JNIOSH

Abstract of Special Research Report (RR-32)

National Institute of Occupational Safety and Health, Japan

Theoretical Analysis of Electric Field Produced by Underwater Leakage Current (2nd Report) --Effects of a Solid Body on the Field Intensity--

RR-32-1
Tatsuo MOTOYAMA, Eiki YAMANO and Ryuji TANAKA

: This is one of a series of research work concerning the prevention of underwater electric shocks.
    In this report the authors give the results of theoretical analysis of the intensity of underwater electric field which will be affected by the existence of a certain solid body in water.
    In respect of the solid body, its electric conductivity, size and configuration contribute to the intensity of underwater electric field in conjunction with electric conductivity of water itself.
    Effects of each of the above factors on the field intensity are theoretically evaluated, particularly in terms of a ratio E = E s / E 0, where E and E 0 are electric field intensities in, and prior to, the existence of a solid body in water, respectively.
    Main conclusions obtained are as follows :
  (1) The effect of electric conductivity (σ0) of a solid body could be evaluated in relation to electric conductivity (σw) of water. The maximum value of Es will be approximately 1.5 or 3.0, depending upon whether the ratio (σ0w) is much smaller or larger than 1 ;
  (2) While the size of a solid body does not affect the maximum value of E s, the range of effects would be expanded with the increase of the size ;
  (3) Configuration of a solid body has complicated effects on the electric field intensity, particularly in respect of its sharpness ; and
  (4) Direction and non-uniformity of electric field should be also taken into account for the evaluation of underwater electric field.

A tree dimensional computational analysis of the wind loads on canvason or panel covered over the scaffold structure around a tall building

RR-32-2
Kinichi KINOSHITA

: For the purpose of the prevention of accidents caused by falling articles from a tall building under construction and others, canvases or panels are put up over a wide range on the outside of scaffold structures which built around the building. In these circumstances there have often happened fall-down accidents of the scaffolds by a strong wind.
    In this paper, six geometries of scaffold structures that are shown in Figure-1 were taken as models, and a numerical analysis has been conducted to predict wind loads on the canvases or panels in strong wind.
    The MAC method was employed to solve a set of governing equations for the three dimensional incompressible turbulent flow, and the turbulent quantities were treated with a two-equation model. A characteristic point in this computational treatment is that air flow process and wind pressure are carefully considered in the narrow space between the outer wall of a building and the canvas on the scaffold.
    From the results of computations, contour maps of coefficient of wind load on the canvas or panel are obtained for the model structures, and they are shown in Figures 18,19,21,22,24 and 25.
    The maps show that strong negative pressure arises in the space between the front side canvas to the wind and the outer wall of the building and locally at the neighborhood of front corners of scaffold.
    Numerical solutions are compared with experimental observations in a wind tunnel (Case 2) and on full-scale scaffoldings in natural strong wind environment (Case 6).
    The results show that both computational and experimental values of the coefficient of wind load are nearly the same, and the author concludes that the computational method described in this paper will be useful for practical applications.

A study on Time Intervals between Occupational Accidents (II)

