Electricity as a source of energy, with all its advantages, has such a serious drawback as an increased danger to humans. Therefore, in the development of various electrical systems to protect people from electric shock, great attention is always paid.
Almost everyone knows that electrical devices must be grounded for safe operation. For this purpose, a special screw is provided on the metal elements of their case, Figure 1, marked with a characteristic icon, and the sockets have one or two additional grounding contact.
Why even good grounding does not always protect a person
Nevertheless, it so happens that even with a fully functional grounding, when touching metal body of equipment or metal objects of the building structure, a person receives electric shock. It is usually much weaker than touching a bare wire or busbar directly, but it is nevertheless not a pleasant experience. Moreover, in severe cases, it can even end in death.
Why is this happening? To understand the physical processes, let us turn to Figure 2, which shows two electrical installations A and B grounded according to the norms and equipped with protective circuit breakers Z.
Suppose that the integrity of the insulation is broken in installation A and a voltage of 220 V appears on its case. This voltage creates a current I1, which will be drawn to earth through the earth circuit with resistance R1. Let the current I1 be so small that the circuit breaker 3 does not react to it and the unit A is not de-energized. All this together means that in this case, a voltage of 220 V will be constantly present on the case of unit A.
Installation B is fully operational, for it we have I2 = 0, i.e. for the voltage on the case of this device, it is true UB = 0. As a result, when parts of the body touch the buildings of installations A and B at the same time, a person is under the potential difference UA - UB = 220 - 0 - 220 V and receives an electric shock.
What provides potential equalization?
How to protect a person from electric shock in such cases if it is impossible to reduce the sensitivity threshold of the circuit breaker 3 due to the increased risks of false triggering? Technically, the simplest way is to organize an additional conductive connection between the frames of the A and B installations with a conductor, which is shown in Figure 3 in red. Due to this, a voltage of 220 V will appear on the housings of both installations A and B, but there will be no potential difference and, in contrast to the previous case, with the simultaneous touching of A and B, we have UA - UB = 220 V - 220 V = 0 V, as a result of which the electric shock is not will happen.
In terms of protection efficiency, potential equalization is inferior to grounding, because it is not able to trigger the automatic machine Z. Therefore, the system that implements it is referred to as additional means of ensuring electrical safety.
Varieties of alignment systems
In the process of forming the potential equalization system, all of them are connected to each other and to the grounding bus. metal elements of the building structure (primarily pipes of all communications and fittings reinforced concrete). This system is considered basic. In bathrooms, where the danger of electric shock increases due to leaks, additional alignment is performed. To do this, the pipes, the body of the bathroom, the washing machine and the electric water heater are connected to the grounding conductor of a specially dedicated box, Figure 4 with separate wires.
Conclusion
It is always advisable to use earthing and potential equalization systems together. Only then is the highest level of protection guaranteed.