Grounding protection measures for the hottest brid

2022-07-23
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Grounding protection measures for bridge crane and gantry crane

1. The understanding of "grounding" in gb60bt-85 standard, the main basis for maintenance work is the safety code for cranes, printed in GB (hereinafter referred to as "the code"). The regulations are specified in the grounding structure section: "the crane metal structure must have reliable electrical connection; the crane working on the track can generally pass through the wheel, and the development strategy of efficient and green plastic granulator and track grounding shall be implemented", It is also specified that "the cross section of the grounding wire shall not be less than 15 °. The Rockwell hardness (HRC) and Brinell hardness (HB) of Jinan test metal shall be used" For equal hardness, 0 square flat steel or 10 square copper wire shall be specifically distinguished and connected by welding. In general, the metal structure of the crane and the metal shell of all electrical equipment, pipe trough, metal sheath of cable and cab are reliably connected with the crane wheel. Therefore, the focus of crane grounding maintenance should be on the connection between crane running track and grounding wire. According to the requirements of the "Regulations", the following four items are considered as qualified grounding devices:

1) the grounding resistance at any point on the crane is not greater than 4 ohms

2) the cross section of the grounding wire shall not be less than the requirements of the "Regulations", that is, the flat steel shall not be less than 150mm, and the copper wire shall not be less than 10mm

3) the grounding wire and grounding body, and the grounding wire and crane track must be reliably welded

4) metal jumper wire is welded at the joint of two rail joints of crane track

however, there are two titles in the "Regulations" that have not been described in detail, and there have been disputes in the maintenance. The author has a point of view:

a) "Regulations" only mention "grounding". In the electricity with neutral point grounding, grounding or neutral connection should be adopted. Many factories adopt the three-phase four wire power supply mode with neutral point directly grounded. According to the principle of protective grounding and relevant regulations, if the crane working in this kind of power distribution simply adopts grounding protection, it can not completely limit the leakage voltage within the safe range. On the other hand, in case of single-phase leakage of the crane power supply, the grounding short-circuit current is not enough to cause the line protection device on the crane to act. The gasket shall be made of red copper or aluminum alloy, and the dangerous voltage will exist for a long time. Therefore, when the power supply of the crane is from the three-phase four wire system power supply with neutral grounding, only neutral protection can be used, but grounding protection cannot be used

b) there shall be at least several grounding points in the unified span. According to the premise of the user, the crane running track is long and short. There are more or less cranes in the same span. I think there should be at least two grounding points, and the grounding points of long rails should be increased accordingly. After long-term use of the crane track, due to poor fixation or lateral dislocation, it is easy to cause the grounding wire to be disconnected from the track weld and the jumper wire between the rail seam. Therefore, it is reliable to have two or more grounding wires

2. The grounding (neutral connection) protection store computer receives and processes the data and draws the curve in the title

1) without grounding device

2) there is no metal jumper at the rail joint

3) there is only one grounding point in the whole span

4) the section of grounding wire is smaller than the scale

5) adopt protective neutral connection

3. The following is a description of the above shortcomings:

1) there is no grounding device, which includes: the crane track has no metal connection to the ground or zero line, although there is a connection, it has been welded or the grounding wire has been disconnected; There is connection, but the grounding body is unqualified. Some people also put the steel rail on the metal rail bearing beam, which has a metal pillar. Therefore, it is considered that the steel rail forms a qualified grounding through the rail bearing beam and pillar. In fact, this is not reliable. Because there is no reliable welding between the steel rail and the rail bearing beam, the pressure plate is pressed on the rail bearing beam, and even there is a paper pad in the middle, and the rail bearing beam and the pillar are coated with paint

2) there is no metal jumper wire at the rail joint, which is connected only by the fishplate at the rail joint. Due to corrosion and other reasons, this is unreliable. Some rail joints do not have plywood, and the rail gap is 10mm wide, which obviously does not play the role of leakage protection. Another case is that the grounding wire is fixed to the rail clamp bolt, some are welded to the retaining iron at the end of the crane track, but the retaining iron is not welded to the rail, These phenomena are not conducive to safety

3) the cross section of the grounding wire is smaller than the scale. Generally speaking, the grounding wire should preferably be flat steel. However, many units use round steel. The "specification" only states that "flat steel with a cross-section of not less than 150mm should be used for grounding wire connection". The cross-section of round steel should not be defined. It may also be 150mm. That is, the diameter of round steel should not be less than 13.8mm, but most units use 10mm Round steel, and some units even use 4mm Round steel. The section is obviously too small

4) there is only one grounding in a single span, which can be divided into two cases:

there is only one grounding wire from the crane running track to the earth; Although there are more than two grounding wires, only one is effective, and the rest have no protection effect. There are many reasons why it does not work. For example, the resistance value of the grounding body is too large. It is not 4 ohms, some are 20 ohms, and the maximum is 62 ohms; Some units only use an aluminum wire buried in the soil as the grounding wire, which is very easy to loose; Some grounding wires are actually broken or welded

5) adopt protective neutral connection. In the low-voltage neutral grounding power, adopt neutral connection protection. However, during the maintenance, it was found that some units connected one crane to neutral protection and the other crane to grounding protection, which is not conducive to safety. Because the electrical equipment of the neutral system can no longer be grounded

there are many reasons for unqualified grounding, but the main reason is that the understanding of grounding is not accurate enough, or the welding of grounding wire is ignored after track renovation. Of course, there are also some enterprises that do not put the electrical safety of cranes in an important position because of their small scale. In fact, when electric leakage occurs to the crane, protective grounding or neutral connection is a major event to ensure personal safety, which should be paid enough attention by the user unit. According to the relevant national standards and regulations, a regular inspection system should be established to strengthen management and avoid personal injury and death accidents caused by electric leakage of the crane

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