Electricity is
essential to modem life, both at home and on the job. Some employees work with
electricity directly, as is the case with engineers, electricians, electronic
technicians, and power line workers. Others, such as office workers and
sales-people, work with it indirectly. As a source of power, electricity is
accepted without much thought to the hazards encountered. Perhaps because it
has become such a familiar part of our surroundings, it often is not treated
with the respect it deserves. Electric shock occurs
when a person’s body becomes part of 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 both wires of the electric circuit, one wire of an energized
circuit and the ground, or a metallic part that has become "hot" by
contact with an energized conductor. The three electrical
factors involved in an electric shock are resistance, voltage, and current.
Figure
1. Electric shock danger sign
The lower body resistance, the greater the potential electric shock
hazard. Body resistance can be divided into external ( skin resistance) and
internal ( body tissues and blood stream resistance). Resistance
to the flow of electricity is measured in ohms and varies widely. It is
determined by three factors: the nature of the substance itself, the length and
cross-sectional area (size) of the substance, and the temperature of the
substance.Some substances, such as metals, offer very little resistance to the
flow of electric current and are called conductors. Other substances, such as
bakelite, porcelain, pottery, and dry wood, offer such a high resistance that
they can be used to prevent the flow of electric current and are called
insulators. Dry wood has a high resistance, but when saturated with water its
resistance drops to the point where it will readily conduct electricity. The
same thing is true of human skin. When it is dry, skin has a fairly high
resistance to electric current; but when it is moist, there is a radical drop
in resistance. Pure water is a poor conductor, but small amounts of impurities,
such as salt and acid (both of which are contained in perspiration), make it a
ready conductor. When water is present either in the environment or on the
skin, anyone working with electricity should exercise even more caution than
they normally would. However, in
a small number of instances, the consequence is death from cardiac arrest, or
from ventricular fibrillation (where the heart muscle beats in a spasmodic and
irregular fashion) or from respiratory arrest. The
magnitude of the current is the applied voltage divided by the impedance of the
body.
The overall circuit impedance will comprise the body of the casualty and
the other components in the shock circuit, including that of the power source
and the interconnecting cables. For this reason, the voltage applied to the
body, which is commonly known as the touch voltage, will often be lower than
the source voltage. The impendance of the body is determined by the magnitude of the touch voltage ( there being an inverse relationship between impedance and voltage) and other factors, such as the wetness of the skin, cross-sectional area of contact with the conductors, and whether or not the skin is broken or penetrated by the conductors.
Sources :
Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.1-2
