One of the features of the Soviet apartment system was the absence of 380 V sockets in it: such a voltage was possible only in public buildings and in production. Now such a strict division is absent and, if necessary, it is quite legally possible to organize a 380-volt input even into an old apartment, cottage and private house.
What was the reason for the ban on the input of 380 volts into apartments and houses?
The ban on the use of 380-volt wiring was complex.
Let's name only the main reasons that underlie it:
- 3-phase wiring is initially simply more expensive to implement, and the cost of its mass creation is noticeable would increase the cost of public housing under construction and reduce the commissioning of new and so scarce square meters;
- Due to the higher voltage and technical complexity, 3-phase wiring is less reliable in operation, and the consequences of emergencies on it are on average more severe;
- 3-phase wiring is more difficult to install, modify and repair by a non-specialist, including due to the greater number of options for connecting individual wires;
- 3-phase wiring in its original form is very sensitive to such common faults as zero burn-off.
The last reason is not obvious and requires additional clarification.
Why is zero burning out dangerous?
In the simplest case, 4 wires are connected to the output of a 3-phase voltage source, regardless of its design, three of which are phase, and one is zero. To obtain a voltage of 220 V, two of them are enough: zero and phases. At the same time, to ensure a more or less uniform load on the source, a common zero and one of the phases are brought into neighboring apartments. Figure 1 depicts this case in an extremely simplified form.
Next, we need one more elementary scheme at the level of a school physics course. It is shown in Figure 3 and is necessary to understand the voltage distribution in a network with two loads connected in series.
For this circuit with R1 = R2 we have U1 = U2, and U1 + U2 = E. When R1> R2, the voltages also become unequal: U1> U2. For further consideration, it is important that the equality of loads of neighboring apartments should be considered as an exceptional phenomenon.
Let the current flow circuit in the neutral wire break between the 3-phase voltage source and point F in the diagram in Figure 1, from which the zero is wired to individual apartments. This phenomenon is known as zero burn-off. As a result, the circuit will take the form shown in Figure 3. Simple reasoning shows that the circuits in Figure 2 and Figure 3 are equivalent.
For the circuit in Figure 3, we have U1 + U2 = E = 380 V and for example R1 >> R2 all this voltage will be applied mainly to the load R1. The switched on consumers of apartment 1 will burn out immediately or with some delay, and a voltage close to 380 V will appear in the free sockets.
Why is the ban on 380 V not relevant for the residential sector now?
Over the three decades that have passed since the end of the existence of the USSR, technology has stepped forward. High-speed voltage relays RG, highlighted in Figure 4 and connected to each phase conductor, become effective protection against zero burn-out.
The relay constantly monitors the voltage value and, if it exceeds a certain limit value, it de-energizes the network, which protects the receivers from failure.
In private houses, it will also be useful to re-ground the neutral wire. Its introduction is useful in that in an emergency situation there is no sequential switching on of loads R1 and R2.
Conclusion
The ban on the supply of 380-volt voltage to households during the Soviet era had a serious technical justification. time with high-quality wiring and the use of appropriate protective technology, the implementation of this prohibition has lost relevance.