How Shocks Occur | Electrical Hazards

Electricity travels in closed circuits, and its normal route is through a conductor. Electric shock occurs when the body becomes a part of the electric circuit. The current must enter the body at one point and leave at another. Electric shock normally occurs in one of three ways. Individuals, while in contact with the ground, must come in contact with:

  1. Both wires of the electric circuit,
  2. One wire of an energized circuit and the ground, or
  3. A metallic part that has become “hot” by contact with an energized conductor.
The metal parts of electric tools and machines may become energized if there is a break in the insulation of the tool or machine wiring. The worker using these tools and machines is made less vulnerable to electric shock when there is a low-resistance path from the metallic case of the tool or machine to the ground. This is done through the use of an equipment grounding conductor, a low-resistance wire that causes the unwanted current to pass directly to the ground, thereby greatly reducing the amount of current passing through the body of the person in contact with the tool or machine. If the equipment grounding conductor has been properly installed, it has a low resistance to ground, and the worker is protected.

Severity of the shock

The severity of the shock received when a person becomes a part of an electric circuit is affected by three primary factors:
  • Amount of current flowing through the body (measured in amperes),
  • Path of the current through the body, and
  • Length of time the body is in the circuit.
Other factors that may affect the severity of shock are the frequency of the current, the phase of the heart cycle when shock occurs, and the general health of the person.

The effects of electric shock depend upon the type of circuit, its voltage, resistance, current, pathway through the body, and duration of the contact. Effects can range from a barely perceptible tingle to immediate cardiac arrest. Although there are no absolute limits or even known values that show the exact injury from any given current, the table below shows the general relationship between the degree of injury and amount of current for a 60-cycle hand-to-foot path of one second’s duration of shock.

The table also illustrates that a difference of less than 100 milliamperes exists between a current that is barely perceptible and one that can kill. Muscular contraction caused by stimulation may not allow the victim to free himself or herself from the circuit, and the increased duration of exposure increases the dangers to the shock victim. For example, a current of 100 milliamperes for three seconds is equivalent to a current of 900 milliamperes applied for .03 seconds in causing ventricular fibrillation. The so-called low voltages can be extremely dangerous because, all other factors being equal, the degree of injury is proportional to the length of time the body is in the circuit.
A severe shock can cause considerably more damage to the body than is visible. For example, a person may suffer internal hemorrhages and destruction of tissues, nerves, and muscles. In addition, shock is often only the beginning in a chain of events. The final injury may well be from a fall, cuts, bums, or broken bones.

Effects of electric current in the human body 
1 Milliampere
Perception level. Just a faint tingle.
5 Milliamperes
Slight shock felt; not painful but disturbing.
Average individual can let go. However, strong involuntary reactions to shocks in this range can lead to injuries.
6-25 Milliamperes (women)
Painful shock, muscular control is lost.
9-30 Milliamperes (men)
This is called the freezing current or “let-go” range.
50-150 Milliamperes
Extreme pain, respiratory arrest, severe muscular contractions[*]. Individual cannot let go. Death is possible.
1,000-4,300 Milliamperes
Ventricular fibrillation. (The rhythmic pumping action of the heart ceases.)
Muscular contraction and nerve damage occur. Death is most likely.
Cardiac arrest, severe burns and probable death.
[*]If the extensor muscles are excited by the electric shock, the person may be thrown away from the circuit. Source: W.B. Kouwenhoven, Human Safety and Electric Shock, Electrical Safety Practices, Monograph, 112, Instrument Society of America, p. 93. (Papers delivered at the third presentation of the Electrical Safety Course given in Wilmington, DE, in November 1968.)

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