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July 19, 2021
Grounding protection of MH bridge and gantry cranes
1. The main basis for the understanding and inspection of "grounding" in the GB60BT-85 standard is the "Safety Regulations for Lifting Machinery", printed GB6067-85 (hereinafter referred to as "the Regulations"). The regulations stipulate in the section on the grounding structure: "The metal structure of the crane must have a reliable electrical connection; the crane working on the track can generally be grounded through the wheels and the track", and it also stipulates that "the grounding wire should be connected with a cross-section of not less than 150 square meters. Flat steel or 10 square copper wires are connected by welding. Under normal circumstances, the metal structure of the crane and the metal shells, pipe grooves, metal skins of cables and the driver's cab of all electrical equipment are reliably connected to the wheels of the cart. So. The focus of the grounding inspection of the crane should be on the connection between the running track of the cart and the grounding wire. According to the requirements of the "Regulations", it is qualified to have the following four grounding devices at the same time:
1) The grounding resistance at any point on the crane is not more than 4 ohms.
2) The cross-section of the grounding wire is not less than the requirements of the "Regulations", that is, the flat steel is not less than 150mm, and the copper wire is not less than 10mm.
3) The grounding wire and the grounding body, and the grounding wire and the cart track must be reliably welded.
4) Metal jumpers are welded at the joints of the two rails of the cart track.
There are two issues in the "Procedures" that have not been explained in detail, and there have been disputes during the inspection.
a) In the "Regulations", only "grounding" is mentioned. In the neutral-grounded power grid, whether to ground or connect to zero should be used. Many factories adopt a three-phase four-wire system where the neutral point is directly grounded. According to the principle of protective grounding and related regulations, if the cranes working in this kind of power distribution grid simply use grounding protection, it cannot completely limit the leakage voltage within a safe range. On the other hand, one-phase grounding short-circuit current when the crane power supply is single-phase leakage, this ground short-circuit current is not enough to cause the line protection device on the crane to operate, and dangerous voltage will exist for a long time. Therefore, when the power supply of the crane is taken from a three-phase four-wire power supply with a neutral point grounded, only zero-connection protection can be used, but grounding protection cannot be used.
b) There should be at least several grounding points in the same span. According to the conditions of the user, the running track of the cart may be long or short. There are also many cranes in the same span. I think there should be at least two grounding points, and the grounding points of the long track should be increased accordingly. Because the rail of the cart is used for a long time, due to poor fixation or lateral dislocation, it is easy to disconnect the ground wire from the track welding place and the jumper wire between the rail seams, so two or more ground wires are required to be reliable. of.
2. The problems of gantry crane grounding (zero connection) protection
1) No grounding device;
2) There is no metal jumper wire at the rail joint;
3) There is only one grounding point in the whole span;
4) The cross-section of the grounding wire is smaller than the standard;
5) Use protection to connect to zero;
3. The following is an explanation of the above shortcomings:
1) No grounding device It includes: the cart track has no metal connection to the ground or the neutral line, although there is a connection but the welding has been opened or the grounding wire has been disconnected; there is a connection, but the grounding body is unqualified. Some put the steel rail on the metal rail beam, which has metal pillars, so they think that the rail is properly grounded through the rail beam and the pillar. In fact, this is unreliable. Because there is no reliable welding between the rail and the rail beam, a pressure plate is used to press on the rail beam, and there is even a paper pad in the middle, and the rail beam and pillars are painted.
2) There is no metal jumper wire at the rail joints, and the rail joints are only connected by fishplates. This is unreliable due to corrosion and other reasons. Some rail joints do not even have a splint, and the width of the rail joints is 10mm. Obviously there is no leakage protection. In some cases, the ground wire is fixed to the rail anchor bolt, and some ground wire is welded to the stop iron at the end of the cart track, and the stop iron and the rail are not welded. These are urgent. All are not conducive to safety.
3) The cross-section of the grounding wire is smaller than the standard. Generally speaking, flat steel is used for the grounding wire. However, many units use round steel. The "Regulations" only states that "flat steel with a cross-section of not less than 150mm should be used for the grounding wire connection". The cross-section of the round steel is not specified, so it may be 150mm. That is, the diameter of the 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 cross section is obviously too small.
4) There are only one grounding points in a single span: there is only one grounding wire from the track of the cart to the ground; although there are more than two grounding wires, only one is effective, and the rest are no longer protective. . There are many reasons why it doesn’t work. For example, the resistance value of the grounding body is too large. It is not specified to be 4 ohms, some reach 20 ohms, and the larger reach 62 ohms; some units only use an aluminum wire to bury shallowly in the soil. As a grounding wire, it is easy to loosen; some grounding wires are actually broken or unwelded.
5) Use protection to connect to zero. In the low-voltage neutral grounded power grid, zero protection should be adopted. However, during the inspection, it was found that individual units connected part of the crane to zero protection and the other part of the crane to ground protection, which is not conducive to safety. Because the electrical equipment of the zeroing system can no longer be grounded.
There are many reasons for the unqualified grounding, but the main reason is that the understanding of grounding is not accurate enough, or the welding of the grounding wire is neglected after the track refurbishment. Of course, some companies have not put the electrical safety of cranes in an important position due to the small scale of the company. In fact, when the gantry crane has a leakage situation, the protective grounding or zero connection is a major event to ensure personal safety. It should be paid enough attention by the user. According to the relevant national standards and regulations, a regular inspection system should be established, management should be strengthened, and the cause should be eliminated. Personal injury and death accidents caused by crane leakage.