Causes of unstable voltage
To increase the main line voltage, a wiring fault condition in the house can "work".Due to the detachment of a common zero cable for some reason, a "zero burn" occurs, resulting in adjacent phases taking on a dangerous voltage of 360 volts.
The voltage drop can also occur due to more simple reasons, for example, the simultaneous connection of several powerful electrical appliances or malfunctions in the operation of substations.
Of course, the increased voltage is much more dangerous than the low one, but it is necessary to fight both with the one and the other in order to avoid breakdowns of equipment. To do this, you must first measure the voltage level, and then pick up a device that can cope with the problem. 
To whom to trust?
The reasons for the electricity supply drops can be different. Sometimes the voltage jumps barely noticeable, but there are also severe cases when one phase gives out 290V, another 200, and the third only 160. In this case, a cheap Chinese machine is unlikely to cope. We need to either buy expensive European options, or trust ourselves and make a reliable automatic device to protect against voltage fluctuations with our own hands.
Consider why you should not save on the quality of your own network security.
Design controversy of the inexpensive purchase automaton
If you can inspect the device from the inside, we see the microcontroller PIC16F676.The board has 14 outputs, operates without strapping, is inexpensive( about $ 1), has onboard the ADC.In general, the controller is very good and can serve as an excellent basis for a reliable machine, if you take into account and correct all its shortcomings. Image Automatic
The board does not have enough leads, hence the developers' mistakes. Switching on the relay in the original machine is done through the screen scan detector. To smooth the pulsations, the electrocondenser acts as a detector filter. And theoretically this inexpensive design, of course, copes with the task, but in practice it is far from the truth. When the mains voltage is out of range, the screen flashes, the capacitor slowly discharges, and only after that the relay trips and disconnects the load. The time allotted for the whole process is enough to burn the smart electronics of modern technology.
And on the viscera of such a machine voltage jump will be so that it will only be discarded. 
How to make a breaker against self-tilt?
If there is a soldering iron and skillful hands, it is quite easy to create a reliable defender of the network inside the housing. It is enough to follow the exact 
scheme. As a result of the work and assembly of all the details( cheap, by the way), this is the
device. Preliminary tests for a period of six months or more have shown the reliability of such a homemade machine. However, if there is little experience in such subtle works, it is better not to take any risks and buy a ready-made, but expensive and reliable automatic device for protection against voltage drops.
