Electrocution and Electrical Shock
Electrocution or electrical shock occurs when an electric current flows through a person's body. The severity of the shock depends on factors such as the body's resistance, the current's magnitude, and its frequency.
Ohm's Law and Body Resistance
The current that flows through the body is determined by Ohm's Law:
I = E/R
Here, I represents the current flowing through the body, E is the voltage across the body, and R is the body's resistance. Body resistance varies among individuals, meaning the same voltage level may have different effects on different people. Generally, a person's hand-to-hand resistance ranges between 1,000 and 2,000 ohms, but babies, children, and some individuals have lower resistance levels.
Current and Risk of Electrocution
The risk of electrocution and electrical shock depends primarily on the current. The threshold for perception is about 100 microamps (0.0001 Amps). The National Electrical Code (NEC) considers 5 milliamps (0.005 Amps) to be the safe upper limit for both children and adults. However, this so-called safe limit does not guarantee freedom from injury, as a person may still harm themselves or others due to their nervous system's reaction to an electrical shock.
The "Let-Go" Limits
Severe electrocution and shock risks occur above the "let-go" limits, which vary by gender. For 99% of females, this limit is above 6 milliamps, averaging at 10.5 milliamps. For 99% of males, the limit is above 9 milliamps, with an average of 15.5 milliamps. Exposure to 60 Hz currents greater than 18 milliamps across the chest can cause the diaphragm to contract, leading to suffocation. Although specific data for females and children is unavailable, it is assumed that suffocation occurs at lower current levels.
The Role of Frequency in Electrocution
The frequency of the electrical current is just as vital as its magnitude when assessing electrocution and electrical shock injuries. Humans and animals are most vulnerable to frequencies between 50 and 60 hertz. The heart's internal nerve signals function at around 60 hertz, and ventricular fibrillation can occur when a 60 hertz current from an electric shock disrupts the heart's natural rhythm. This interference causes the heart to lose its pumping ability, resulting in death. Ventricular fibrillation can happen at current levels as low as 30 milliamps for a two-year-old child and 60 milliamps for adults. Most adults experience ventricular fibrillation at hand-to-hand currents below 100 milliamps (0.1 Amp).
Tolerance to Different Frequencies
Interestingly, humans can tolerate 10 times more current at direct current (DC) and 1000 hertz than at 50 or 60 Hz. Electro-surgical devices operating above 100,000 Hertz can pass high currents through the body without affecting a patient's heart or breathing. It's worth pondering if Murphy's Law played a role in setting the American power line frequency at 60 Hertz and the rest of the world at 50 Hertz. All the current limits mentioned in the growfaq articles are based on power line frequencies of 50 or 60 hertz.
Electrocution Injuries and Physical Evidence
Electrocution injuries may or may not exhibit physical evidence. The presence of burns or other skin damage depends on the current density at the point where the current enters or exits the body. Electrocutions at 110 VAC rarely cause skin damage unless the contact point is small or the person has sensitive skin. However, at higher voltages, more intense currents flow through the body, increasing the likelihood of skin damage. In such cases, entrance and exit wounds are common, but not always present.