Electricity is dangerous-plain and simple. The world has become electrified and electricity has almost universally improved the standard of living around the world. However, when we put electricity into our homes, work places, and more, we are taking a risk that the protection system won't fail and that we will remain safe.
As your everyday experience should tell you, the safeguards put in place in the United States have massively mitigated the risk of electrical shock. Things like grounding, high-grade insulation, and an ever-improving knowledge of electrical design all make a huge difference ensuring our electrical safety. Even in homes with wiring much older than the design life of 25-30 years, it's not uncommon to see the electrical system still working robustly.
Still, there's always the potential that things go wrong. A few missteps and you could be looking at a painful shock when you touch something like a receptacle or a light switch in your home. But what decides whether that shock is life-threatening or just a simple inconvenience? Dalziel's equation, as described in IEEE 80, applies to this situation:
Where:
I is the current that can safely pass through the body in milliamps (mA)
116 is a conservatively low constant determined by Dalziel in their research
t is the time in seconds that the electrical shock is endured
The amount of current that can safely pass through the body is nonlinearly related to how long the shock occurs. The form of this equation could have been derived from Ohm's Law. The power lost in a resistor is proportional to the square of the current. If we think of the human body as a big resistance, then this all makes sense.
Dalziel's formula says that for a 1 second shock duration, the maximum current that can pass through a person safely is 116 mA. That's only .116 A, a really small current flow. Load calculations assume that a typical household 120V receptacle has about 1.5A flowing through it. Yikes! So how can anything like this be considered remotely safe?
Well there are two key pieces of information I have not included in the above assessment:
The resistance of the human body is usually modeled as 2000 Ohms. In reality, the human body could have a much higher resistance when skin is dry. 120V across a 2000 Ohm resistance would only produce about 60 mA of current, around half of what is described above.
1 second is an extremely long time for a fault to clear. In areas where a shock is most likely to occur, ground fault circuit interruptors (GFCI) are commonly used to mitigate the risk of a shock. These devices will trip in MUCH less than 1 second at 116 mA.
Put those together and we realize that the threat of dying from a shock in your home, with a properly designed electrical system, is not that high. The situation would require numerous failures of design against the code and often some particularly bad luck by the person being shocked.
What about at a higher voltage, like the medium voltage distribution lines that bring power to your step-down transformer? Even for a lower MV system, like 7.2kV, the current that could pass through the body touching line-to-line would be 3.6A. Using Dalziel's equation, the fault would need to be cleared in .001 seconds to ensure the person's safety. This level of speed is impractical. Generally, it's not possible to protect somebody from direct contact with an energized medium voltage conductor. But, we can put in safeguards to prevent this contact from taking place and ensure that line-to-ground faults, where current flows through the earth and not directly through a person, remain safe. This is the standard of safety that has been adopted across the world for electrical systems.
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