In the near future, there is often a user asking questions about the installation of lightning protection box, especially the significance of the Kevin type connection. According to our experience, combined with the requirements of the standard specification, give us a brief description.
1 according to GB50057-94 (2000 EDITION) "building lightning protection design code" the requirements of article 6.4.4: to make the maximum power
The surge voltage is low enough and the lead wire at both ends of the surge should be the shortest. When the lead is long, resulting in a large voltage, when possible, can also be used in the diagram of the D, C diagram wiring (Figure C that is Kevin type wiring). 2 according to GB50343-2004 "building electronic information system lightning protection technology standard" the requirements of article 5.4.1.5: surge protection
The wire should be straight, and the length should not be more than 0.5m.
3 according to the requirements of section 5.0.3 of the YD/T5098-2001 "Communication Bureau (station) lightning overvoltage protection engineering design code":
The power connecting line length between boxtype SPD terminals and phase line and the zero line, but if the wiring is difficult, wire length, appropriate to relax the light of the specific conditions, but its cross-sectional area should be increased; SPD grounding line length should be less than 1m, and should be near the ground. According to the above standard specification, installation of lightning protection box, select the installation location and the wiring way to try to lead the two ends of the shortest do, its purpose is to reduce the harm of excessive lead and the additional residual pressure of the equipment, to avoid reducing the protective effects of SPD on equipment.
The following example shows that the use of ordinary parallel connection and the use of Kevin type wiring way to protect the effect of different equipment: A point for the AC distribution box.
B room grounding bus.
Assuming the A point to the distance of the lightning proof box is 1 meters, the inductance of the L1 is about 1uH. Assuming the lightning protection box to the B point of the distance is 5 meters, the inductance of the L2 is about 5uH.
Switching power supply AC input side of the residual voltage (residual pressure) = L1 residual pressure + anti mine box of the residual pressure + L2 residual pressure, and not only the lightning protection box of the residual pressure.
Assume that the lightning current through the lightning protection box is 20KA: the residual voltage of the lightning protection box is 1500V
The residual pressure of L1 = L1*di/dt = 1uH*20KA/10uS = 2KVL2 = = L2*di/dt = 5uH*20KA/10uS = 10KV
Then the final switching power supply AC input side of the residual voltage (residual pressure) = 2 + 1.5 + 10KV = 13.5KV. Far greater than the lightning protection box 1500V voltage, but also far more than the switching power supply 2500V voltage, the result of loss of lightning protection effect, leading to the switching power supply will be damaged by lightning. A point for the AC distribution box. B room grounding bus.
After using the Kevin type method, although the A point to the lightning proof box for a distance of 6 meters.
Switching power supply AC input side of the residual voltage (residual pressure) = anti mine box residual pressure. That is to say, the length of L1 and L2 is changed to 0. Assume that the lightning current through the lightning protection box is 20KA: the residual voltage of the lightning protection box is 1500V.
The residual pressure of L1 = L1*di/dt = 0uH*20KA/10uS = 0KV. The residual pressure of L2 = L2*di/dt = 0uH*20KA/10uS = 00KV.
Then the final switching power supply AC input side to get the residual voltage (residual voltage) = 1.5KV.
Basically equal to the lightning protection box 1500V residual pressure, but also far less than the switch power supply 2500V voltage, the results of the lightning protection effect is very good, the switch power supply will not be damaged by lightning.
Of course, a good method of Kevin type wiring method is to reduce the SPD lead, if the conditions used should be used as far as possible, but not all places can be used this way, it is the biggest flaw in the distribution system due to the large capacity of bus line diameter (or the copper), great SPD the terminal capacity is limited, unable to make this connection. In addition, because the power line first to SPD, then from SPD to distribution equipment or electrical equipment, inevitably increase two joints, if not handled properly, such as poor contact, contact oxidation and loose contact, resulting in heating current is high, can cause power outages, can also cause severe burns serious accidents, there is precedent for this lesson. Therefore, Kevin type wiring is not a panacea, but also need to be selectively used according to the actual situation.
Here we list a common parallel connection mode and the use of Kevin connection mode in the specific application of the base station environment: The ordinary parallel connection, because before installation without careful consideration of the installation location and the wiring, results of SPD wire length is far greater than the requirements of the standard requirements of 0.5m, residual pressure and residual pressure and residual voltage switching power supply terminal is equal to the lightning protection box plus 6 9m conductor, lightning protection box would not protect the effect of the. Many base stations to install lightning protection box, after the lightning lightning box also moves (a lightning count), but the switching power supply will be damaged by lightning, misleading users in the factory that lightning box capacity is too small, then the installation of lightning protection box of large capacity, improve the actual effect is not obvious. It can be said that this is not a great relationship with the installation of lightning protection box. Above is an example of the use of Kevin type wiring, so that there are several advantages: 1 distribution systems and equipment grounding wire without change, the amount of small.
2 lightning protection box in the front end can protect all the load behind, the most comprehensive protection.
The 3 lightning protection box is installed beside the total grounding bus wire, the shortest, the minimum residual pressure, lightning inflow channel is the most reasonable, the best protection.
4 grounding of all equipment are connected to the main earthing busbar, a reference potential, without risk potential counterattack. Of course, the example is the ideal case, if the power wire diameter is too large, the distribution line is not long enough, outdoor circuit breakers, lightning protection box without proper installation space, the implementation will hinder the Kevin connection.
We should grasp the overall situation, reasonable design, follow the standard, flexible application, careful construction, strictly regulate engineering. Kevin type wiring in the project to be used, in order to get the best protection effect, but ignore the specific application environment, blindly emphasize the use of Kevin type wiring, is an impractical way.