The electrical wiring of a country house or summer cottage is a technical system of increased danger, when design and implementation of which the requirements of regulatory documents. One of these prerequisites is the implementation of a grounding system that effectively protects person and equipment in case of failure of the insulation of live parts of the wiring itself and electrical equipment.
Typical household wiring diagram and the need to switch to TT circuit
In the bulk of cases, electrical wiring in multi-apartment residential buildings is built with the union of land and zero in the introductory panel.
This scheme successfully combines:
- high efficiency of protection against electric shock to users due to the simplicity of the hardware detection of short circuits and the drop in insulation resistance below the safe limit;
- moderate implementation costs.
The situation is changing noticeably in individual housing construction, when the delivery of electricity is carried out via an overhead line, and consumers are switched on "by a loop". With a fairly frequent break in the neutral wire of the air, the entire current generated by electrical devices of real estate objects for cliff (buildings B and C in Figure 1), begins to flow through the combined system of zero and ground of objects before the cliff (in this case, the only building A).
This is shown schematically by an arrow in the same diagram. Such a system is not designed for such high currents and, due to this, may fail. Moreover, in severe cases, an accident with a neutral wire can even lead to a fire.
It is possible to guarantee the normal performance of the wiring in such emergency situations only the transition to a rigid separation of the neutral and grounding wires, which is implemented by the TT-type system shown on Figure 2. The ground wire in it is connected to the local ground on the site and is not combined with zero anywhere. With this approach, any fault currents are immediately discharged to earth and do not pass through the earthing system of neighboring buildings.
A feature of the residual current device in networks with separate zero and ground
The TT-type protection system functions normally only with a two-stage RCD system, arranged in a star-shaped scheme. A general simplified diagram of connecting RCDs and automatic machines is shown in Figure 3.
The essence of this structure is that:
- a common RCD is provided at the input (indicated as RCD 300), the rated operating current of which should not be less than the threshold of the VA input automaton;
- a separate 30mA RCD is installed in each line, designated as RCD 30.
With a large number of beams, for reasons of economy and ease of operation, it is practiced to use a general RCD of the second level for each group of two or three individual lines.