In this study, the combustibility of MgH2 powder （Dv,50: 18, 40, 72 µm） of three sizes was conducted. As a result of the experiment, the lower limit of explosion concentration was 25 to 60 g /m³, the MIE was 2.1 to 24 mJ, and the ignition temperature in the deposited state was 370 to 390 ℃. The ignition temperature in the suspended dust state was 500 to 580 ℃, which was lower than that of the Mg powder used for comparison （610℃）. In the hot surface ignition, Mg powder ignites first after the desorption of hydrogen, and then ignites the premixed gas of hydrogen and air retained in the powder layer. Regarding the ignition of Mg powder, it is considered that the desorbing hydrogen destroys the protective oxide film from the inside of the powder, which makes it easier to react with oxygen on the new surface, leading to ignition. As a result of examining the effect of moisture on the ambient temperature with respect to heat generation, it should be stored at a temperature lower than 50 ℃ at most.

Hydrogen can be considered as an energy carrier used to reduce CO2 emissions that affect greenhouse gases. As one of the efficient hydrogen supply forms, there is a supply system using pipelines. However, considering the spread of this system, there is a risk of accidents due to hydrogen leakage, and ensuring its safety is an important issue. Until now, there are few papers that consistently evaluate the diffusion in the ground and in the atmosphere of hydrogen when it leaks from buried pipes assuming actual operation. Therefore, it is strongly required to examine the safety risks under these conditions. In this paper, we organized the basic knowledge about the diffusion behavior of hydrogen when it leaked from the buried pipe by experiments, and presented the applicable analysis method. This knowledge is expected to be useful when examining the safety risks of system that supply hydrogen by pipelines in the future.

The number of “drawn into a machine” accidents caused by machinery in the manufacturing industry is the highest ^1）. Focused and prioritized measures are required to reduce “drawn into a machine” lost-time accidents with serious sequelae and fatal accidents. Regarding the prevention of occupational accidents by machinery, there are many research reports on mechanical equipment protection measures and human factors of workers. According to the Ministry of Health, Labor and Welfare report ^2）～5）, the number of accidents of workers with short years of experience is large, including not only the younger age group but also the middle-aged and older age groups. In this paper, I conducted multivariate analysis on the age and factors of occupational accidents, focusing on the occupational accidents that were “drawn into a machine” by workers with short years of experience. As a result, I found that the factors of occupational accidents differ depending on the age group. As one of the measures to prevent occupational accidents, it is necessary to provide education and guidance in consideration of the age group of inexperienced workers.

The incendivity of abnormal discharges occurring from a nozzle-type bipolar electrostatic ionizer was experimentally examined through the discharge amount using three methods（ i.e., EN type probe, IEC type probe, JNIOSH type probe）. Prior to the main experiments, a calibration test was performed with all the three methods, referred to as preliminary experiment. Then, for the main experiment, corona discharge needle of the ionizer was contacted with the discharge electrode （metal ball, 25 mm in dimeter） attached to each probe. As a result, the values obtained using the three methods were almost the same, and their reliability was confirmed in the calibration test. Among them, the JNIOSH method is the easiest and has the smallest error. It was also confirmed that all the charge amount obtained in this study did not exceed 30 nC （absolute value） which is a safe level and is the threshold for the risk assessment in the industry process using electrostatic ionizer. Therefore, it was suggested that the ionizer used in this experiment is extremely safe. However, when the generation of corona ions was suppressed from the discharge needle, the maximum value was 32.38 nC （absolute value）, which is not within the safe level. In order to not hinder the production of corona ions, regular maintenance is required.

To prevent explosion accidents, it is necessary to isolate the ignition source from the explosive atomosphere. In order to establish spatial isolation, electrical equipment must be made as explosion-proof where the explosive atmospheres may exist, according to the risk assessment for the prevention from becoming an ignition source. The designating places where the explosive atmospheres may exist, is called as “Zone” in the International Electrotechnical Commission （IEC）. In this paper, we introduce the concept of functional safety by considering the failure rate of equipment related to the release, dilution, and ventilation of explosive atmospheres for indicating the “Availability of Ventilation” in qualitative expressions when determining “Zone”. A method of calculation, which based on the downtime from the functional safety, is proposed. The “Availability of Ventilation” can be quantified by relating it to the mean failure rate of equipment.

When the explosion vent opens due to thermal runaway of the lithium-ion secondary battery placed in the environmental test chamber, and the open state is maintained, new hazards may be produced by continuous flame emissions, repeated gas explosions, and toxic gas outflows. This paper proposes a hinged door type explosion vent （explosion door） that opens at a set pressure and closes by its own weight to control these hazards. Next, the hazard control effects of the proposed system is discussed from the results of its operational experiments.