Modern dwelling, along with running water, ventilation and heating, must necessarily have electrical wiring, the presence of which provides the necessary level of comfort. With all the advantages of electric current as a source of energy, electrical wiring belongs to equipment increased danger and the protection of users from electric shock is always given increased Attention.
Principles of protecting the user of electrical appliances from electric shock
A person receives an electric shock from household wiring in two main cases:
- when in contact with a bare wire or contact that is energized;
- when touching a metal part of an electrical installation during insulation breakdown, when the conductive elements of the structure themselves are energized.
The first potential hazard is minimized by various design measures during development electrical receiver (reliable insulation, interlocking system, etc.), the main means of protection against insulation breakdown - chassis grounding. The latter is a forced connection of the conductive elements of the housing with the grounding bus and provides a guaranteed current drain to the ground. A dedicated PE conductor is intended for the implementation of grounding.
To further enhance the efficiency of grounding, an RCD is used, Figure 1. This component is usually installed on a DIN rail in a panel and is an automatic device that controls the difference in currents flowing in the phase and neutral wires.
The RCD operation diagram is illustrated in Figure 2. Based on the first Kirchhoff's law, this chain always satisfies the condition I1 = I0 + I3. In normal mode I3 = 0 and I0 = I1. When a significant leakage current appears or when the wire is touched, a current I3 appears, as a result of which we have I0 < I1. With a certain difference in these currents, a decision is made about a breakdown grounded to the case or touching a phase wire, the machine is triggered and de-energizes the controlled part of the wiring.
The strong side of the circuit Figure 2 is the operation of the RCD even in the event of a breakdown or a drop in the insulation resistance of both the outlet and the wires themselves (conventionally not shown) below a certain limit due to the forced connection of the conductive body (highlighted by a dotted line) of the load from the wires RE. In this case, contact with the conductive parts of the installation may not occur.
What happens if there is no ground wire in the outlet? Important facts
In old houses, there is no PE wire and users are protected according to the PEN scheme, connecting the grounding contact of the socket (if any) with the neutral wire. In this situation, a reasonable question arises about the advisability of installing an RCD in the wiring. The answer to this question will be yes and is illustrated in Figure 3.
A careful comparison of the resulting structure with Figure 2 shows their significant coincidence, i.e. proximity of basic properties. The only significant difference is that during insulation breakdown, when the current-carrying case of the load is energized, the machine will not work.
In such a situation, for the RCD to operate, it is required to touch the case by a person through whose body the current flows to the ground. Naturally, certain unpleasant sensations are possible, but the risks to life and health become noticeably smaller. However, one should prepare for the fact that the probability of a false triggering of the machine will increase, which is determined precisely by the absence of a full-fledged earth in the protective circuit.
Conclusion
From the foregoing, it follows that the advantages of introducing an RCD into the wiring of old houses exceed certain inconveniences from its presence. The most radical solution in terms of electrical safety to the standard level is the creation of normal wiring with a PE conductor during a major overhaul.