BK Precision 4011A Instruções de Operação descarregar pdf (Página 43)

Appendix A: Safety

201031

Other injuries may be indirectly caused by electrical accidents, such as burns from exploding oil-

immersed switchgear or transformers.

Although electric shock is normally associated with contact with high-voltage alternating current

(AC), under some circumstances death can occur from voltages substantially less than the nomi-

nal 120 volts AC found in residential systems. Electric shock is caused by an electric current

passing through a part of the body. The human body normally has a high resistance to electric

currents, so a high voltage is usually required to cause lethal currents. This resistance is almost

all at the skin, however, and when the skin is wet its resistance is much lower. When a person is

hot and sweaty or is standing in water, contact with 120 volts or less may well cause a fatal

shock.

Electric shock is not a single phenomenon but is a disturbance of the nerves by an electric cur-

rent. A current through a part of the body such as the arm or leg will cause pain and muscle con-

traction. If a victim receives an electric shock from grasping a live conductor, a current of greater

than 15 to 30 milliamperes through the arm will cause muscle contractions so severe that the vic-

tim cannot let go. Similar currents through leg muscles may cause sudden contractions that make

the victim jump or fall, possibly resulting in further injuries or death. Prolonged contact of more

than a minute or so may cause chest muscles to contract, preventing breathing and resulting in

suffocation or brain damage from lack of oxygen.

Death by electric shock is most often attributed to ventricular fibrillation, which is an uncon-

trolled twitching or beating of the heart that produces no pumping action and therefore no blood

circulation. Unless corrective action is taken, death follows quickly from lack of oxygen to the

brain. While the amount of current that will cause fibrillation depends on several variables, 0.5 to

5 amperes through the body will normally cause the very small current (approximately 1 mA)

through the heart that is sufficient to cause fibrillation in most people. Larger currents than this

through the heart cause contraction or clamping of the heart muscle, resulting in death unless

corrective action is taken.

Electric burns may be caused by electric currents flowing in or near parts of the body. Such bums

are similar to bums from ordinary heat sources, except that those caused by high-frequency cur-

rents are generally deeper and take longer to heal the other bums. Electrocution will often leave

severe burns at the points where the current entered and left the body.

Source of Electric Shock

Since electric shock is caused by an electric current flowing through a part of the body, the prob-

lem may be prevented by not allowing the body to become part of any electric circuit. For this

reason, one needs to understand how a person might inadvertently become part of an electric cir-

cuit, and the most likely way is through accidental grounding.

Electric circuits may be classified as either grounded or ungrounded. Grounded circuits are safer

for most conditions, since the voltages at all points in the circuit are known and it is easier to pro-

tect against fault conditions. The disadvantage of a grounded circuit is that a person standing on

a non-insulated floor can receive a shock by touching only one conductor.

Almost all electric power generation, transmission, and distribution systems are grounded to pro-

tect people and equipment against fault conditions caused by windstorms, lightning, etc. Resi-

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