals with smoke filled up the whole space quickly because no smoke extraction systems as in [41-71]. This is clearly demonstrated in above with no or inadequate fire serv ices installations. Additional f ire services installations such as long throw spri nklers are installed [41,42,48,5 2]. Fire safety mana gement [51,55,60 -71] is a key part in providing safety, with more inspections to see combustibles put outside the cabins . Even the combustibles inside the shops were limited in some termi nals. Note that higher punis hment [55,60 -67] to responsible persons on safety issues in the Hong Kong Special Administrative Regions (HKSAR) in early 2023 . Enhancing safety culture [62-66,68] in retenting top management after big fires as in Beiji ng 2023 is a good practice for the HKSAR to learn, instead of just slight punishment with few thousand US dollars as in tower crane collapsed with causality . Nothing proceeded on collapse of a university green roofs [49,50] . Safety issue s must be responsible by top management to execute effect ive fire safety management schemes . Recent serious incidence led to retention of responsible persons on falling down of external finishes in July 2023. Research on positive pressure ventilation (PPV) [53,56 ] must be explored on firefi ghting and rescue in atrium full of smoke . Anyway, HKSAR is moving towards a position of caring labou rs more on safety issue s after smooth transition to mainland in 1997. 18 Acknowledgement This work descr ibed in this pape r was s upported by a grant from the Fundamental Research Funds for the Central Universi ties in China . 19