There, she saw a white SUV on the grass of a house. Water spewed from the broken hydrant – about 2,000 gallons per minute – and downed power lines carrying a charge of 4,800 volts lay along the ground, charging the water around the exposed wires.
Most people understand that electricity is dangerous but often they don’t understand how dangerous. I started the power series and I keep meaning to go back and continue it, and one of these days I will. For now though I’m going to try and explain just the specifics to try and help people understand what is going on.
To start off with, if you are in a vehicle and it strikes a power pole or a power distribution box. Stay In Your Vehicle! Your vehicle will actually help protect you from the voltages. If you can, back away from the downed conductors and then call 911. In the incident of hitting a power distribution box, odds are the line will fault and a breaker will open. However, be advised that doesn’t mean your safe. The majority of faults in the distribution system are momentary. Because of this fact there is an auto reclosing circuit which will reclose the line and energize it. The fault will remain and the circuit will open again. Usually after the 3rd shot failed they will lock out the re-closer. Still, stay in your vehicle, don’t risk it unless there is a more pressing danger to your life such as a vehicle fire. If you do need to leave your vehicle, use my notes at the bottom.
Now, if you see someone get in an accident with a power pole or distribution box, keep your distance! If you see the line arching or sparking, keep your distance as much as possible. Immediately call the utility and they can de-energize the lines. While the power company wishes to clear faults when they occur on the power system there is a particular kind of fault that is very hard to detect. Not only is it hard to detect, but it becomes exceedingly difficult when you’re in the distribution system.
Detecting this fault is difficult because there is not a lot of current involved which is one of the big items that is watched for protection. The reason there is less fault current, especially at the distribution level is the distance from generation and the fact it is in parallel with numerous other loads. Especially the further you get down a distribution line.
What is confusing in this case is the combination with water with the fault. Normally high impedance faults are found on things like asphalt or in the desert on dry sand. Where the ground itself isn’t a good conductor. In this case water is an excellent conductor, however I suspect that the water was mainly on asphalt which was acting as an insulator and this fault was way down the distribution line.
So here’s basically what I figure happened. When the line went into the water, all the water was basically at a potential of 4800 volts. The water would have a low impedance so there wouldn’t be much of a voltage drop across it. However, there is a massive drop between the water and the actual earth ground. As someone stepped into the water they bridged a circuit with their body, especially if they stepped off of grass over a curb into the water.
Your body and electricity don’t really mix. It takes only a mere 11mA to stop your heart, it isn’t voltage that kills you, it’s the current. Your bodies natural resistance tops out at around 100 kilo ohms at the skin, if it’s dry and you’re not sweating. Internally it’s only about 300-1000 ohms. If you get your skin wet the resistance drops.
With dry skin if you were to grab on to a 4800V line you would get 48mA through your body approximately, that would be more than enough to kill you, though that would depend on the exact path taken through the body. Wet, the current will be easily measured in Amps. If some how it doesn’t stop your heart, odds are you will be lit on fire as your body will be dissipating approximately 27 kW of energy.
So everyone please listen to me, stay away from downed conductors. Yes people may be injured, people may need help, but it will do no good for them for you to injure yourself in the process. In fact it will divert resources from helping them to helping you.
Now if some how you find yourself near a downed conductor, do not walk towards it or away from it! Do the following:
- Keep both feet as close together as possible. This will keep your feet at approximately the same electrical potential and will limit current flow.
- Hop both with both feet away from the downed conductor. Again, do not walk. At most shuffle. See there’s a reason they made you learn the bunny hop in PE in school!
- If you need to exit a car. Jump clear of the vehicle without touching the vehicle and the ground at the same time.
- Keep your arms held tightly to your sides.
The explanation of this is think of the ground as a high impedance resistor. Over a resistor is a voltage drop. The further the distance on the ground from one point to another the bigger the voltage drop. Your body has a much lower resistance than the ground, and if you step, that difference in potential will form a circuit through your body. If you are not comfortable hopping, then shuffle away.