Gantry crane grounding system of measurement and testing

"Tower crane technical conditions" and " bridge gantry crane  technical conditions" and other standards provides that: all electrical equipment on the  Steel crane ,  normally uncharged metal, metal pipes, security lighting, low voltage side of the transformer and other required end grounded, and the ground resistance of less than 4Ω (repeated grounding resistance ≤ 10Ω). ① main grounding resistance grounding resistor and the resistance of the ground electrode itself; ② the surface of the ground electrode in contact with the contact resistance between the land; ③ around the earth electrode resistance is the most important ground electric resistivity of the earth around the orange.
    In practice, lifting machinery rarely used TN-S system (three-phase five-wire), because most of the construction site is difficult to achieve this standard, the majority of the TT system or rr system (three-phase four-wire
Three-phase three-wire), which requires not a simple system does not meet the criteria determining eligibility, but should be required to ensure safe protective grounding resistance ≤ lOΩ.
    Currently, the grounding system testing methods are mainly two kinds:
    (1) Electricity shake table
    The table is currently the most commonly used measuring instruments, mainly used to measure ground resistance of a point, where A represents the measurement object, B represents potential electrode. c represents the current electrode. Power (for the exchange, the electrochemical reaction is generated by DC; the AC frequency of the power system is easily induced phase separation using a frequency other than the frequency, usually 1KH less). Measuring the potential difference between AB V, inflow current of the earth I (A), the grounding resistance is the V / I, an important feature of the potential drop is two auxiliary electrode does not affect the measurements. Ying C current electrical grounding resistance plus take charge circuit, the earth will affect the magnitude of the current inflow. However, when the current value changes,
Because it is proportional to the potential difference tiB Kun also changes, the measurement result V / I unchanged. However, the potential drop method and the auxiliary electrode B, C beating positional. Shown in Figure 2, the two are too close to each other will affect the potential, will also affect the accuracy of measurement.
    (2) Clamp resistance measuring instrument
    The law is soon to appear more advanced instruments, generally used to measure a system grounding resistance value. The instrument will generate an AC voltage signal E, E is a known quantity, the coil current measurement instrument measured a current value, whereby the loop resistance of E / I,. If the instrument displays <10Ω, you can determine where the grounding resistance (10Ω, if found instrument displays dozens Ω, then the need for further measurements judgment, but if the instrument displays the value is very high, it is necessary to check whether this ground is defective. If the TN-S system, with the vise jaws ground, it can be measured; if it is three-phase four-wire, grounding and repeated ground distribution system constitutes a loop. vise protective grounding wire can; if it is three-phase three-wire, Direct measurement of the ground wire is invalid because no grounding distribution  hoist gantry crane  and connected to the device, therefore, by forming a loop. only measure the ground resistance of the grounding.
    You should note that the instrument is used to measure a loop, if the point of no ground loops, you need to pull a grounding conductor and a grounding resistor smaller column (net) are connected to form a loop, otherwise undetectable data. Be mistaken not grounded.
    In summary, in the detection of lifting the grounding system should be based on the specific beat by land. Select the correct measurement method. Accurate measurement of earth resistance can be a separate shake table, the measurement system should be used throughout the earth resistance clamp meter to ensure the electrical safety of lifting machinery.