Miyerkules, Oktubre 12

Electrical Hazards

Current is the killing factor of Electricity

Three Categories of Electrical Hazard


ELECTRICAL SHOCK

To some people low voltage means low hazard. Actually, low voltage does not necessary means low hazard, because potential difference is only a factor making up dangerous effect of electricity. The terms high voltage and low voltage are relative terms. The term low voltage is really deceiving. The extent of injury from electrical shock depends on these factors:

• The amount of current conducted through the body
• The path of the current through the body
• The length of time a person is subjected to current


The amount of the current depends on the potential difference and resistance. The effect of low current on the human body ranges from temporary mild tingling sensation to death. An electrical shock can injure you in either or both of the following:

• A severe electrical shock can stop the heart, the breathing muscles or both
• Heating effect of the current can cause severe burns, especially at point where the electricity enters and leaves the body

Effect of Electric Current on the Human Body


Current is the KILLING factor in electrical shock. Voltage is only important because it determines how much current will flow through a given body resistance.


Nominal Resistance Values for Various Parts of the Human Body


Human Resistance


Electrical Shock can happen in Three Ways


1. A person may come in contact with both conductors in a circuit
2. A person may provide a path between an ungrounded conductor and the ground
3. A person may provide a path between the ground and a conducting material that is in contact with an ungrounded conductor

Effects of Electric Shock

Entrance Wound - High resistance of skin transforms electrical energy into heat, which produces burns around the entrance point (dark spot in center of wound). This man was lucky, the current narrowly missed his spinal cord.

Involuntary Muscle Contraction - This worker fell and grabbed a power line to catch himself. The resulting electric shock mummified his first two fingers, which had to be removed. The acute angle of the wrist was caused by burning of the tendons, which contracted, drawing the hand with them.

Internal Injuries - This worker was shocked by a tool he was holding. The entrance wound and thermal burns from the overheated tool are apparent.

Thermal Contact Burns - Current exited this man at his knees, catching his clothing on fire and burning his upper leg.

Potential Difference





Equipment Grounding

Ground - A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.

Step and Touch Potential

1. Step Potential

    
• Current flow on the surface of the ground for some distance around the point where the earth become energized.
• Step potential is caused by the flow of fault current through the earth. The person closer to the ground rod or grounded device, the greater the concentration of the current and higher the voltage.
• The current flow creates a voltage drop as it flows through the earth’s surface and a person standing with their feet apart bridges a portion of this drop thus creating a parallel path for the current flows as shown the illustration.
• The wider apart a person’s legs are, the larger the voltage difference across the body. Protection from the step potential hazard is to stay in the zone of equipotential while working. Being alert to this hazard is the best defense. This means that person standing near the point where fault current enters the earth may have a large potential difference from foot-to-foot.
• The potential difference over the same span will less and less as the span is move away from either fault current entry point or the fault return point at the source.
• The voltage drops as you move away from the point of contact. If one part of your body touches a high voltage zone while another part of your body touches a low voltage zone, you will be come a conductor for electricity.
• This is why you should shuffle away from the line, keeping your feet close together as illustrated.
To escape from the potential step hazard: feet shuffle away from the line or keep your feet close and have a short hops.

2. Touch Potential


Touch Potential is a problem similar to step potential. It involves a fault current flow in the earth establish a  potential difference between the earth contact point and some nearby conductor structure or hardware. 



ELECTRICAL BURNS

Burns caused by electric current are almost always third-degree because the burning occurs from the inside of the body. It means that the growth centers are destroyed. Electric-current burns can be especially severe when they involve vital internal organs.

Current Duration

The amount of energy delivered to the body is directly proportional to the length of time that the current flows; consequently, the degree of trauma is also directly proportional to the duration of the current. Three examples illustrate this concept:

1. Current flow through body tissues delivers energy in the form of heat. The magnitude of energy may be approximated by J = I 2Rt Where: J = energy, joules , I = current, amperes , R = resistance of the current path through the body, ohms , t = time of current flow, seconds If sufficient heat is delivered, tissue burning and/or organ shutdown can occur. Note that the amount of heat that is delivered is directly proportional to the duration of the current (t).

2. Some portion of the externally caused current flow will tend to follow the current paths used by the body’s central nervous system. Since the external current is much larger than the normal current flow, damage can occur to the nervous system. Note that nervous system damage can be fatal even with relatively Short durations of current; however, increased duration heightens the chance that damage will occur.

3. Generally, a longer duration of current through the heart is more likely to cause ventricular fibrillation. Fibrillation seems to occur when the externally applied electric field overlaps with the body’s cardiac cycle. The likelihood of this event increases with time.

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