Guo Jiwei, Communication Construction Department, Xuzhou Telecommunication Bureau
The good grounding of communication equipment is an important guarantee for the normal operation of the equipment, especially for precision communication equipment in telecommunications networks such as switches, optical transceivers, and computers. The ground wire used by the equipment is usually divided into a working ground (power ground), a protective ground, and a lightning protection ground. Some equipment also has separate signal grounds to isolate strong and weak electric grounds to ensure that digital weak signals are not subject to strong electric ground waves. The main effect of these ground wires is to provide power circuits, protect the human body from electric shock, and also shield the internal circuits of the equipment from external electromagnetic interference or prevent interference with other equipment.
The grounding method of the equipment is usually to bury conductors such as metal grounding piles and metal nets, and the conductors are connected to the ground wire row or cabinet in the equipment through the cable. When multiple devices are connected to the same ground conductor, it is usually necessary to install a ground bar. The location of the ground bar should be as close to the ground pile as possible. The ground wires of different devices are separately connected to the ground bar to reduce mutual influence.
Generally, the grounding resistance of the device should be as small as possible, and the requirements for grounding resistance should be given in the device manual. The grounding resistance of the device includes the wiring resistance from the ground wire row in the device to the general ground wire row in the equipment room, the resistance from the total ground wire row to the grounding pile, the resistance between the grounding pile and the ground (ground resistance), and the connection resistance between each other. In this case, the resistance (earth resistance) between the grounding pile and the ground is the most important variable part. The total resistance of the parts other than the ground resistance is always less than 1Ω in most cases.
1. Measuring principle of ground resistance
There are many factors that affect the grounding resistance: the size (length, thickness), shape, number, buried depth, surrounding geographical environment (such as flat ground, ditches, slopes are different) of the grounding pile, soil moisture, texture, etc. In order to ensure a good grounding of the device, it is indispensable to use the instrument to measure the resistance of the ground, and the hand-cranked ground resistance meter and the clamp-shaped ground resistance meter are commonly used.
1. Measurement principle of manual ground resistance meter Manual ground resistance meter is a more traditional measuring instrument, its basic principle is to use the three-point voltage drop method, as shown in Figure 1. The measurement method is to drive two auxiliary test piles on the ground of the grounding pile (temporarily called X) of the measured ground line. The two test piles are required to be located on the same side of the ground pile, and the three are basically on a straight line. , An auxiliary test pile (called Y) close to the measured ground pile is about 20 meters away from the measured ground pile, and an auxiliary test pile (called Z) far away from the measured ground pile is about to be measured The pile is about 40 meters. During the test, rotate the crank according to the required speed, the tester generates electrical energy through the internal magneto, and "sinks" current between the ground test pile X and the remote auxiliary test pile (called Z). A voltage can be obtained between the geodetic pile X and the auxiliary ground pile Y, and the instrument can calculate the ground resistance of the grounded pile measured by measuring the current and voltage values.
2. Measuring principle of clamp-type ground resistance meter Clamp-type ground resistance meter is a novel measurement tool, it is convenient, fast, and its appearance is exactly like a clamp-type ammeter. It does not need auxiliary test piles when testing, just go to the measured ground line. Clip, you can get the measurement result in a few seconds, which greatly facilitates the measurement of ground resistance. Clamp ground resistance meter also has a great advantage that the ground resistance of the equipment in use can be measured online without cutting off the power supply of the equipment or disconnecting the ground wire.
The circle next to E and I in the circuit represents the ring bayonet of the clamp-on ground resistance meter, Rx is the ground resistance of the ground wire pile under test, R1, R2. . . Rn is the ground resistance of other grounding points in the distributed grounding system. This figure can be further equivalent to FIG. 3. During measurement, the clamp-on ground resistance meter uses the principle of electromagnetic induction to send a constant voltage E to the cable under test through the loop formed by its front end bayonet (with an electromagnetic coil), and this voltage is applied to the circuit shown in FIG. 3 In the middle, the ground resistance meter can measure the current I in the loop through its front-end bayonet at the same time. According to E and I, the total resistance in the loop can be calculated, namely: E / I = Rx + 1 / (1 / R1 + 1 / R2 +. ... + 1 / Rn)
1 / (1 / R1 + 1 / R2 + ... + 1 / Rn) is R1, R2. . . In a distributed multi-point grounding system, the total resistance of Rn in parallel is usually Rx >> 1 / (1 / R1 + 1 / R2 + ... + 1 / Rn), ">>" means "far greater than" assuming the above conditions If it is established, the measured ground resistance Rx = E / I.
