Residual current devices. The principle of limiting the time of exposure to current. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Residual current devices. The principle of limiting the time of exposure to current. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Current Devices

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The basis of the action of the protective shutdown of the RCD is limitation principle (due to quick shutdown) duration of current flow through the human body when it touches the elements of the electrical installation under voltage.

Another equally important property of the RCD is its ability to protect against fires and fires that occur at facilities due to a malfunction of electrical equipment.

It is known that more than a third of all fires occur precisely as a result of the heating of conductors by short-circuit currents. Short circuits, as a rule, occur due to insulation defects, earth faults, current leakage to earth.

Reacting to the leakage current, the RCD disconnects the electrical installation from the power source in advance, before the leakage develops into a short circuit, thereby preventing unacceptable heating of the conductors and subsequent ignition of the electrical installation.

Functionally, an RCD can be defined as a high-speed protective switch that reacts to the difference in currents (differential current) in the conductors supplying electricity to the protected electrical installation. The principle of operation of an RCD of a differential type is based on the use of an electromagnetic vector current adder - a differential current transformer.

Comparison of the current values of two or more (in four-pole RCDs - four) currents in amplitude and phase is most effective, that is, with a minimum error, carried out electromagnetically - using a differential current transformer (Fig. 14.14).

The principle of limiting the time of exposure to current
Rice. 14.14. The main blocks of the RCD: 1 - differential transformer that; 2 - threshold element; 3 - actuator 4 - testing circuit; 5 - power contacts

Total magnetic flux in the core - F_Σ proportional to the difference of currents in the conductors, which are the primary windings of the transformer i_L и i_N, induces the corresponding EMF in the secondary winding of the current transformer, under the action of which current i flows in the secondary winding circuit_Δ also proportional to the difference of the primary currents.

Extremely high quality requirements are imposed on the magnetic core of the current transformer of an electromechanical RCD: high sensitivity, linearity of the magnetization characteristic, temperature and time stability, etc. For this reason, for the manufacture of current transformer cores used in the production of RCDs, special high-quality amorphous (non-crystalline ) iron.

Author: Koryakin-Chernyak S.L.

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