JAPANESE REPORTS ON ELECTRICAL FIRE CAUSES

2. Glowing Phenomenon at the Contact of Different Kind of Metals

Y. Hagimoto, K. Kinoshita, T. Hagiwara
Summary of Annual Meeting of Japan Society for Safety Engineering

Abstracted by translator

Copper wire, brass wire and iron wire were used in all combinations. DC 100V and AC 100V power supply were used. The electrical energy at the glowing contact was measured. The energy at the contact and their increasing rate were compared.

Translation begins here

1 Introduction

A loose connected contact sometimes results in "glowing contact" which cause abnormal heat.(ref. 1, 2) This experiments were carried out to examine the electrical characteristics at the contact of different kind of metals.

2 Method of experiments

Experimental arrangement is shown in figure 1. Copper, brass and iron were used for this experiment in every combinations (Cu-Cu, Cu-Fe, Fe-Fe, Fe-Br, Br-Br and Br-Cu). These contacts were connected to DC 100 V or AC 100V and variable resistance in series. 'ON and OFF' at the contact was repeated until glowing started.

3 Results of experiments

1) Similarity of these phenomena

Wave forms of the AC voltage across the contacts of every combination shown in figure 3 were similar to that of Cu-Cu contact.

In case of any combination of metals, glowing path, where current flows, was observed on the glowing part between two metals.

2) Component of the glowing part

Major component of the glowing part are shown in table 1. This table shows that these parts are the oxidized metal connected to (+) pole of the power source.

Table 1.
Current Combination of Metals Component
AC Cu - Cu Cu2O, CuO
DC Cu (+) - Br (-) Cu2O, CuO
DC Cu (+) - Fe (-) Cu2O, CuO
AC Br - Br ZnO, Cu2O, CuO
DC Br (+) - Cu (-) ZnO, Cu2O, CuO
AC Fe - Fe Fe3O4, Fe2O3
DC Fe (+) - Cu (-) Fe3O4, Fe2O3
Major component of the glowing part
(analyzed by X-ray diffraction)

3) Electrical characteristics

Wave forms shown in figure 3 indicates that the contact of the conductor metal and oxidized metal (glowing part) has rectifying characteristics.

DC voltage across various contacts are shown in figure 4. (Horizontal axis is time (minutes) and vertical axis is voltage (V). Japanese letters in the figure are as follows.)

Fig. 4 Transition of DC voltage drop across the contact (unavailable)

4) Electrical heating

Transition of DC voltage in case of DC 2A shown in figure 4 are gathered in figure 5. It shows that the voltage drop across the contact depends upon the kind of metal connected to (+) pole.

Electrical energy dissipated at the contact on hour after the starting of glow are shown in figure 6.

5) Breeding speed of the glowing part

Cu: several mm / hr

Br: 1/10 of several mm / hr

Fe: negligible

Breeding speed of copper connected to (+) pole has a maximum value at 6 Amps as shown in figure 7.

6) Fire hazard

Iron starts glowing more easily than copper or brass. But the electrical energy dissipated at the glowing copper oxide or brass oxide is larger than that dissipated at the glowing iron oxide as shown in figure 4.

This phenomena can hardly be found until a fire starts because the voltage drop at the contact is only several Volts and most electrical appliances are not affected by it.

Small current ( AC 0.5A ) can start this phenomena and the maximum temperature at the glow, although which is very small, will exceed 1000 C.

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