1 on the problems of
1.1 without making ground
Mainly as follows: (a) the crane metal structure (or carts track) and then zero trunk or earth without any connection; (2) in the metal structure factory building, mistakenly believe that the carts bearing rail track on a metal beam constitutes ground protection. First, the rail beam metal surfaces coated with paint, secondly, carts beam between the rail tracks and rail clips or hooks with bolts links, sometimes in order to absorb vibration noise, still bearing rail carts and metal beams track mats rubber gasket between people, therefore, carts and metal bearing rail track between beams simply constitute grounding protection. In this case, in the event of collision Housings phase metal structure will cause the crane (or carts track) charged to form a phase-to-ground voltage, that once the contact electric shock occurs.
1.2 Grounding chosen does not match
Inspection found some cranes used is directly grounded neutral point of the three-phase four-wire distribution system, while the metal structure (carts track) grounding type taken not to protect access to zero, ie, TN earthing system, but it is protected ground, that TI 'grounding system; also found a transformer from the power supply to take the crane then zero protection for individual devices from the protection of the zero line network and the measures taken to protect the earth. This is wrong, the consequences of doing serious, on the one hand, it can not completely control the leakage voltage within the safe range, on the other hand, when the crane power single-phase leakage, the grounding line short-circuit current protection device is not enough to cause operation, as shown in Figure 1.
Under normal circumstances, low voltage power supply system phase voltage 220 V, power supply neutral grounding resistor and crane grounding resistance is generally no more than 4 Q (here considered by 4Q), the use of ground fault protection device A shell hit a phase short circuit occurs , its ground current
At this time, the electrical equipment enclosure to ground voltage
This voltage is dangerous for people, while 27.5 A ground current sufficient to cause operation of circuit protection devices, ie devices and the neutral line long dangerous voltage, the zero line of the dangerous voltages applied to the entire distribution system used to protect access to zero all the electrical equipment enclosure, so that all the devices connected to the contact person has zero risk of electric shock.
1.3 Grounding cable connection is not reliable
Mainly as follows: (1) ground cable at the track plate bolts; (2) ground cable at the end of the retaining rail carts iron; (3) grounding cable with grounding or carts track virtual access , Desoldering, etc., resulting in no real visible ground, would not achieve the proper protection.
1.4 Grounding cable cross-section selection does not match
GB6067-1985 "crane safety regulations" (hereinafter referred to as "Regulations") provides: Ground wire connection should use no less than 150 mm2 cross-section of 10 mm2 copper or steel. The reality is that many units of choice is to do with round grounding cable, according to the "Regulations" provisions, if the choice of round do ground cable, then the bar diameter should not be less than 13.8mm, but most of the units are used is ￠ 10 or less round, and even individual units ￠ 4 round, resulting in a long period of poor grounding crane state.
1.5 Through grounding crane rail carts
Carts track joints without making reliable electrical connection. GB50256 - 1996 "crane electrical equipment installation engineering construction and acceptance of electric device," states: at the joint between the tracks, should be electrically connected across. But some rail joints connecting plates alone, and some rail joints did not even plywood, and even some rail joints also left a gap of 10mm, resulting orbit earth touch.
(2) proper grounding
(1) crane power supply for the neutral point directly grounded low-voltage systems, the overall structure of the metal grounding type should adopt the TN or TT system (shown in Figure 2a, shown in Figure 2b). TN earthing system used to protect the zero line in the introduction of metal structures (or carts track) before, if the neutral grounding device from the power supply or the protection of ground zero line other than 50m should do repeat ground, which can reduce leakage equipment enclosure for ground voltage, reduce leakage after Zero Line enclosure shock hazard, increases in short-circuit current phase, accelerated action on the line protection device, shortening fault duration, or when the zero line is disconnected, the device loses leakage any protection. In the TT earthing system and the neutral point ungrounded voltage power supply system, ie IT earthing system (Figure 3), the crane metal structure (or carts track) should be taken to protect the ground. Simply take ground protection crane leakage protection device must be set, and in accordance with grounding resistance and leakage protection device operating current is not greater than the product of 50V, otherwise a short circuit occurs when a phase, leakage to ground voltage equipment enclosure will reach 110 V, this voltage on the people who are still very dangerous (figure Symbol: neutral grounding resistor; crane grounding resistance; body resistance; repeated grounding resistance; Z impedance).
(2) ground cable and grounding, grounding cable and metal structures (or carts track) must be reliable soldering. Not the line, which ensure the metal structure and the ground conductor continuity between devices.
(3) ground cable should be selected puzzle section is not less than 2 150 r 10 r puzzle flat or copper, with round grounding cable to do, its diameter should not be less than 13.8 mm.
(4) By grounding crane rail carts, carts rail joints should be a reliable electrical connection.
3 Conclusion
Good grounding to prevent damage to the insulation of electrical equipment leaving the personal danger of electrical shock, personal safety is a major event in the relationship, so crane units must attach great importance to and strengthen routine inspection and maintenance to ensure that the crane metal structure grounded in good state, to avoid electrical equipment leakage caused personal injury from occurring.
