Application of protective grounding in production
The occurrence of electrical accidents in the production process will not only cause personal injury and property loss, but also serious electrical accidents will bring heavy economic losses and even casualties. Electrical accidents account for a large proportion of work-related accidents. The death toll from electric shock in my country accounts for about 5% of all accident deaths. Due to the invisible, inaudible and invisible characteristics of electricity, it brings certain difficulties to the protection of electrical accidents. In general enterprises, the grounding protection of electrical equipment is a feasible and effective measure to prevent electrical accidents. Three aspects should be done well for protective grounding and zero connection.
In order to correctly judge the degree of danger of touching live objects outside the human body and the safety performance of the circuit, the characteristics and parameters of the circuit must be analyzed.
1. Classification of routes
Electrical circuits can be divided into simple circuits and three-phase circuits. Simple circuits are divided into DC circuits and single-phase AC circuits; three-phase circuits mainly refer to three-phase circuits commonly used in industry.
2. Ungrounded grid
For a neutral and ungrounded system, when the insulation is good, it is safer for the human body to touch a single phase line; when the insulation is poor, there are serious safety hazards.
3. Grounded grid
The grounded power grid is not as safe as the ungrounded power grid under normal conditions; when the insulation is damaged, the electric shock voltage is higher than 220V, and the fuse may work.
(1) Protective grounding
1. Principle and type
Principle: Protective grounding is a safety technical measure to prevent indirect contact with electricity; it is to tightly connect the metal part of the ground voltage that may appear in the case of a fault with the earth, and limit the fault voltage on the equipment to a safe range. In a system where the neutral point is not grounded, if there is no protective grounding on the electrical equipment, when a part of the equipment is damaged and the human body touches the equipment shell, there is a danger of electric shock. After the electrical protection grounding is adopted, the ground short-circuit current flows along the two paths of the grounding body and the human body at the same time. The resistance of the grounding body is generally less than 4Ω, and the resistance of the human body is about 1000Ω. By the shunt of the grounding body, the current passing through the human body is very small, so that accidents in the event of electric shock can be avoided.
Types of protective grounding: protective grounding is divided into working grounding and safety grounding; working grounding is divided into grounding with the earth as a conductor and grounding to maintain the safe operation of the system; safety grounding includes protective grounding to prevent electric shock, anti-static grounding, Lightning protection grounding and shielding grounding.
2. Scope of application
Scope of application "In a system where the neutral point is not grounded, when there is no electricity under normal conditions, and the insulation is damaged, the shell is short-circuited, or other failures occur, the metal parts of the electricity that may be charged and their accessories are grounded. protection.
(2) Protection connection
1. Principle and type
The zero-protection connection is generally used in conjunction with protective devices such as fuses and automatic switches. When a short-circuit occurs to the shell, the short-circuit current flows from the phase line through the shell to the neutral line (neutral line) and returns to the neutral point. Because the resistance of the fault circuit is very small, there is a large enough fault current to make the protection device on the line act quickly, so that the faulty device can disconnect the power supply and play a protective role.
The type of protection zero connection: the protection zero line and the working zero line are one line (TN-C system), the protection zero line and the working zero line are separated (TN-S system); the protection zero line and the working zero line are part of It is public (TN-C-S system).
2. Claim
(1) The reactance of the line is not easy to be too large, to ensure that there is a large enough short-circuit current when leakage occurs, so that the protection device on the line can act quickly;
(2) Fuse and switch are never allowed to be installed on the protective zero line;
(3) It is not allowed for individual equipment to be grounded and not connected to zero in the same system.
3. Scope of application
The protective zero connection is only applicable to the low-voltage power grid with the neutral point directly grounded.