Abstract of Technical Note (TN-81)

National Institute of Occupational Safety and Health, Japan

Thermal Stability and Decomposition Hazard of Sponge Blowing Agents

TN-81-1

Shigeru MORISAKI

: The thermal decomposition of six sponge blowing agents was investigated with a pressure differential scanning calorimeter and a pressure thermobalance to evaluate their thermal hazard. Decomposition heats and kinetic constants in an atmosphere of inert gas or air were obtained using thermal analysis methods.
A positive gas pressure was employed in the elucidation of the decomposition behavior of these substances because some of them evaporate during linear heating at atmospheric pressure. The decomposition heats and exothermic onset-temperatures of most of the sponge blowing agents tended to increase with a pressure increase in nitrogen or air. The evolution heats, ranging from about 150 to 500 cal/g (in air at 1 atmosphere), were increased to a range of about 250 to 1000 cal/g by a pressurized air atmosphere of 5.3 MPa. Under pressure thermogravimetry conditions the rate of decomposition increased with increasing gas pressure, suggesting a distinction in the decomposition mechanism. The influence of heating rate on the decomposition heat and weight change is also discussed, and kinetic constants such as activation energies at 1 atm are compared with those obtained in pressurized gas atmospheres.

The Influence of Mounting Method of Safety Guard for Portable Grinder on the Protective Effect --Cases on Safety Guards for Depressed Center Wheels--

TN-81-2

Soichi KUMEKAWA

: Mounting methods of safety guards for portable grinders are divided roughly into two methods.
    One of them is that a safety guard is just mounted on the grinder with bolts. And another one is that a mounting set plate fixed as one body on the safety guard is mounted on the grinder with tightening bolts.
    In the case of the latter mounting method, the safety guard slips off from the grinder or moves from the fixed position to guard when the grinding wheel of portable grinder comes to burst.
    Then broken pieces of depressed center wheels with safety guards mounted by several methods are observed with a high speed moving picture camera and the influence of mounting methods of the safety guards on the protective effect are investigated.
    The following becomes clear from the results of our experiments.
  1) Mounting method of a safety guard for a portable grinder should provide for prevention to slip off from the grinder.
  2) In the case of mounting method with the set plate, the mounting mechanism of the set plate influences sensitively on moving angle of the safety guard from the fixed position. Especially, the safety guard of set plate type with a hinge joint moves largest angle when a grinding wheel comes to burst.
  3) When a grinding wheel comes to burst, the larger is moving angle of the safety guard from the fixed position, the bigger is total weight of broken pieces which fly out into the area to guard.
  4) In order to keep the protective effect of the safety guard, it should prevent for large moving angle of the safety guard that broken pieces which remained within the safety guard fly out for the area to guard.
  5) For a grinder operator in the guarded area, area of angle 30 degree from the edge of the safety guard in the wheel rotating direction is most dangerous from flying broken pieces.

On the Safety of Horizontal Lifeline Employed in Assembling or Demolition of Scaffolding

TN-81-3

Katsunori OGAWA and Yoshitada MORI

: Dynamic characteristics of horizontal lifeline holding body with safety belt employed in assembling or demolition of frame scaffold which is applied with the aim of prevention of falling accidents were studied in this report.
    Two kinds of experiments were undergone; one is to act a static load on the middle of the lifeline under various conditions, the other is to drop a sandbag connected to lifeline.
    Experimental conditions are as follows :
  1) Lifeline : φ9 mm wire rope and φ16 mm nylon rope.
  2) Length of lifeline : 10, 15 and 20 m
  3) Weight of sandbag : 75 kg
  4) Safety belt is A type and 1.5m long lanyard
    The results of the experiments are summarized as follows :
  1) As the initial tensile force of the lifeline increases, the impact load of the falling body decreases.
  2) In the lifeline of wire, the tensile force of lifeline becomes bigger, when the stiffness of the parts suspending the lifeline increases. However, the impact load of the falling body is not influenced.
  3) In the lifeline of nylon, both the tensile force of lifeline and the impact load of the falling body are not influenced by the stiffness of that parts.
  4) The descent distance of lifeline is about 8% of its length in wire and about 30% in nylon.
    For the impact load of the falling body, the tensile force of the lifeline etc., the empirical formulae are shown in chapter 6.

Analysis of Fatal Electric Shock Accidents Caused by Contacts with Distribution Line

TN-81-4

Kenji ICHIKAWA

: In Japan, electric shock accidents in industries have recently decreased. In the meantime, those accidents which are caused by contacts with electric distribution lines account for about 40 percent of all electric shock accidents.
    In order to establish effective preventive measures against these accidents, it is necessary, as a first step, to investigate actual conditions of occurrence of these accidents. In this report, features, tendencies and factors of these accidents are described on 211 cases made available for data analysis which had been reported by the Labour Standard Offices throughout the country during the period 1973-1975.
    The outlines of this investigation are as follows ;
  1) The most frequent accidents by job were seen on electrical working and building construction sites:
    a) A large number of electrical accidents were caused by touching high voltage lines. There seems to be no seasonal tendency of these accidents.
    b) Most fatal workers were at their twenties and their thirties in age.
  2) Electrical accidents occurred on building construction sites are largely concerned with work handling a car loaded with a crane. For example, even if a crane boom is not moving, the wire rope might touches on a nearly hot line as a result of swinging of the hanging hook or materials, and accidents would happen frequently.
  3) In electrical working on electric poles, there were many cases that electric current flowed from hand to hand during working.
  4) Accident factors, regarding the accidents occurred on electric poles, are studied with a method of "Fault Tree Analysis".