It seemed like a typical electrical job. The owners of a house trailer hired an appliance repairman to take a look at their washing machine.
Unfortunately, the repairman chose to fix the unit alone and at night. The next morning, they found him dead — with his head in contact with the metal trim around the kitchen counter and one hand placed in the working mechanism of the washing machine. After pulling the washer out from under the counter and working on it while plugged into a nearby electrical outlet, the repairman had suffered the stress of a continued flow of current.
Although it's often left unsaid, the fact is house trailers have a strong potential for electrical accidents. As in this case, a trailer's lack of inherent grounds make it easier than usual to have electrical circuits with serious flaws. After the accident, the lawyer representing the deceased's wife hired me to determine fault. Before I came on the scene, early investigators concluded:
This initial information gave me a place to start. First, I needed to determine what role, if any, the timer played in the accident, since two of the earlier investigators strongly disagreed about the status of the "ground" inside the trailer.
Next, I asked to examine the washing machine's timer. Much to my surprise, they gave me the timer with permission to disassemble it without witnesses. Although I felt uncomfortable modifying physical evidence, counsel for both sides agreed. So I disassembled the timer, checking the connections between various parts. I found the timer was a mechanical, clock-driven unit with cams that made various switches close and open in an appropriate sequence to cause the various cycles of the washer to function. The device also included a bimetallic disc adjacent to a heating resistor through which the main current flowed. When this disc heated sufficiently, it moved the timing switch into the off position, thereby providing overcurrent protection. This is the same movement used by the operator of the washer to turn the unit on or off by pushing or pulling the control knob.
After some inspection, I found the timer cams and bimetallic disc (both mounted on the extended timer shaft) slightly displaced. This caused an unintended contact to occur at some settings that electrically connected the frame of the washer to one side of the supply circuit. Depending on the arrangement of the "extension cord" connecting the trailer to the external supply and the power cord of the washer, you could connect the frame of the washer to the nominally hot side of the supply circuit. In the other position, the frame of the trailer connected to the hot side, and the washer would be grounded. Obviously, this made for quite a dangerous situation.
With its ungrounded supply, there was no basis for sufficient current to flow in the trailer to cause breaker operation. In fact, the trailer didn't even have partial grounding through the water in the nonmetallic water line. Had water gone from the washing machine through the rubber hose to the waste line system, it might have served as a ground and caused breaker operation.
The disagreement between the other two investigators about "ground" is more difficult to explain. One of them based his entire analysis on the reasonable presumption that the trailer was at ground potential. The other was certain the trailer was hot, except for the washer. I decided they were both correct in some sense. Depending on the orientation of the "extension cord" connecting the trailer to its external power source, the frame of the trailer would be at or near ground potential or at approximately 115V different potential from ground. These statements refer to "ground" as the potential of the soil. Parked on rubber tires, the trailer remained insulated. However, once the inspector stepped inside holding a neon-type testing unit, tests would always indicate the trailer's frame was at ground potential.
Based on my examination of the timer, I had no doubt it was the cause of the electrical problem (besides the fact that the trailer was improperly grounded). However, there was no evidence to suggest the timer manufacturer was entirely at fault. Why? An unknown person had serviced the timer after it left the manufacturer's control. A more prudent design of the timer to prevent unauthorized servicing would also have been helpful. You also can't blame the manufacturer for the fact that the trailer's safety features had been bypassed by the use of the "groundless" extension cord.
Finally, the victim worked on the unit alone at night, without disconnecting it from the energized outlet.
Thorn is a Fellow of the National Academy of Forensic Engineers.