Understanding of Grounding in GB60BT-85 Standard The main basis for the inspection work is the “Safety Regulations for Lifting Machinery”, namely GB6067-85 (hereinafter referred to as the regulations). In the section of grounding structure, the regulations stipulate that the metal structure of thegrúamust have a reliable electrical connection. Forgrúas operating on rails, they can generally be grounded through the wheels and rails. It is also specified that the grounding wire connection should preferably use a flat steel with a cross-sectional area of not less than 150 square millimeters or a copper wire of 10 square millimeters, and the connection should be made by welding. Under normal circumstances, the metal structure of the crane, the metal casings of all electrical equipment, the pipe grooves, the metal sheaths of cables, and the driver’s cab are all reliably connected to the wheels of the cart. Therefore, the focus of inspecting the crane’s grounding should be on the connection between the running track of the cart and the grounding wire. According to the requirements of the regulations, the following four conditions must be met simultaneously for a qualified grounding device:
The grounding resistance at any point on thegrúashould not be greater than 4 ohmios.
The cross-sectional area of the grounding wire should not be less than the requirements of the regulations, that is, the flat steel should not be less than 150 square millimeters, and the copper wire should not be less than 10 square millimeters.
Both the connection between the grounding wire and the grounding body and the connection between the grounding wire and the cart track must be reliably welded.
There should be a metal jumper wire welded at the joint of the two rails of the cart track.
There are two issues in the regulations that are not described in detail and have caused disputes during the inspection. Here are my views:
a) The regulations only mention grounding. In a power grid with a grounded neutral point, it is unclear whether to use grounding or zero connection. Many factories adopt a three-phase four-wire power supply mode with a directly grounded neutral point. According to the principle of protective grounding and relevant regulations, for cranes operating in such a power distribution grid, simply using grounding protection cannot completely limit the leakage voltage within a safe range. Por otro lado, when there is a single-phase leakage of the crane’s power supply, the grounding short-circuit current of one phase is not sufficient to cause the line protection device on the crane to operate, and the dangerous voltage will exist for a long time. Therefore, when the crane’s power is taken from a three-phase four-wire power source with a grounded neutral point, only zero connection protection can be used, and grounding protection cannot be used.
b) Regarding the minimum number of grounding points within the same span, due to the different conditions of the using units, the lengths of the running tracks of the carts vary, and the number of cranes within the same span also varies. I believe that there should be at least two grounding points, and the number of grounding points for long tracks should be increased accordingly. Because after long-term use of the cart track, due to poor fixation or lateral displacement, it is very easy for the welded joints between the grounding wire and the track as well as the jumper wires between the rail joints to become unsoldered. Therefore, having two or more grounding wires is more reliable.
Problems Existing in Grounding (Zero Connection) Protection
There is no grounding device.
There is no metal jumper wire at the rail joint.
There is only one grounding point within the entire span.
The cross-sectional area of the grounding wire is smaller than the standard.
Protective zero connection is used in an improper way.
Explanation of the Above Shortcomings
a) The absence of a grounding device includes situations such as no metal connection between the cart track and the ground or the neutral wire, or although there is a connection, it has been unsoldered or the grounding wire has been disconnected; there is a connection, but the grounding body is unqualified. Some people think that when the rail is placed on a metal rail bearing beam with metal supports, the rail forms a qualified grounding through the rail bearing beam and the supports. In fact, this is unreliable. Because there is no reliable welding between the rail and the rail bearing beam. The rail is pressed on the rail bearing beam by a pressure plate, and there may even be a paper pad in between, and both the rail bearing beam and the supports are painted.
b) Without a metal jumper wire at the rail joint, relying only on the fishplate connection at the rail joint is unreliable due to reasons such as rust. In some cases, there is no splint at the rail joint at all, and the rail joint width is 10mm, which obviously cannot play a role in leakage protection. There are also situations where the grounding wire is fixed to the rail pressure plate bolt, or the grounding wire is welded to the stop iron at the end of the cart track, but the stop iron is not welded to the rail. These phenomena are all detrimental to safety.
c) When the cross-sectional area of the grounding wire is smaller than the standard, generally speaking, flat steel is the best choice for the grounding wire. Sin embargo, many units use round steel. The regulations only state that the grounding wire connection should preferably use a flat steel with a cross-sectional area of not less than 150 square millimeters, and there is no regulation on the appropriate cross-sectional area of the round steel. We might as well also use 150 square millimeters as a reference, that is, the diameter of the round steel should not be less than 13.8mm. But most units use 10mm round steel, and some individual units even use 4mm round steel, and the cross-sectional area is obviously too small.
d) There are two situations for having only one grounding point within a single span: there is only one grounding wire from the running track of the cart to the ground; although there are more than two grounding wires, only one is effective, and the others no longer play a protective role. There are many reasons for this ineffectiveness. Por ejemplo, the resistance value of the grounding body is too large. The regulation requires it not to be greater than 4 ohmios, but some reach 20 ohmios, and the largest is 62 ohmios; some units only use a single aluminum wire buried shallowly in the soil as the grounding wire, which is very easy to loosen; and some grounding wires are actually broken or unsoldered.
e) Regarding the use of protective zero connection, in a low-voltage power grid with a grounded neutral point, zero connection protection should be adopted. Sin embargo, during the inspection, it was found that in some individual units, some cranes use zero connection protection while others use grounding protection, which is not conducive to safety. Because electrical equipment in a zero connection system cannot be grounded again.
The reasons for unqualified grounding are various, but mainly due to inaccurate understanding of grounding or neglect of the welding of the grounding wire after the track is renovated. Of course, in some cases, it is because the enterprise is small in scale and does not attach great importance to the electrical safety of the crane. In fact, when there is a leakage in the crane, protective grounding or zero connection is a major matter to ensure personal safety, and the using units should pay sufficient attention to it. According to relevant national standards and regulations, a regular inspection system should be established, management should be strengthened, and personal injury and death accidents caused by crane leakage should be eliminated.
