Lightning Protection Knowledge for Communication Equipment

The lightning protection issue of communication station facilities in the power system remains a prominent problem. Accidents of communication station facilities being struck by lightning overvoltage occur frequently, disrupting normal communication at communication stations and directly threatening the safe operation of the power grid.

(1) The grounding resistance of communication stations has not been measured once before the rainy season each year as required. In particular, the main communication station on the office building shares a grounding device with the building. The measurement of grounding resistance is the responsibility of the electrical team of the property management department, but in reality, no one is in charge. The grounding resistance has not been measured for a long time, with no records kept, and the main communication station does not follow up either. Some communication stations on mountains have relatively high grounding resistance, reaching over 30Ω, exceeding the standard by 3 times. The smaller the grounding resistance, the lower the overvoltage value, and the higher the lightning withstand level. High grounding resistance results in a low lightning withstand level, making the station susceptible to lightning strikes and damage to communication equipment. Therefore, attention must be paid to the grounding resistance value of the grounding devices of buildings and communication stations, and it must be measured once before the rainy season each year. The grounding resistance value should be within the standard range: less than 5Ω in general areas and less than 10Ω in high soil resistivity areas. It is best to have professionals from the testing institute perform the measurement, as measuring grounding resistance requires certain professional knowledge and is generally not easy to measure accurately. If high grounding resistance exceeding the standard is found, the cause should be carefully analyzed and studied. If the resistance value is indeed high, measures to reduce grounding resistance should be studied and implemented. In particular, for mountain communication stations where grounding resistance exceeds the standard by too much, specialized research institutions must be invited to assist in studying and proposing improved lightning protection measures to reduce grounding resistance.

(2) Faraday cage grounding system for the communication equipment room. The regulations require that the grounding down-conductors of buildings and communication equipment rooms may utilize the main steel reinforcement of the building structure. The upper and lower connection points of the steel reinforcement should be welded with lap welds, and the upper end should be welded to the roof lightning protection device, the lower end to the grounding grid, and the middle to the equalizing pressure net or ring grounding busbar on each floor, forming an electrically connected Faraday cage grounding system. Most units did not raise this requirement during construction, and it was not considered in the architectural design. Checking the architectural design drawings, it is impossible to determine whether the steel reinforcement is welded, whether there is an equalizing pressure net or ring grounding busbar in the equipment room, or to find the design and construction records of the concealed lightning protection grounding system. Some units had to retrofit ring grounding busbars and equalizing pressure nets in the equipment room afterwards. This is a lesson, indicating that in the past, leaders of the communication department did not attach importance to this work. When constructing new buildings, they did not directly participate in the design of the equipment room construction, did not raise the requirement for the cage structure of the lightning protection grounding system, and did not participate in the inspection and acceptance of concealed works in the equipment room. Attention should be paid to the lightning protection of the equipment room, and involvement should begin from the design stage of the equipment room to ensure that the cage grounding grid of the equipment room meets regulatory requirements. For those that did not have it originally, it must be retrofitted in the equipment room.

(3) The grounding bolts of equipment metal enclosures, metal frames, and metal sheaths of various cables should be bonded using silver-containing epoxy resin conductive adhesive. Most units do not do this in their equipment rooms because they doubt the necessity of this measure. If the grounding system is well implemented but the equipment enclosures and metal frames are not well connected to the grounding system, with poor contact, when overvoltage is introduced, the equipment is not protected under an absolute zero potential, and it may be damaged. Therefore, good grounding of equipment must be ensured. Bolts used for grounding must be bonded with silver-containing epoxy resin.

(4) To reduce external lightning electromagnetic interference, in addition to adopting Faraday cage shielding measures in the equipment room, outdoor power cables, communication cables, tower light cables, and other cables must be shielded before entering the communication equipment room.

1. Overhead wires should be changed to shielded cables after being led down from the terminal pole. Before entering the indoor area, they should be horizontally buried directly for more than 10m, with a burial depth greater than 0.6m, and both ends of the shielding layer should be grounded. If non-shielded cables are used, they should be run through galvanized iron pipes and buried horizontally and directly for more than 10m, with both ends of the iron pipes grounded.

2. Outdoor communication cables should use shielded cables, with both ends of the shielding layer grounded. Before entering the indoor area, the cables should be horizontally buried directly for more than 10m.

3. Power cables (wires) and communication cables (wires) in the equipment room should preferably use shielded cables or be laid in metal conduits.

These measures are very necessary for preventing interference. Units that have not adopted these measures in the past should make gradual improvements in a planned manner. For some non-shielded cables, an alternative approach of using two half-section iron pipes or metal pipes combined as a cable shielding layer has been adopted. This is a makeshift method for non-shielded cables already in operation, and its feasibility is for reference.

(5) Grounding of spare cable pairs entering the equipment room. To prevent induced lightning from causing back-strikes at the open ends of conductors and damaging equipment, the regulations require that all spare cable pairs on the distribution frame be grounded at the distribution frame. Grounding spare pairs to prevent overvoltage back-strikes is well-founded and should be implemented. The method is simple: connect the wire ends of the spare pairs together and connect them to the grounding body of the distribution frame with a single conductor.

(6) Limiting the amplitude of intruding lightning overvoltage to prevent lightning damage accidents.

1. Lightning arresters should be installed on the three phases to ground on the terminal pole of the high-voltage distribution line, and lightning arresters should be installed on both the high-voltage and low-voltage sides of the distribution transformer.

2. Zinc oxide lightning arresters should also be installed on the three phases to ground at the input end of the power distribution panel or rectifier in the equipment room. The positive terminal of the DC power supply should be grounded on both the power equipment side and the communication equipment side, and the negative terminal should be connected to varistors on both the power room side and the communication equipment room side.

3. The protective performance of the distribution frame should be good, and it should be equipped with limiting devices such as varistors to suppress overvoltage on cable lines.