Why Can Someone electrocuted ?

As we know, electricity requires a complete path that electricity can flow. This is why the shock resulting from static electricity is only a momentary jolt: the flow of electrons occurs in a short time to charge the two objects are equal. Momentary electrical shock is generally not harmful. 
Without the contact points between the body with the flow in and out, there can be no risk of electric shock. This is why birds can safely perched on top of a high-voltage power line without electrical shock. Because the only bird perched on one point of contact in the circuit.

In order for electrons to flow through a conductor, there must be a voltage to "use it". Voltage, as we know, is the amount between the two points. So there is no voltage value at only one point, because the bird perched on one point on the circuit, so there can be no voltage drop on the bird's body and the current can not flow through the bird's body. The point? But the bird has two legs. Yes, even though the bird has two legs, thus making two points on the circuit, but the second leg of the bird perched on the same wire. So there is no voltage difference is generated on the bird's body, and there is no "driving force" for the current to flow through the bird's body. 
Maybe you find it difficult to believe that the bird is perched on a cable is not electrocuted. Like the birds, if you are sure to touch only one point / touch the same cable at the same time, you will be safe. But this is not true. Unlike birds that can fly, humans usually standing on the ground when they touch the electrified wires. In many uses, one end of the electrical system with one side connected to ground / earth ground, so that when someone looks just touching a point on the wire / cable but the actual person making the contact between two points on the circuit (ie the earth wire with ground ).
Symbol grounding in the picture is a picture of three increasingly smaller lines placed down on the bottom left of the image sequence, so that the feet of the men also "hit stun". In real life, grounded on the electrical power system consists of several kinds of metal conductors embedded in the ground to make maximum contact with the earth. The conductors are electrically connected to an appropriate connection point on the circuit with thick wire. Ground connection is through the legs of the victim, as his feet hit the ground / earth. 

Maybe some questions in the minds of the students: 

If the existence of a ground point on the circuit makes a person susceptible to electric shock, why it should have a ground circuit? Instead of a series without becoming safer ground? 
A person who uses an electric shock footwear. If rubber and fabric material is an insulator (insulator), why they wear footwear that does not protect the victims of electric shock? 
How good conductor can conduct electricity? If you could be electrocuted because the electrical current can pass through the earth, why not use the earth as a conductor of electricity? 
The answer to question number one is, the existence of a point "ground" is intentional in a series intended to ensure that one side of the circuit safe to touch. Note if the victim in the diagram above touching the bottom of the resistor, it will not happen even if it touches the feet of the ground / soil:
Because the bottom of the circuit is actually connected to ground via a grounding point under the left circuit, so the conductor on the bottom created a series of common (similar) electrically with earth. Because there will be no voltage between the common point, there will be no voltage on the wire is touching the bottom, and they will not be electrocuted. For the same reason, the wire that connects the circuit by grounding rods are usually left bare (not covered insulator), so that the metal object to touch it would be equally electrically common with the earth. 

Grounding the circuit to ensure that there is at least one point in the circuit that are safe to touch. But what happens when it is not in-circuit grounding at all? Would not someone be safe to touch a point on the series as did the bird? Ideally is yes. But in practice, no. Try to see if the circuit has no ground at all.

Despite the fact that the foot is touching the ground, but a single point on the circuit it be safe to touch. Because there is no complete path (circuit) formed through the agency of the bottom to the top of the voltage source, there can be no current flow through that person. However, everything is going to change in the event of unintentional grounding, like a tree trunk touching the wire so as to make the grounding path to earth:

Unintentional connection between the earth wire (ground) is called a ground fault (ground fault). Ground fault can be caused by many things, including dirt formed on insulatornya wire (forming a mud path for current from the conductor to the pole, and to the ground, when it rains), ground water filter embedded, and birds perched on a wire, bridge the path to poles with their wings. Of the many cases of ground faults, they tend to be unpredictable. In the case of tree trunks, no one is guaranteeing wire which is in contact with a tree branch. When a tree is in contact with the upper wire in the circuit, this will make the top of the wire to be safe to the touch but the bottom is to be a hazard if touched, like the opposite of the previous scenario where the trees touch the underside of the circuit:
With the dial on the top of a tree branch wire on the circuit, the wire becomes grounded conductor on the circuit, means electrically connected to earth ground. So there is no voltage between the wire above the ground, but there is full voltage between the bottom and the ground wire. As mentioned earlier, a tree limb is the only possibility of a ground fault on the power system. 

See the power system without ground and without contact with a tree branch, but now there are two people who touched the wire at one point.
With each person standing on the ground, touching different points on the circuit, an electric current is created passing lanes one person, past the earth, and passed the others. Although each person thinks they can safely touch a point on the circuit, but the combination of their actions create a deadly scenario. In effect, one person made ​​a mistake ground that makes others insecure. This is why an ungrounded power system is dangerous: the voltage between a point on the circuit with a ground (earth) may be unpredictable, due to a ground fault can occur at any point and at any time. The only character who secured safely in all scenarios and this is a bird, which has absolutely no connection to earth ground. By connecting a point in the circuit to earth ground (grounding circuit download), the security can not be guaranteed at any point in the circuit. It is more secure than the ground did not have at all. 

For the second question, it has a leather shoe in the electrically insulating properties to protect users from electric shock through his legs. However, most of the design of shoes in general does not really safe electrically, part of the shoe tread is too thin and the substance of the material is not appropriate. Also, moisture, dirt, or conductive salts produced from the sweat makes the shoes are no longer safe. There are some shoes that are made specifically to address electrical hazards, the soles are made of thick rubber so that it can withstand the electrical current wearer working on the electrical circuit is operating, but the use of these shoes must be completely clean, dry so that it can work effectively. Thus, the usual footwear is not enough to guarantee protection from electric shock. 

Research on conduction and contact resistance between parts of the human body with the point of contact (such as ground) indicates values ​​different. As shown in this data. 

Contact the hand or foot, covered by rubber: 20 MΩ generally 
Contact the feet using shoe soles of leather (dry): 100 kΩ to 500 kΩ 
Contact the feet using shoe soles of leather (wet): 5 kΩ to 20 kΩ 
As you can see, not only better than rubber on the skin as an insulator, but the presence of water in these materials will reduce the insulation resistance value in a very large number. 

The answer to the third question, dirt / dust is not a good conductor. Impurities are too poor as a conductor for electrical current flow. However, it will drain a little current to injure or even kill a human, so, even a very small value of conductivity in the dust can produce a deadly electric current path when there is a large enough voltage. 

Some ground surfaces are better insulators than others. For example, asphalt is oiled, has a resistance value greater than dirt or rocks. However, the concrete tends mimiliki low resistance because it is composed of water and electrolyte materials (chemicals that are conductive).

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