When a cable carrying a high tension electric current say 15 kV, as in the case of a neon sign, touches a wooden board which is also in contact with an earthed conductor at another point, an electric current flows through this board and causes it to ignite. Wet boards are more easily ignited because electricity readily passes through them.
Fires due to this phenomenon are sometimes seen in our country, especially just after rain. It has happened only when the applied voltage is of the order of several kV, and applied voltages of 100 V or 200 V have been believed to be too low for this.
However, an applied voltage of even 100 V or 200 V has been found to be sufficient for causing a fire if the wooden board contains a small quantity of NaCl. The specific resistance ρ of wooden boards which had been immersed for several days in a water solution of NaCl of the percentage concentration p (grams per 100 grams of solution) was measured to be
ρ=2.7 p^-2/3     kΩ · cm ………………………… (1)
while they were wet, showing that these wet salted (salt-soaked) boards belong to the class of conductors when p is not small.
The ignition of salted wood due to electricity depends not only on the various factors such as the size and the shape of the material, the distance between electrodes, the voltage (V) applied, and the percentage concentration (p) of the solution, but on the shape of the electrodes. In cases when an electrode makes point-contact with the board, such as when the end of a cable touches the board, or a cable comes in contact with a nail in the board, the current density in the immediate vicinity of the point electrode is very high, and the board is more easily ignited.
When the applied voltage is sufficiently high, the narrow region which has been dried up at first around the pole and has changed to an insulator, is unable to support the whole voltage and is subjected to dielectric breakdown. As a result, a thin highly conducting carbonized (graphitized) canal is formed in this region. Then the current is concentrated on the top of the canal, which fact results in the drying up around the canal top and the extension of this thin canal into this newly dried region. Thus the canal continues to extend until it reaches the other electrode. An electric current up to 10 A flows through this communicating canal and the heat produced causes the board to catch fire.
When the voltage is not high enough, the speed of growth of the canal is so small that the regions other than around the canal top are also dried up and the current is stopped by the cooperation of all these dried regions. The canal can no longer grow after that.
In this case, however, if water is poured on this board, the canal grows again until the water vanishes. Thus, after pouring water several times, the canal reaches the opposite electrode and a heavy current begins to flow just as in the former case.
If the voltage is below this value, the slow evaporation of water continues and the current dies away gradually, leaving no effect of especial interest.
The voltage V below which the canal would not grow was studied using rectangular-prism-shaped wooden boards, each 7cm×2cm×1cm. Measurements were made with various percentage concentrations p of the NaCl solutions in which these boards were dipped, the two electrodes being nails driven into the boards 5cm apart. The results are summarized as follows :
p^0.3V=const. ………………………… (2)

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The Japan Fire Protection Science Association has made four small-size experimental furnaces, types 1～4 (Fig. 1 a～d) for the fire resistance test of various building materials, and has tested their properties. The experimental test shows the following points :
1. Type 1 (for the use of city gas) has excellent properties.
2. Type 2 (for the use of city gas) which was made as a low cost device shows poor characteristics.
3. Type 3 (for the use of propane gas) has very good properties, like Type 1.
4. Type 4 (for the use of city gas) is made for the fire resistance test of ceiling materials. However, there remain various matters to be investigated and examined for this furnace.

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Experimental studies are reported on whether cigarette butts dropped on shingle roofing can cause fires. The test has been carried out under various conditions with the apparatus shown in Fig. -2.  Table-2 shows the experimental results.
From this, we can conclude :
1. The possibility that the roofing will catch fire is much greater when the wind velocity is 3m/sec than it is when the wind velocity is 0 or 6m/sec.
2. When a cigarette butt is laid on the roofing in the way labeled IV (Fig. 1), the possibility of ignition is much greater more than in the other cases I, II, III, V (Fig. 1).
3. When the wind velocity is 3m/sec, a cigarette butt of 2cm length has a greater possibility of ignition than one of 3cm length. However, when the wind velocity is 0 or 6m/sec, the cigarette butt of 3cm has the greater possibility of ignition than one of 2cm length.
4. It is not clear whether the possibility of ignition depends on the brand of cigarette.

