What is in this article?:
- Forensic Casebook: The Case of the Missing Ground Wire
- SIDEBAR:Electrical Circuit Phenomena
Grounding problems on utility pole lead to death of a cable television technician.
SIDEBAR:Electrical Circuit Phenomena
Because the ground wire was not connected to ground and cut in two places, the connection between the end of the ground wire above the cut section and the ground was through the wood pole. The dry wood (the weather was dry at the time of the accident) in the pole presented a high resistance to electric current so that very little current flowed via this path. In other words, not enough current flowed to cause the protective devices that protected the power circuit to trip and interrupt the current. Thus, the arc between the phase conductor and the ground wire could have continued indefinitely. Had the ground wire not been cut in two places, it would have provided a safe path to ground and should have tripped protective devices.
As shown in the first Figure, when the victim contacted the energized ground wire above the cut sections and the grounded cable support wire at the same time, the current flowed into his hand touching the energized ground wire, through his body, out his other hand to the cable TV support cable, and then through the cable TV ground wire to ground. The burn marks on his hands confirmed this path.
The resistance of the human body for electrical safety purposes is considered to be about 1,000 ohms, according to IEEE Std 80-2000. Before the switch is closed in the above circuit, simulating a person contacting the energized ground wire while being grounded through the cable TV support cable, the voltage across the resistance representing the wood pole is about 880V, as measured by the electric utility. When the switch is closed, the body resistance shorts out the wood pole resistance, because it is so much smaller in value, which allows current to flow through the body. When this happens, the voltage across the body resistance will drop from whatever the open-circuit voltage on the ground wire was (880V or higher) to a lower value due to the higher current flow from the source through the body resistance (click here to see Figure). It only takes about 70mA to 300mA of current to kill a human. Thus, it only takes about 70V to 300V to cause a potentially fatal current to flow in the body.