## 2 Design of sectionalizing Devices  
The work [37] describes our experience of development and usage of switching devices for sectionalizing 0.4 kV electrical networks and network redundancy. The manufactured devices were introduced in the electric networks of Orel region, the Russian Federation, in 2015-2016 and are currently working there. Figure 1 illustrates the mentioned sectionalizing units. The gained experience revealed the strengths and weaknesses of adopted technical solutions. So, it was confirmed that the choice of vacuum contractors as _SU_ switching devices had been the right decision, since it had enabled _SU_s to successfully isolate network sections under single-phase short circuits. On the other hand, the successful manual reactivation of _SU_s after triggering revealed the need to equip the devoted _SU_s with an  
Figure 1: Photos of sectionalizing units installed in Orel electrical networks [37]. automatic reclosing function. In addition, the disadvantages of the presented _SU_s also include the lack of equipment for operating mode monitoring and remote control. As a result, the personnel of power grid companies learn about _SU_ triggering if the disconnected consumers call them. This leads to an unreasonable increase in power supply outage time. In order to eliminate the above disadvantages, in the Mezenka electrical network, it was proposed to install the modernized _SU_ in accordance with the block diagram presented in Figure 2. In the diagram, a sectionalizing unit is combined with electricity metering, power quality control, control of the amount and time of voltage outages. The diagram provides for the possibility of _SU_ remote control and data transmission from _SU_ to a dispatcher of power grid company. According to Figure 2, the diagram of _SU_ contains input switching element of manual control (ISEMC 1), switching element of remote control (SERC 2), output switching element of manual control (OSEMC 3), control unit of switching element of remote control (CUSERC 4), control unit of sectionalizing unit (CUSU 5), data receiving and transmitting unit (DRTU 6), and uninterruptible power supply unit (UPS 7). Load break switches are used, ISEMC 1 and OSEMC 3. They provide manual switching of power circuits. A vacuum contact is used, SERC 2, which enables switching SC currents. In fact, SERC 2 is the power contact of a contactor. CUSERC4 is the control device of vacuum contactor. CUSERC4 enables SERC 2 to be switched on and off in accordance with the commands from CUSU 5. CUSU carries out the functions of monitoring, control, accounting, and management. With the help of this unit, the sectionalizing unit monitors the operating modes of device and the operating mode parameters of _SU_ electric circuit, including the current and voltage before and after the SU. Monitoring SERC 2 position allows drawing conclusions about the causes for its triggering based on the comparison of facts of changes in current and voltage parameters as well as changes in SERC 2 position. CUSU 5 also monitors and takes into account the time of voltage outages both in the circuit before the SU and in the power circuit of device itself. This makes it possible to obtain reliable data on PS outage time for consumers as well as to analyze SU operation. CUSU  
Figure 2: Block diagram of SU combined with electricity metering and power quality control. 5 carries out technical electricity metering and implements the automation functions of SERC 2 control including automatic reclosing of SERC 2. The functions of CUSU 5 also allow switching SERC 2 off in case of short circuits and overloads. In case of SU local control, a command to switch it off is also given with the help of CUSU 5. For this purpose, a signal is sent from CUSU 5 to CUSERC 4, which, in turn, switches SERC 2 off by stopping the power supply to the SERC coil. Then, the voltage in circuit behind SERC 2 disappears. Additionally, the disconnection of SU can be carried out using the commands that are sent to CUSERC 4 from DRTU 6 and processed by CUSU 5.