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In Part 1, we first discuss the general problems regarding the performance test of flame proofing materials, giving our comments on the various testing method published in the past, after which a new testing method developed by the authors is outlined.
The method called the Fire Research Institute Method is composed of a burning test, hygroscopicity test, tension test and corrosion test, and can be readily applied to the performance test of thin materials such as cloth, paper and thin wooden materials. The procedures of the tests are as follows ;
1. Burning test
As shown in photo 1, a test fabric (31×22cm) dried at 50°C for 24 hours, is fixed on the steel tambour (Fig. 3) and kept in the test cabinet at an angle of inclination of 45° (Fig. 1). Next the underface of the test sample is heated by the flame of an alcohol lamp. The arrangement of fabric and flame is shown in Fig. 2. For the determination of flame proofing ability, the ignition-lag of fabric, charred area as a function of heating time, and period of after-flame or after-glow are measured by means of a stop watch and a planimeter.
2. Hygroscopicity test
Test samples (5×2cm) dried under the same condition as in the burning test, are put into a chamber at a temperature of 30°C and relative humidity of 79.5%, In this case, a saturated aqueous solution of ammonium chloride is used to obtain the above condition. The increase in weight due to the absorption of moisture is measured by means of an analytical balance, and the hygroscopicity ratio is determined according to the following formula :
F=(ΔW/W) / (ΔW₀/W₀)
where F is the hygroscopicity ratio and W is the weight of a dried sample treated with the flame proofing agent while W₀ is the weight of the same material in the untreated state, and Δ is the increase due to the absorption of water vapour under the given condition. If F is greater than 1.50, the treated fabric is too hygroscopic.
3. Tension test
The tensile strength of wet test fabrics is determined using the “ Schopper ” type tension meter. A rectangular test strip (1.5×18cm) is cut from the sample fabric its two sides to be parallel and perpendicular to the fibre. The tension decrease of the test strip in each direction is calculated from the following formula.
Tensile strength = {(T₀-T)/T₀}×100(%)
Where T₀ and T are the values of tensile strength of strips of the untreated and treated fabrics.
4. Corrosion test
For the corrosion test, metal tubes (diameter 3.8cm, length 10cm) made of brass and iron are prepared. A test fabric (10×12cm) is twined around the tube which is covered by a sheet of filter paper and the ends of the fabric are stitched together with yarn of the same kind. Thus the surface of the tube comes in contact with the fabric through the medium of the paper. Then the metal tube, is put into a chamber saturated with water vapour at 30°C. After a week, the tubes are taken out and the degree of corrosion of the metal surface is observed qualitatively with naked the eye. The interposed paper is used for keeping the tube surface away from the corrosive fabric.
If a rust spot of finite area appears on the metal surface, the fabric is defined to be corrosive.

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Performance tests of cloth and paper treated by commercial flame proofing agents were carried out by means of the Fire Research Institute Method described in the last paper. The results are given in Fig. 1-12. and Table 1-2.
According to the results, the charred area of a material treated by good agents approaches a limiting value with increasing of the heating time, While that of a material treated by poor agents increases rapidly with heating time. The diagram for the (charred area) vs (heating time) can be used for the selection of good flame proofing agents.
It is remarkable that the materials which were ignited by heating for a short period of few seconds, were found to withstand heating for a long period of twenty or thirty seconds. Some typical examples of this phenomenon are shown in Figs. 2, 5 and 7. It seems that these results give a valuable suggestion regarding the mechanism of the flame proofing effect.

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Using a small board of a material and according to the usual method of JIS A 1321, it may be possible to measure the ignitability of the material. It is impossible, however, to know if it gives rise to the phenomenon called “ flash over ” which is called by the ignition of the accumulated decomposed gases from the material and is most hazardous in an actual fire.
To test this risk of flash over, three pieces of boards were used and were constructed in a frame of two adjacent walls and a ceiling, and were heated according to the standard temperature-time curve.
Another merit of this method is that the air supply, while heating, can be made more similar to that in an actual fire than by other methods like “ Tunnel Test ”, etc.
The sizes and the construction of the boards tested were as follows :
(a) Two adjacent vertical walls each 0.9m×1.8m and a ceiling 0.9m×0.9m.
(b) Two adjacent vertical walls 0.6m×1.5m and 0.3m×1.5m, and a ceiling 0.3m×0.6m.
(c) Two adjacent vertical walls 0.45m×0.9m and 0.15m×0.9m and n ceiling 0.45m×0.15m.
The observed time when the flash and flash over started using various kinds of materials are tabulated as follows :
Material              Flash      Flash over
Plywood            5min. 30sec.    6min. 30sec.
Non-ignitable plywood      6min. 30sec.   6min. 40sec.
Non-ignitable particle board   6min. 10sec.   6min. 50sec.
P.V.C.laminated gypsum board  8min. 0      8min. 20sec.
P.V.C.laminated metal      6min. 30sec.   6min. 50sec.
                7min.0

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