Communication equipment grounding resistance and its measurement method

Communication equipment grounding resistance and its measurement method

Good grounding of communication equipment is an important guarantee for the normal operation of the equipment, especially for precision communication equipment in telecommunication networks such as switches, optical terminals, and computers. The ground wires used by the equipment are usually divided into working ground (power ground), protection ground, and lightning protection ground. Some equipment also has a separate signal ground to isolate strong and weak grounds to ensure that digital weak signals are protected from strong ground surges. The main functions of these ground wires are: providing power supply loops and protecting the human body from electric shocks. In addition, they can shield the internal circuits of the equipment from external electromagnetic interference or prevent interference with other equipment. The way to ground the equipment is usually to bury conductors such as metal grounding piles and metal meshes, and the conductors are then connected to the ground bar or casing in the equipment through cables. When multiple devices are connected to the same grounding conductor, a ground bar is usually required. The location of the ground bar should be as close to the grounding pile as possible, and the ground wires of different devices are connected to the ground bar separately to reduce mutual influence.

Generally, the grounding resistance of the equipment should be as small as possible, and the equipment manual should give the requirements for grounding resistance. The grounding resistance of the equipment includes the connection resistance from the internal grounding row of the equipment to the main grounding row of the machine room, the resistance from the main grounding row to the grounding pile, the resistance between the grounding pile and the earth (ground resistance), and the connection resistance between each other. Generally, the resistance between the grounding pile and the earth (ground resistance) is the most important variable part. In most cases, the total resistance of other parts except the ground resistance is always less than 1Ω.

1. Measurement principle of ground resistance

There are many factors that affect grounding resistance: the size (length, thickness), shape, quantity, burial depth of the grounding pile, the surrounding geographical environment (such as flat land, ditches, and slopes are different), soil moisture, texture, etc. In order to ensure good grounding of the equipment, it is necessary to use an instrument to measure the ground resistance. Commonly used measuring instruments are hand-cranked ground resistance meters and clamp-type ground resistance meters.

1. Measurement principle of hand-cranked ground resistance meter

The hand-cranked ground resistance meter is a relatively traditional measuring instrument. Its basic principle is to use the three-point voltage drop method, as shown in Figure 1. Its measurement method is to drive two auxiliary test piles into the ground on one side of the ground pile (temporarily called X) of the measured ground wire. The two test piles are required to be located on the same side of the measured ground pile, and the three are basically in a straight line. The auxiliary test pile (called Y) closer to the measured ground pile is about 20 meters away from the measured ground pile, and the auxiliary test pile (called Z) farther away from the measured ground pile is about 40 meters away from the measured ground pile. During the test, the crank is turned at the required speed, and the tester generates electrical energy through the internal magnetic motor, "injecting" current between the measured ground pile X and the farther auxiliary test pile (called Z). At this time, a voltage can be obtained between the measured ground pile X and the auxiliary ground pile Y. The instrument can calculate the ground resistance of the measured ground pile by measuring the current and voltage values.

2. Measurement principle of clamp-on ground resistance meter

The clamp-type ground resistance meter is a novel measuring tool. It is convenient and fast. Its appearance is similar to that of a clamp-type ammeter. No auxiliary test pile is needed during testing. You only need to clamp the ground wire to be measured and the measurement result can be obtained in a few seconds, which greatly facilitates the ground resistance measurement. Another great advantage of the clamp-type ground resistance meter is that it can measure the ground resistance of the equipment in use online without cutting off the power supply of the equipment or disconnecting the ground wire.

The rings beside E and I in the circuit represent the ring-shaped clamp-type ground resistance meter, Rx is the ground resistance of the ground wire pile to be measured, and R1, R2... Rn are the ground resistances of other grounding points in the distributed grounding system. This figure can be further equivalent to Figure 3. During measurement, the clamp-type ground resistance meter uses the principle of electromagnetic induction to send a constant voltage E to the measured cable through the ring formed by its front-end clamp (with electromagnetic coil inside). This voltage is applied to the loop shown in Figure 3. The ground resistance meter can simultaneously measure the current I in the loop through its front-end clamp. According to E and I, the total resistance in the loop can be calculated, that is: E/I=Rx+ 1/(1/R1+1/R2+... +1/Rn)

1/(1/R1+1/R2+…+1/Rn) is the total resistance of R1, R2…Rn in parallel. In a distributed multi-point grounding system, usually Rx>>1/(1/R1+1/R2+…+1/Rn), “>>” means “much greater than”. Assuming the above conditions are met, the measured ground resistance Rx=E/I.

