What is indirect contact with electric shock? -Solutions - Huaqiang Electronic Network

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When the electrical equipment is in normal operation, its metal casing or structure is uncharged. When the electrical equipment is damaged and the grounding short-circuit fault occurs (commonly known as “carrying the shell” or “leakage”), the metal casing will have a voltage. When the human body touches the accidental electrified body, an electric shock will occur. This is an indirect contact with electric shock. The so-called contact voltage electric shock is an indirect contact electric shock.
1. When the grounding fault current enters the location near the ground potential distribution, when the electrical equipment encounters a shell failure, the conductor breaks down, or the line insulation breakdown causes a single-phase ground fault, the current flows through the grounding body or the falling point of the conductor into a hemispherical shape. As shown in Figure 9-19(a). Since the soil layer near the current entry point has the smallest flow-dissipation cross section and exhibits a large flow-dissipation resistance value, the ground current will generate a large voltage drop per unit length of the flow path, and the current flow away from the current entry point. The hemispherical section increases as the distance from the current entry point increases, and the corresponding flow resistance decreases gradually. The voltage drop of the ground current on the flow resistance also decreases. Thus, there are different potential distributions in the soil around the current entry site and at various points on the ground surface, as shown in the potential distribution curve in Figure 9-19(b). The curve shows that the potential is the highest at the point of current entry. As the distance from this point increases, the ground potential decreases first and then slowly. At 10m away from the current entry point, the potential has dropped to the potential of the current entry point. %. At the ground 20m away from the current entry point, the cross section of the diffuse hemisphere is already quite large, and the corresponding flow resistance is negligible, or the current in the ground is no longer generating a voltage drop here. It can be considered that the ground potential is zero, the electrician The so-called "ground" in technology refers to the ground at this zero potential (not the ground within 20m around the current). Generally speaking, the grounding voltage of the electrical equipment refers to the potential difference of the charged body to the zero potential point.
2. Contact voltage and contact voltage electric shock When the electrical equipment is grounded due to insulation damage, if the two parts of the human body (usually the hands and feet) touch the outer casing and the ground of the leakage device at the same time, the two parts of the human body are at different ground potentials. The potential difference between them is called the contact voltage. In the electrical safety technology, the person standing in the horizontal direction of the leakage device is 0.8m, and when the hand touches the casing of the leakage device 1.8m away from the ground, the potential difference between the two points of the hand and the foot is the calculated value of the contact voltage. The electric shock caused by the contact voltage is called the contact voltage electric shock.
The magnitude of the contact voltage varies with the position of the body standing point. The farther the human body is from the ground, the higher the contact voltage is.
3. When a stepping fault and stride voltage electric shock occurs in an electrical equipment, the person walking on the potential distribution area around the grounding current entry point (with the current entering the center as the center and the radius of 20 m) will have different feet. The potential difference between the two legs (the average person's step is about 0.8 m) is called the step voltage. Let the potential of the front foot be U1, and the potential of the rear foot be U2, then the step voltage Ustep=U1-U2, obviously the closer the body is to the current input point, the higher the step voltage it is subjected to.
When the human body is subjected to striding voltage, current will form a loop from one foot to the other through the ankle to the other. The symptoms of the electric shock are numbness, twitching, and falling to the ground. After a fall, the current may change path (such as from head to toe or hand) and flow through important organs of the body, making it fatal.
It must be pointed out that striding voltage electric shock can also occur in other occasions, such as near the grounding fault point of the overhead conductor or near the wire breakage point, and the ground near the lightning protection grounding device.
The contact voltage and stride voltage are related to the ground current, soil resistivity, equipment grounding resistance and human body position. When a person wears a boot shoe, the contact voltage and stride voltage of the human body will be significantly reduced due to the voltage drop on the insulation resistance of the floor and the boot shoe. Therefore, it is strictly forbidden to operate the electrical equipment barefoot.