Electricity: The Basics
What affects the flow of electricity?
Electricity flows more easily through some materials than others. Some substances such as metals generally offer very little resistance to the flow of electric current and are called ” conductors.” A common but perhaps overlooked conductor is the surface or subsurface of the earth. Glass, plastic, porcelain, clay, pottery, dry wood, and similar substances generally slow or stop the flow of electricity. They are called ” insulators.” Even air, normally an insulator, can become a conductor, as occurs during an arc or lightning stroke.
How does water affect the flow of electricity?
Pure water is a poor conductor. But small amounts of impurities in water like salt, acid, solvents, or other materials can turn water itself and substances that generally act as insulators into conductors or better conductors. Dry wood, for example, generally slows or stops the flow of electricity. But when saturated with water, wood turns into a conductor. The same is true of human skin. Dry skin has a fairly high resistance to electric current. But when skin is moist or wet, it acts as a conductor. This means that anyone working with electricity in a damp or wet environment needs to exercise extra caution to prevent electrical hazards.
What causes shocks?
Electricity travels in closed circuits, normally through a conductor. But sometimes a person’s body — an efficient conductor of electricity — mistakenly becomes part of the electric circuit. This can cause an electrical shock. Shocks occur when a person’s body completes the current path with:
both wires of an electric circuit;
one wire of an energized circuit and the ground;
a metal part that accidentally becomes energized due, for example, to a break in its insulation; or
another ” conductor” that is carrying a current.
When a person receives a shock, electricity flows between parts of the body or through the body to a ground or the earth.
What effect do shocks have on the body?
An electric shock can result in anything from a slight tingling sensation to immediate cardiac arrest. The severity depends on the following:
the amount of current flowing through the body,
the current’s path through the body,
the length of time the body remains in the circuit, and
the current’s frequency.
This table shows the general relationship between the amount of current received and the reaction when current flows from the hand to the foot for just 1 second.
Effects of Electric Current in the Human Body
|Below 1 milliampere||Generally not perceptible|
|1 milliampere||Faint tingle|
|5 milliamperes||Slight shock felt; not painful but disturbing. Average individual can let go. Strong involuntary reactions can lead to other injuries.|
|6–25 milliamperes (women)||Painful shock, loss of muscular control*|
|9–30 milliamperes (men)||The freezing current or ” let-go” range.* Individual cannot let go, but can be thrown away from the circuit if extensor muscles are stimulated.|
|50–150 milliamperes||Extreme pain, respiratory arrest, severe muscular contractions. Death is possible.|
|1,000–4,300 milliamperes||Rhythmic pumping action of the heart ceases. Muscular contraction and nerve damage occur; death likely.|
|10,000 milliamperes||Cardiac arrest, severe burns; death probable|
* If the extensor muscles are excited by the shock, the person may be thrown away from the power source.
Source: W.B. Kouwenhoven, ” Human Safety and Electric Shock,” Electrical Safety Practices, Monograph, 112, Instrument Society of America, p. 93. November 1968.