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Application of protective grounding and neutral connection in production -- determine the correct protective measures (3)

(I) protective grounding

1. principle and species at the end of October

principle: protective grounding 1 Keeping the whole material testing machine clean is a safety technical measure to prevent indirect electric shock; It is to closely connect the metal part of the ground voltage that may occur under fault conditions with the ground, and limit the fault voltage on the equipment to a safe range. In the system with ungrounded neutral point, if there is no protective grounding on the electrical equipment, when part of the insulation of the equipment is damaged and the human body touches the shell of the equipment, there is a risk of electric shock. After the protective grounding is adopted for the electrical, the grounding 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 4 Ω, while the resistance of the human body is about 1000 Ω. Through the shunt of the grounding body, the current passing through the human body is very small, which can avoid the occurrence of electric shock accidents

types of protective grounding: protective grounding is divided into working grounding and safety grounding; Working grounding is divided into using large enterprises to have a wake-up understanding of this problem to do wire grounding and maintain the safe operation of the system grounding; Safety grounding includes protective grounding, anti-static grounding, lightning protection grounding and shielding grounding to prevent electric shock

2. scope of application

scope of application "in the system with ungrounded neutral point, the metal parts and their accessories of the electrical equipment that are likely to be charged shall be grounded for protection in case of insulation damage, shell touching short circuit or other faults that are not charged under normal conditions.

(II) Protective grounding type

1. principle and type

protective neutral connection is generally used in conjunction with fuses, automatic switches and other protective devices. In case of shell touching short circuit, the short-circuit current flows from the phase line through the shell to the zero line (neutral line) and returns to the neutral point. Since the resistance of the fault circuit is very small, if there is enough fault current to make the protection device on the line act quickly, so that the fault equipment is disconnected, immediately instruct the relevant person in charge to follow up and solve the power supply to play a protective role

types of protective zero connection: the protective zero line and the working zero line are one line (TN-C system), and the protective zero line and the working zero line are separated (TN-S system); Part of the protective zero line and the working zero line are public (TN-C-S system)

2. Requirements

(1) the reactance of the line is not easy to be too large, so as to ensure that there is enough short-circuit current in case of leakage, so that the protective device on the line can act quickly

(2) fuses and switches are never allowed to be installed on the protective zero line

(3) in the same system, individual equipment is not allowed to be grounded and not connected to zero

3. application range

protective neutral connection is only applicable to low-voltage power with neutral point directly grounded

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