Yike electric power tells you that the design of power supply and distribution system should strive for simple system, reasonable setting of protection level and distribution stage, so as to ensure personal safety, reliable power supply, advanced technology and reasonable economy. Code for design of power supply and distribution system (GB 50052-2009), 4.0.6 "the power supply and distribution system shall be simple and reliable, the distribution series of the same voltage level shall not be more than two levels, and the low voltage shall not be more than three levels." The power distribution level is different from the protection level. The protection level refers to the number of upper and lower protection switches. The distribution of a circuit into several circuits through the power distribution device is called primary distribution. For the power distribution device, the combination of the main incoming switch and the feed out switch is called primary distribution, which does not change the distribution level because the switch is a circuit breaker, fuse or disconnector.

If the power supply and distribution system has too many distribution stages, it will not only be inconvenient for management and frequent operation, but also increase the possibility of accidents due to component failure and operation error due to too many series components. The distribution series of the same voltage in the high-voltage distribution system is two-stage, for example, from the 10kV general substation or regional substation to the 10kV distribution substation, and then from the 10kV distribution substation to the distribution transformer, the 10kV distribution series is considered as two-stage. The distribution level of low-voltage distribution system is three levels, for example, from the low-voltage distribution panel of the substation with 380V at the low-voltage side to the distribution panel in the distribution room, from the distribution panel to the power distribution box, and from the power distribution box to the terminal electrical equipment, the 380V distribution series is considered to be three-level. The number of protection stages should not be too many, which will make the selection of upper and lower levels difficult to achieve. Therefore, the distribution series and protection series should be set according to the power supply reliability, management requirements and convenient operation and maintenance of the distribution coefficient. The selectivity of protection appliances at all levels should meet the requirements of the system for power supply reliability.

Take the low-voltage distribution system as an example to illustrate the distribution series and protection series. The low-voltage distribution system diagram is shown in the following figure. In the figure, al1, Al2 (Al3) and AL4 (al5) distribution boxes are 380V main distribution box, distribution box and terminal distribution box of a building. A1 switch and B1, B2, B3, B4 switches constitute the first level of power distribution. Similarly, it can be seen that the low-voltage distribution system is a three-level distribution system. In al1 distribution box, A1 switch and B1, B2, B3, B4 switches are upper and lower switches. If the switch is a circuit breaker, the selectivity between upper and lower circuit breakers should be considered. B1 switch and C1 switch belong to the same level switch. If B1 switch and C1 switch are circuit breakers, then B1 and C1 are circuit breakers of the same level, so the selectivity between them is not needed to be considered because the blackout range caused by B1 or C1 tripping is the same, so the selectivity between them is not needed to be considered. Similarly, C1 and E1 are the same level switches. When the terminal line of F1 switch is short circuited or overloaded, the most desirable thing is that F1 will trip within the agreed time, and its superior switches E1, D1, C1, B1, A1, etc. will not trip. If F1 fails or does not meet the selectivity, the superior switch will trip, or F1 and the superior switch will trip at the same time. This phenomenon is called override trip, which will expand the scope of power outage and cause unnecessary economic losses.

For the upper and lower protective appliances installed in distribution lines in code for design of low voltage distribution (GB 50054-2011), 6.1.2, the action characteristics of Yike electric power should be selective, and coordination among all levels should be possible. Partial selective or non selective cut-off can be adopted for protective apparatus of non important load It is known that selectivity must be considered for important loads, and partial selectivity or no selectivity can be considered for non critical loads. The selectivity between the upper and lower circuit breakers can be realized by current setting value, time, energy and ZSI (regional selective interlocking). At the same time, in order to better realize the selectivity between the upper and lower circuit breakers, the protection series can be reduced to make the circuit breaker setting more convenient and the system more concise. For example, disconnectors can be used for C1, C2, E1, E2 in the system diagram above. Selectivity is achieved to a certain extent, it is difficult to ensure that 100% of circuit breakers in the whole building can achieve selectivity.