RR-32-3
Shigeo HANAYAS

: The accident frequency rate has been widely used as one of the measurement of safety performance over a long period of time. The accident frequency rate is defined as the number of occurrence of occupational accidents for a certain unit of man-power or employee hour exposure. Since the accident frequency rate, which implies the potential of accident risk in a working place, is closely related to the number of occurrence of accidents, its statistical evaluation depends mainly upon the analysis of the frequency of occurrence of accidents in a fixed interval of time, e.g., poisson distribution.
    This study, in the meanwhile, takes into consideration the fluctuating time intervals between occupational accidents, regarding them as a useful measurement of safety performance in working places having a certain accident risk. In particular, emphasis was placed upon the statistical treatment for the extreme values of the time intervals between accidents, i.e., the smallest and the largest value among the observed time intervals between occupational accidents.
    The frequency distributions of so called extreme values as the smallest and the largest value in a random sample size (n) with a continuous density can be easily derived from the probability density function of order statistics as shown in Eqs. from (1) to (6).
    According to the result of the several observational investigation of accidents to date, it is recognized that occupational accidents are taking place at random, so that the time intervals between successive accidents can be represented by exponential distribution given by Equation (7), to at least a rough approximation. Also parameter of exponential distribution can be connected to the accident frequency rate as given in Equation (10).
    Hence, in order to verify whether the smallest or the largest time period among the observed time intervals between accidents is significantly short or long period of time, statistical evaluation of the extreme values of the time intervals between accidents such as the calculation of the probability of the extreme value corresponding to a certain accident frequency rate or estimation of the extreme values with reference to a probability, can be easily achieved by making use of the extreme value distribution functions represented by Eqs. (8),(9) and (14),(15).
    Extreme value analysis can also be applied to a problem of discovering the collective time periods between accidents of a work place having many independent industrial groups simultaneously. Namely, the collective time intervals between successive accidents of the whole groups can be solved by way of the smallest value of the time intervals, and it finally becomes another type of exponential distribution whose parameter is to total all groups' accident frequency rates together as shown in Eqs. (12) and (13). Similarly, time distribution of the period in which accidents have taken place in all groups, can be treated as the largest value of the time intervals and eventually expressed by the product of each group's distribution function exhibited as Eqs. (17) and (18).
    The estimation of the parameter of exponential distribution is sometimes difficult in dealing with the maximum and minimum values of the time periods between accidents. Hence, in this study, in order to avoid the disadvantage of estimation of the parameters of distribution function in the process of statistical evaluation, the extreme value ratios (the ratio of the smallest/largest value of the time intervals to the total summed-up hour of time periods between accidents) were derived as Equations (19),(20) and (22),(23). These distribution functions can be used for calculation of the extreme value ratio with reference to a probability without knowing the parameters of exponential distribution. An example using actual accident data is given to illustrate how to examine whether or not the smallest and the largest time intervals between accidents are significant values, by means of the distribution function of the maximum and minimum time ratio derived here.
    In conclusion, the author proposes that maximum and minimum time intervals between accidents can be utilized as a useful yardstick to express the safety performance in working places. They can be especially used for the analysis of homogeneity of the time series of accidents as well as the characteristics of the time intervals in a work place where many individual groups exist. Another advantage in relation to the use of the maximum and minimum values of periods is the simplification of the calculation in conducting the statistical evaluation of the time intervals. Particularly, maximum and minimum time ratio distribution can be employed in the analysis of the extreme values of the time periods without knowing the parameter or the accident frequency rate in working places.

Stability on the Saddle-type Beam for Powered Platform

RR-32-4
Yoshimasa KAWAJIRI and Yoshitada MORI

: Recently, the temporary powered platform came into use as hanging scaffold in many working sites for the maintenance of exterior buildings or structures. The saddle-type beam, as shown in Fig.1, is used to suspend a working platform of powered platform from the rooftop of buildings. This is a kind of spacer which is put on the rooftop parapet of building to keep constant space between the wall of building and a working platform.
    This beam, in its usual use, is not anchored to the parapet, therefore it may turn round or fall away from the parapet if the method of installation is inappropriate. Accidents on powered platform, this caused by the beam, are on the increase, and it is reported workers were wounded or killed as the result of such accidents.
    In the meantime, provisions for installation of the beam are not contained in the relevant Japanese laws and regulations for occupational safety, and the need for such is keenly felt.
    The purpose of this study was to clarify conditions of stability of saddle-type beam.
    Following theoretical analysis, equations were obtained to stabilize essentially the saddle-type beam, and from the result of a computer simulation for those equations and model experiment, the conditions of essential stability for the saddle-type beam are summarized as follows. (see also Fig.11)
      (1) I2 /I1 > 1
      (2) αm = -49.7 (I2 /I1) - βm + (113 - 123)      αm, βm : in degree
    (1) is for the condition of the beam itself and (2) is for its installation.

