In many cases, the installation of a genset or UPS device implies the necessity of modernisation of the entire power supply system. It is a relatively simple case, when dealing with new institutions and modern projects based on designers’ evaluations of the type of necessary equipment and methods of interconnections within the whole system. Unfortunately, the majority of installations is based on already overexploited systems of unequal distribution of voltage. The issue is even more important, because outdated electrical systems compel customers to buy more and more powerful devices, far exceeding the requirements, and the results of such practices can be very harmful.