The Case of the Torrid Telephone
Kenneth Russell, Professor Emeritus, MIT, Cambridge, MA -- Design News, April 6, 2003
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Telephones are fairly benign instruments, made unpleasant mostly by the ubiquity of telemarketers on the opposite end of the line. But many years ago one of Ma Bell's instruments turned downright homicidal.
A yacht owned by a wealthy Boston-area businessman caught fire, and the size of the resulting insurance claim merited serious investigation. The cause of the fire was a mystery. The yacht was berthed at the time and the owner had just been aboard and had found nothing amiss. The insurance investigator did find that the telephone cord, which had been jammed between the lifeboat and a bulkhead, showed signs of fire damage. The fire seemed to have started from this region. The damaged cord, and the mystery of the fire origin were turned over to me for a study of the materials aspects of the case.
USA Today recently referred to people such as myself as "trained tin kickers." I have also been referred to as a "metal bender." I rather like either sobriquet. My job is to grub around wreckage in hopes of finding the cause of the accident. If I ruin a suit of clothes, so be it. I will just up my bill enough to buy new duds.
The phone cord had clearly seen high heat, as evidenced by the fact that part of the plastic insulation had melted and blackened. I wanted to study the inside, but was somewhat leery about cutting into the cord, which in those days was owned by the telephone company. Lily Tomlin's classic skits as the telephone operator, Ernestine, on Rowan and Martin's Laugh In made clear the depth of the phone company's disapproval of any such "abuse of the instrument." But I decided to play ignorant and, if need be, offer to buy a new cord.
The cord contained three insulated copper wires, each of which was enclosed in a plastic sheath. The wires were color coded with green, red, and yellow plastic insulation. The yellow and red wires were blackened, even at regions where the outer insulation was in good order. The heat had clearly come from within, but how and why?
The cord had been pinned against the bulkhead at a point about 10 ft from the plug. A short between the red and yellow wires at this point would have given a current path of about 20 ft, with an electrical resistance of about 8 ohms. The ringing circuit on a telephone operates with about a 100V, as has come as a shock to those in contact with this circuit during a ring. Passage of 100V through an 8-ohm resistance gives about 1,200W, a bit more heat than is produced in an electric iron. So, if the wire were shorted, and if the voltage persisted for perhaps a minute, there was more than enough heat to cause the fire.
These were the days before answering machines, so callers tended to let the phone ring long enough for their party to reach the phone and answer. A minute's ringing wasn't unreasonable. But, how could the phone company allow such a short to cause a fire? There have certainly been shorted phone wires in residences, but I had never heard of one causing a fire. Common sense demands that the circuit be protected by a fuse, presumably at the switchboard.
I ultimately got up in court and gave my findings. Predictably, I was berated by the phone company lawyer for "abusing the instrument." Then I sat back and listened while my client questioned the phone company expert. The expert reluctantly admitted that the nautical installation had not been fused. I nearly jumped from my seat with a shout of EUREKA! I trusted my results, but until this moment had not believed that such a hazardous installation could have been allowed.
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