https://www.yomiuri.co.jp/dy/national/20061203TDY02004.htm The Yomiuri Shimbun
A fire in August in a company office in Taito Ward, Tokyo, was caused by arc tracking inside a telephone cord, an electrical phenomenon that previously had been thought unlikely to occur, the Tokyo Fire Department said Saturday.
It is the first time the phenomenon has been reported to occur in a telephone cord, causing a fire, as extremely weak electric currents run in such cables.
The arc tracking in the August fire was caused because the plug of the cord was covered with detergent. The phenomenon generally occurs if plugs are covered with dust.
The fire department has called for the public to be careful, as many households will do thorough housecleaning at the end of the year.
According to Asakusa Fire Station, the fire started in the office of a security goods manufacturer on the sixth floor of a building around noon on Aug. 28.
An unplugged telephone cord hanging over a partition suddenly caught fire and a part of the partition was burned.
Initially, the fire station suspected arson as there was no fire hazard in the room. But workers in the office saw sparks fly from the tip of the cord just before the fire.
The fire station's examination found that one end of the telephone cord was connected to the switchboard but the other end was disconnected from the handset and on the floor.
It was also found that cleaning workers picked up the cord and hung it over the partition when they cleaned the floor with detergent the previous day.
Fire station officials believe the plug was soaked in detergent at the time of cleaning and conducted an experiment to reproduce the condition using the same kind of detergent.
About 90 seconds after the detergent was dropped on a plug, the plug discharged smoke. About four minutes later, the officials confirmed the plug produced a flame.
The phenomenon occurs if an electric plug is covered with dust or moisture that lowers insulation performance. Electric currents short out and can generate flames.
Such accidents can occur with electric plugs that have been kept connected for many years behind furniture. But it had been believed that the phenomenon can occur only with electric currents of at least one ampere and 100 volts--equivalent to that of an everyday lightbulb.
If a telephone cord is connected to a telephone exchange via a switchboard, weak electric currents of 15 to 130 milliamperes and 48 volts travel inside the cord.
The fire station concluded the detergent caused a short circuit and arc tracking, resulting in the fire.
In this case, the fire started about 20 hours after the cleaning. The fire station believes the delay between the cleaning and the fire starting occurred because the detergent was diluted with water to a strength of between 5 percent and 10 percent.
As the water gradually evaporated, traces of detergent remained on the surface of the plug and later caused the short circuit, the fire station said.
The maker of the telephone cord also conducted a reconstruction after receiving a notice from the Tokyo Fire Department, and confirmed that detergent, including that used to wash dishes, can cause the phenomenon.
The manufacturer took precautionary measures, such as attaching a notice about the fire risk to the manuals of about 50,000 cords.
The fire department issued a warning about using detergents in housecleaning because more telephone cords than before are used in ordinary houses with the proliferation of personal computers.
The fire department said people should be careful not to let detergent spill on to telephone cords when cleaning floors and walls, and warned that if any detergent does get spilled it should immediately be removed.
(Dec. 3, 2006)
