MDG type bridge and gantry crane grounding protection provisions

 MDG type bridge and gantry crane grounding protection provisions

1, for GB60BT-85 standard "ground" understanding of inspection work was based primarily on " gantry crane  safety regulations", GB6067-85 (hereinafter referred to as "Regulations"). Procedures in the structure of a ground states: "crane metal structure must have a reliable electrical connection; crane working on the track generally grounded through the wheels and tracks", also provides that "ground wire is appropriate section of not less than 150 square Flat or 10 square copper wire, connected by welding. normal circumstances, the crane metal structure and all electrical equipment, metal casing, ducts, cables and metal sheath with carts wheel cab are a reliable connection so  overhead gantry crane  inspection should focus on carts ground orbit and ground wire connection. According to "order" requirements, along with the following four are considered qualified according to device:

A) at any point on the crane grounding resistance not greater than 4 ohms.
2) ground wire cross-section of not less than "order" requirements, namely steel not less than 150mm, copper not less than 10mm.
3) ground wire and grounding, grounding wire and must be reliably welded rail carts.
4) two rail joints rail carts seams welded metal jumper.

But the "rules" There are two issues have not been described in detail in the inspection there had been controversy.

a) "regulations" referred only to "ground" in the neutral grounding grid, should be used or connected to ground zero, many factories using the neutral point directly grounded three-phase four-wire power supply. According to the principle of protection of ground and relevant regulations, in which the power distribution grid work of the bridge gantry crane , if the mere use of ground fault protection, and can not completely restrict leakage voltage within the safety range. On the other hand, a phase to ground short-circuit current when the crane power single-phase leakage, which is insufficient to cause ground fault current circuit protection device on the crane operation, dangerous voltages may exist for a long time. So when the crane's power from the neutral point earthed three-phase four-wire power supply, use only then zero protection, and not with ground protection.

b) should be at least within the same across several ground, according to the conditions of use of units, carts orbit have long or short. In the same span cranes are also more or less, I think it should be at least two grounding, long-track ground should be correspondingly increased. Because carts track after prolonged use, due to the fixed bad or lateral dislocation, it is easy to make the ground wire welding as well as with the track rail sutural jumper Desoldering, so there must be two or more of the ground wire is reliable The.

2, the ground (then zero) protection problems

A) no grounding device;
2) no metal rail joints jumper;
3) Only a full cross-ground;
4) ground wire cross-section is smaller than the standard;
5) used to protect access to zero;

3, following the above drawbacks explain:

1)  hoist gantry crane  without grounding device comprising: carts orbit of the earth or the zero line without any metal connection, although it has been opened to connect the ground wire welding or disconnected; has connections, but grounding failed. Some of them also bearing rail beam on a metal rail, the rail beam with metal pillars, so that the rail through the rail beams and pillars qualified ground, but in fact it is not reliable. Because rail and the rail was no reliable beam welding plate is pressed against the rail beam, and even which also pad pad of paper, while the rail beams and pillars and are coated with paint.

2) rail joints without metal jumper, the rail joints alone fishplates phase, due to rust and other reasons, it is not reliable, some in the rail joints did not even plywood, rail slit width are 10mm, Obviously not achieve leakage protection, and then there is the case: ground plate fixed to the rail bolts, some of the ground wire welding carts track ends in iron block, while retaining no soldering iron rails, these phenomena are is not conducive to the safe.

3) ground wire cross-section smaller than the standard, generally speaking, the ground wire is best to use flat steel. But many units uses round, "order" only explanation "ground wire suitably flat section is not less than 150mm," how much is appropriate to round cross-section is not specified, it may also press 150mm. That is not less than the diameter of round 13.8mm, but most of the units using a 10mm round, and even individual units 4mm round cross-section is clearly too small.

4) Within just a single span ground divided into two cases:

By the carts only one orbit to the earth ground wire; although two or more ground wire, but only one valid, and the rest have no protection. Does not work for many reasons, such as grounding resistance value is too big, not too dry 4 ohms provisions, some reach 20 ohms to 62 ohms maximum; Some units have only a shallow aluminum in the soil As a ground wire, easy to loose; also some ground has been broken or open the actual welding.

5) used to protect access to zero at low neutral grounding grid, then zero protection should be used. However, the inspection found that the part of the individual units crane then zero protection, another part of the crane ground fault protection, which is not conducive to the safe. Since then zero system electrical equipment no longer grounded.

Gantry crane failed causing ground for many reasons, but mainly because of the ground is not accurate enough to understand, or ignored in the track renovations ground wire welding. Of course, some as small-scale enterprises, the electrical safety of the crane did not put in an important position. In fact, the case when the crane when leakage occurs, the protection ground or zero is to ensure personal security barrier of the event, should arouse enough attention to the use of units should be based on the relevant national standards and regulations, establish a regular inspection system, strengthen management, to eliminate leakage due to crane cause personal injury.