Safety Assessment of Automated Production Systems Using Microelectronics (1st Report) --Energy-Transfer Chains and Hazards of Industrial Robots--

RR-32-5
Yoshinobu SATO, Noboru SUGIMOTO and Ikuo MAE

: It is apparent that we have to receive some risks when we use the Industrial robots for production, because several cases of the injury accidents caused by robots have been already given in japan.
    The technologies to apply ME (microelectronics) to the automatic machines or apparatus like industrial robots have greatly progressed and the usage of them are increasing rapidly.
    And it seems that the technologies for the application of ME to the production systems will progress further more in the near future.
    Then it is necessary to achieve the safety assessment of the automated production systems using ME to prevent the accidents that will be caused in the systems.
    In this report, the first step of the safety assessment for the potentially hazardous innovations such as the technique to apply ME to the automated production, the hazard identification method and its application are discussed.
    Firstly we gave the strict definitions for the concepts "hazard" and "the production process".
    Next we examined the patterns of the energy transfer chains that result in injury accidents related to a system element. (Fig.3).
    And we proposed the concepts "Hazards related directly to a system element" and "Hazards related indirectly to a system element" that are derived from the patterns of the energy transfer chains.
    Then for the case study of the application of this hazard identification method, the energy forms which are assumed to be possessed by industrial robots-persons-peripheral elements systems and to produce the hazards to a person were examined and the types of the injury accidents caused by the hazards were identified by using this method. (Table 1)
    And it was concluded that the next step of the safety assessment process of potentially hazardous innovations or systematical analysis of the hazards will be effectively derived from the result of the hazard identification using this method.

Behaviour or two Rupture Disks when a Chamber Constituted by them is Pressurized by a Gas and the Quick Releasing Method of Explosion

RR-32-6
Heizaburo TSURUMI

: A single rupture disk has been ordinarily used as one of the safety devices for pressure vessels. But, in this study, two rupture disks are fitted in series on a vessel as shown in Fig.2. A chamber between the two disks was filled with a pressurized gas which was air, nitrogen or helium. The sample rupture disks were made of flat aluminum plate, and the operating rupture pressure of the disk was 3.78 kgf/cm2 . The chamber gas was pressurized at the maximum 3.5 kgf/cm2 . The volume of chamber was 56.65 × 10-3 litre. The rupture pressure of the disks for the dynamic pressure were measured by the explosion of premixed hydrogen-air gases for each charged gas.
    The behaviour of the rupture disks connected with the charged gas are shown in Fig.3 and Fig.5 through 11. The relation between the rupture pressure, flame velocity and charged gas pressure (back pressure) are given in Fig.8, when the flame velocity is less than approximately 100 m/sec. In the case of helium, even then the back pressure was 3 kgf/cm2, the rupture pressures were less than those of charged nitrogen of the atmospheric pressure. It was shown that the back pressure depended on the density of the charged gas.
    As shown in paragraph 3.1 and 3.2, the rupture pressure of disks depends on the back pressure under the conditions of the statical pressure and dynamic pressure. From the results, the author concludes that a pair of two disks is useful as a safety device under some application conditions. If the charged pressure is varied, a change of the rupture pressure of this system is possible, but the maximum charged pressure of the chamber will not be beyond the operating rupture pressure of the disk.
    As described in paragraph 3.3, before the explosion wave reaches in front of the upstream side disk when an explosion occurs inside of the vessel, if the pressure of chamber is reduced quickly to the atmospheric pressure, the upstream side disk will break at the rupture pressure of the disk without back pressure and the downstream side disk will break successively.
    Photo. 8 shows an example of the operating ability of this system when the charged gas is air and the back pressure is 3.5 kgf/cm2 .
    In order to apply of this quick releasing method of explosion, the condition of explosions velocity and the exhaust times of back pressure were given as shown in Fig.13, Fig.14 and Eqs. (3) to (6).

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