In fact, the clamp meter sends a 1.7 kHz AC constant voltage into the cable through the special electromagnetic converter of its front clamp ring. In the current detection circuit, after filtering, amplification, and A/D conversion, only the current generated by the 1.7 kHz voltage is detected. Because of this, the clamp meter eliminates the tiny current on the ground wire caused by the high-frequency noise generated by commercial AC power and the equipment itself to obtain accurate measurement results. Because of this, the clamp meter has the advantage of online measurement. In fact, the meter measures the impedance of the entire loop, not the resistance, but under normal circumstances, the difference between them is very small. The clamp meter can immediately display the results on the LCD display. When the clamp is not locked, it can display "open jaw" or similar symbols on the LCD.

Due to the special structure of the clamp meter, it can be easily used as an ammeter, and many of these meters also have the function of a clamp meter. On the other hand, although the clamp meter uses a signal of a certain frequency to eliminate interference during testing, when there is a large current on the cable being tested, the measurement will also be interfered, resulting in inaccurate results. Therefore, according to the requirements, the current on the cable should be measured first when using it, and the ground resistance can only be further measured when the current is not very large. Some meters automatically perform noise interference detection when measuring ground resistance, and will give a prompt when the interference is too large to make the measurement impossible.

2. Notes on measuring clamp-type ground resistance meter

From the above introduction, it can be seen that the measurement principles of the clamp-type ground resistance meter and the hand-crank ground resistance meter are completely different. When using the hand-crank ground resistance meter, the grounding pile should be disconnected from the equipment to avoid the equipment's own grounding body affecting the accuracy of the measurement. The hand-crank ground resistance meter can achieve higher accuracy, regardless of single-point grounding or multi-point grounding system; for the clamp-type ground resistance meter, its most ideal application is to be used in a distributed multi-point grounding system. At this time, the grounding piles used in the grounding system should be measured in turn, and the measurement results should be recorded and then compared. For the grounding piles whose measurement results are obviously greater than those of other points, they should be checked carefully. If necessary, the ground pile should be disconnected from the equipment and re-measured with a hand-crank ground resistance meter to expose the bad ground piles.

In a single-point grounding system, the clamp meter should be used with caution. From its working principle, it can be seen that the resistance value measured by the clamp meter is the total resistance in the loop. Only when Rx>>1/(1/R1+1/R2+...+1/Rn), the resistance value is close to the ground resistance of the ground pile we want to measure. This condition is not met in many cases, especially in a single-point grounding system. For an open-circuit ground pile that has been buried but not yet connected to the equipment, its ground resistance cannot be measured by this meter at all. The following points should be noted when using the clamp meter:

1. Pay attention to whether there is single-point grounding, whether the ground wire being tested is connected to the equipment, and whether there is a reliable grounding loop.

Open-circuit grounding piles cannot be measured; grounding loops are unreliable and measurement results are inaccurate (too high). We have encountered this situation in actual use. During the acceptance of the F150 module in our bureau, we used this instrument to check the ground resistance of the ground wire.

We used a clamp-type ground resistance meter to measure at A, B, and C, and found that many local ground resistances were high, especially at C, where many local resistances exceeded 50Ω, and some local resistances were as high as 120Ω. We began to suspect that the measurement results were inaccurate, and later retested with the old three-point test method to confirm this. In this case, since the MDF rack is only grounded by the expansion screws at the bottom of the rack in addition to the ground wire, and the expansion screws are inserted into the indoor ground for less than 10cm, its grounding resistance must be very large, and the total loop resistance measured at C includes this resistance. At this time, the assumptions mentioned in the working principle of the clamp-type ground resistance meter cannot be met, resulting in a large deviation in the measurement results.

2. Pay attention to the measurement position and select appropriate measurement points

Different measurement points will produce different results. For example, the results measured at points A, B, and C in Figure 4 are different and very different. This is not difficult to understand based on the working principle of the clamp-type ground resistance meter. This requires that attention should be paid to the selection of measurement points during use. When measuring, there may be no place to clamp. If conditions permit, the original ground wire can be temporarily disconnected and a section of clampable jumper wire can be temporarily connected for measurement.

3. Pay attention to "noise" interference

Large loop currents on the ground wire can interfere with the measurement, causing inaccurate measurement results or even making the test impossible. In this case, many instruments will display "Noise" or similar symbols.