In fact, the clamp-type ground resistance meter is fed into the cable through a special electromagnetic converter of its front end snap ring. It is a constant AC voltage of 1.7kHz. In the current detection circuit, after filtering, amplification, and A / D conversion, only The current generated by the 1.7kHz voltage is detected. Because of this, the clamp-on ground resistance meter excludes the tiny currents on the ground line caused by commercial alternating current and high-frequency noise generated by the equipment itself to obtain accurate measurement results. It is also because of this that the clamp-on ground resistance meter Only has the advantage of online measurement. In fact, the meter measures the impedance of the entire loop, not the resistance, but they usually differ by very small amounts under normal circumstances. Clamp ground resistance meter can immediately display the results on the screen, when the bayonet is not stuck, it can display "open jaw" or similar symbols on the screen.
Due to the special structure of the clamp-shaped ground resistance meter, it can be conveniently used as an ammeter. Many of these instruments also have the function of clamp-type ammeters. On the other hand, although a certain frequency signal is used to eliminate interference when testing a clamp-on ground resistance meter, the measurement will also be interfered when there is a large current on the cable under test, resulting in inaccurate results. Therefore, according to the requirements, the current on the cable should be measured before use. Only when the current is not very large can the ground resistance be further measured. Some instruments automatically detect noise interference when measuring ground resistance, and give a prompt when the interference is too large to measure.
2. Precautions for the measurement of clamp-on ground resistance meter
It can be seen from the above introduction that the measuring principles of the clamp-type ground resistance meter and the hand-cranked ground resistance meter are completely different. When using a manual ground resistance meter, the grounding pile should be disconnected from the device to avoid the device's own grounding body affecting the accuracy of the measurement. The manual ground resistance meter can obtain higher accuracy, regardless of single-point grounding And multi-point grounding system; for clamp-type ground resistance meters, the most ideal application is for distributed multi-point grounding systems. In this case, the grounding piles used in the grounding system should be measured in sequence, and the measurement results should be recorded before proceeding. In contrast, for grounding piles whose measurement results are obviously greater than those at other points, focus on inspection. If necessary, disconnect the grounding pile from the equipment and retest with a hand-held ground resistance meter to expose the bad grounding pile.
Clamp ground resistance meters should be used with caution in single-point grounding systems, as can be seen from its working principle:
The resistance value measured by the clamp ground resistance meter is the total resistance in the loop. Only when Rx >> 1 / (1 / R1 + 1 / R2 + ... + 1 / Rn), the resistance value is similar to the grounding pile we want to measure Ground resistance, and this condition is not satisfied in many cases, especially in single-point grounding systems. For an open grounding pile that has been buried but not yet connected to the equipment, the ground resistance cannot be measured with this instrument at all.
The following points should be noted in the use of the clamp-on ground resistance meter:
1. Pay attention to whether it is grounded at a single point, whether the ground wire to be tested is connected to the equipment, and whether there is a reliable ground loop.
Open-circuit grounding piles cannot be measured; grounding loops are unreliable and the measurement results are inaccurate (on the high side). We have encountered this situation in actual use. In the acceptance of our F150 module bureau, we used this instrument to check the grounding resistance of the ground wire.
We used clamp-on ground resistance meters to measure at three locations, A, B, and C, and found that many local resistances were too high, especially at the C position. Many locations exceeded 50 Ω, and some were as high as 120 Ω. This was confirmed by retesting with the old-fashioned three-point test method. In this case, because only the expansion screw at the bottom of the frame except the ground wire of the MDF frame is grounded, and the expansion screw is inserted into the indoor floor less than 10cm, the grounding resistance must be very large. The total resistance of the loop measured at the C position includes this resistance. The assumptions mentioned in the working principle of the time-clamp ground resistance meter cannot be satisfied, so the measurement results have a large deviation.
2. Pay attention to the measurement position and select the appropriate measurement point. The measurement points selected are different, and the measured results are different. For example, the measured results at points A, B, and C in Figure 4 are different, and they are very different. According to the working principle of the clamp ground resistance meter, it is not difficult to understand, which requires attention to the selection of measurement points in use. The measurement sometimes encounters a situation where there is no place to clamp. If conditions permit, the original ground wire can be temporarily disconnected, and a clampable jumper can be temporarily connected for measurement.
3. Pay attention to the "noise" interference. The large loop current on the ground wire will cause interference to the measurement, resulting in inaccurate measurement results and even making the test impossible. Many instruments will display "Noise" or similar symbols in this case.
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