】 【Weak current College on elevator grounding system of detection methods.

<br> <br> National safety code for elevators provided: all electrical devices exposed to conductive parts should be reliable earthing or zero; its resistance value should not be greater than 4 Ω. The above specification explicitly the importance of the lift ground protection, .also provides for the elevator grounding resistance value range of promise. Grounding system is good or bad by the size of the grounding resistance to reflect, grounding resistance smaller then the more reliable grounding system. If the resistance is too large, ground protection device cannot effectively .play the role of security protection. Therefore, the grounding system of detection also belongs to the elevator installation engineering completion approval of one of the project. <br> <br> Elevator grounding system mainly refers to the engine room control cabinet, host, cabin ., layers, Rails, and other important parts and earthing connection system, but these parts are grounded in general summary in the engine room of the ground bus. Therefore, the engine room of the ground bus grounding resistance will represent the system's resistance. The so .-called resistance mainly refers to the grounding resistance, grounding resistance and the surrounding soil and underground resistance 3 parts, including the surrounding soil and underground resistance varies greatly, with the surrounding soil, temperature and humidity. At present, the elevator grounding system of the .detection method in the following 2 ways. <br> <br> 1-ground shaking table ZC-8 measurement (method 1) <br> <br> Working principle: <br> <br> The table is by far the most commonly .used measurement instrument ground resistance, it is based on the potential difference calculation principles work for grounding electrodes, P and C were potential and current auxiliary electrode, the measured resistance Rx received between the E and P. Measurement time hand generator output current I, flows .through the inductor coil of a TA, ground electrode E, auxiliary electrode C and form a closed circuit, grounding resistance of the pressure drop on the Rx-IRx; at the same time, the current transformer secondary coils for nI, n is the current .transformer ratio, secondary current through resistor of pressure drop for Rp nIRp, galvanometer measurement of voltage and pressure drop nIRp IRx. When the galvanometer deflection modulation is zero, then there is IRx = nIRp Rx = nRp. Visible, measured by the grounding resistance is .determined by the transformer ratio n and potentiometer resistors is determined by the Rp and Rp auxiliary electrode and the Rc is irrelevant. Advantages and disadvantages: table manipulation simple wide application, without power supply, but less cumbersome to carry; measurement takes select auxiliary electrode, .high-rise building, from the top of the leader to ground through a secondary point measurement. The elevator room mostly on the top floor, so that the table is more difficult to measure. <br> <br> 2-way ring used MODEL .-4141 instrument measurement (method 2) <br> <br> Working principle: <br> <br> The instrument consists of Japan import, is currently more advanced ground measurement instrument, it is based on the principle of the work by calculation, .as shown in Figure 2. When you first connect the measurement using work power U (AC 100 ~ 220V), then the ground table pen contact the detection of ground wire or electric equipment exposed to conductive parts (assumed to be A point), when .the light show the correct time, press the test button on the instrument display total system grounding resistance value. When the test button is pressed, the instrument within and across the network, network form a closed circuit. Assuming that the current is flowing through I ., grounding resistance Rx = Rt + RA + RTA = U / I. With RT-transformer grounding resistance, at the RA as A point-to-ground, ground resistance of RTA for transformer and A soil resistance between the points. If a transformer .and A more distant point interval, the RTA may have a greater impact. Advantages and disadvantages: it is compact and easy to carry, easy manipulation, reading correctly, no need to select the auxiliary electrode, for grounding measurement of high-rise buildings are .particularly convenient. But you need external measurement of power supply and power supply of line does not go through leakage switch will cause leakage protection trip, power is cut off, the measure cannot be completed. <br> 3 differences between the two methods <br .> <br> The above two methods in addition to their respective advantages and disadvantages, the measured resistance value meaning also vary. On Figure 2 of point a, method 1 measured data is just A point independent of grounding resistance RA, has nothing to do .with power transformer; and the measurement of method 2 is A point system grounding resistance, apart from the point of grounding resistance RA, including the power systems of transformer grounding resistance RT. In theory, though different in two ways, but not much difference between .the value should be in standard value 4 Ω. But in the actual measurement is sometimes found in both vary greatly, even dozens of Ω over its specific reasons required further analysis of elevator system grounding system. Different grounding on Earth resistance measurement will have different effects ., the following analysis of the TT and TN-S two common grounding system of the Earth resistance measurement condition: <br> <br> 3.1TT system <br> <br> Electrical equipment exposed to electrical conductive parts connected with power termination locations unrelated .to the grounding electrode. ⑴ If used ZC-8-ground shaking table is measured, its connection method and it works as shown in Figure 3, it is clear that A point grounding resistance to RA (assuming A point is the elevator room total grounding .line-end). ⑵ MODEL-4141-instrument used for measuring, it works as shown in Figure 4, it is clear that A grounding resistance RA + RT + RAT. Visible for TT system, two methods of measurement results is not equal to .the difference between the RT + RTA values. Method 1 measured values meet the requirements, but does not mean that the elevator system grounding resistance also meets the requirements. Only the method 2 to correctly reflect the system's resistance. Therefore, the TT system should choose .method 2 for measurement. <br> <br> 3.2TN-S system <br> <br> The entire system of neutral conductor and protective conductor is separate from, this system is three-phase power supply system of five-wire. ⑴ .If used ZC-8-ground shaking table to be measured, it works as shown in Figure 5, it is clear that A point about the grounding resistance is RT + RTA. ⑵ MODEL-4141-instrument used for measuring, it works as shown .in Figure 6, it is clear that A point grounding resistance similar toPE line of line resistance. Visible for TN system, two methods of measurement results is not the same. If method 1 measured values meet the requirements, must also comply with method 2, .system grounding resistance to comply with the request; if the value of the measurement method 1 does not comply with the requirements, the transformer grounding resistance at RT may, but because of the role of PE lines, method 2 measured value will still be able to .meet and therefore lift system for ground fault protection system remains reliable. Therefore, TN system select the two methods are available, when method 1 does not comply with the time, you can instead use method 2 measurement. <br> <br> So, .we detect elevator grounding system, according to the system of grounding, choosing the right measurement method. For an independent point of grounding resistance measuring instrument can be used, while for ZC-8 whole earth resistance of earthing system should, to the extent possible, .adopting an instrument measuring MODEL-4141. In addition, the elevator's power system should, to the extent possible, adopting TN-S grounding system, to ensure the safe operation of the elevator. <br> <br> <br> <br>.