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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Grounding systems. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook Grounding systems TN-C, TN-S, TN-C-S, TT, IT are shown in Fig. 1-5, respectively. The first letter in the designation of the grounding system determines the nature of the grounding of the power source:
The second letter defines the nature of the grounding of the open conductive parts of the building’s electrical installation:
The letters following through the dash behind N determine the nature of this connection, the functional method of arranging the zero protective and zero working conductors:
Until now, a system similar to TN-C (Fig. 1) has been used in Russia, in which the open conductive parts of the electrical installation (housings, casings of electrical equipment) are connected to the grounded neutral of the source by a combined zero protective and working conductor PEN, i.e. "zeroed out".
This system is relatively simple and cheap, but it does not provide the necessary level of electrical safety. TN-S systems (Fig. 2) and TN-С-S (Fig. 3) are widely used in Germany, Austria, France and other European countries. In the TN-S system, all open conductive parts of the electrical installation of the building are connected by a separate zero protective conductor PE directly to the grounding device of the power source.
When installing electrical installations, the rules prescribe the use of a yellow-green insulation marking for the zero protective conductor PE. In the TN-C-S system (Fig. 3), in the input device of the electrical installation, the combined zero protective and working conductor PEN is divided into a zero protective PE and zero working N conductors.
In the TN-C-S system, the neutral protective conductor PE is connected to all exposed conductive parts and can be grounded multiple times, while the neutral conductor N must not be connected to earth. The most promising for our country is the TN-C-S system, which, in combination with the widespread introduction of RCDs (residual current devices), ensures a high level of electrical safety in electrical installations without their radical reconstruction.
Important note! In electrical installations with TN-S and TN-C-S grounding systems, the electrical safety of the consumer is ensured not by the systems themselves, but by residual current devices (RCDs) that operate more efficiently in combination with these grounding systems and the potential equalization system. The grounding systems themselves (without RCDs) do not provide the necessary safety. For example, in the event of an insulation breakdown on the body of an electrical appliance or any apparatus in the absence of an RCD, this consumer is disconnected from the network by overcurrent protection devices - automatic switches or fuse links. The speed of overcurrent protection devices, firstly, is inferior to the speed of the RCD, and secondly, it depends on many factors: the multiplicity of the short-circuit current, which, in turn, depends on the resistance of the conductors, the transient resistance at the point of insulation damage, the length of the lines, the accuracy calibration of circuit breakers, etc. The presence at the facility of metal cases, fittings, etc., connected to the PE conductor, increases the risk of electric shock, since in this case the probability of the formation of a circuit "current conductor - human body - earth" is much higher. Only RCD provides protection against direct contact. The introduction of the TN-S and TN-C-S systems in European countries, whose experience we are constantly forced to refer to, since the problems under consideration there were solved two decades earlier, also took place with great difficulties. For example, the literature describes a case when an electrician, when connecting one object, mistakenly connected a phase to a protective conductor, which resulted in the fatal injury of several people. In terms of ensuring electrical safety conditions during the operation of an electrical installation, a serious alternative to the above grounding systems is a relatively new, but more and more widely used effective electrical protective agent - double insulation. The achievements of the chemical industry in the production of plastics and ceramics, which have excellent mechanical and electrical insulating characteristics, have made it possible to significantly expand the range of electrically safe electrical appliances and power tools in the "double insulation" design, when using which the type of grounding system in terms of ensuring electrical safety conditions does not matter.
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