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Remote switch based on RCD. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies

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The residual current device (RCD) performs the functions of protecting a person from electric shock and electrical wiring from fire. A simplified diagram of the RCD and the connection of the load to it are shown in fig. 1.

The principle of operation of the RCD is based on comparing the current in the wires "phase" (L) and "zero" (N). The RCD contains a current transformer T1 with two primary windings I and II, one secondary winding III, as well as a diode VD1, a relay K1 with two break contacts K1.1 and K1.2, combined with a mechanical switch SA1. The RCD also contains elements used to check its serviceability: the SB1 "Test" button and the resistor R1.

Under normal conditions, the current difference in the L and N wires is zero. In the event of a current leakage, for example to "earth", this current difference becomes non-zero. If it exceeds the threshold, the voltage on the relay winding K1 becomes sufficient for it to operate, as a result of which the contacts K1.1 and K1.2 open and disconnect the load from the network. After that, the load can be switched on only by the switch SA1, mechanically connected to the relay contacts. They can also turn off the load manually.

Remote switch based on RCD
Fig. 1

The serviceability of the RCD is checked by pressing the SB1 button, as a result of which a differential current is artificially created through the resistor R1 and the winding I of the transformer T1, sufficient to operate the relay K1 and turn off the load. If, when you press the SB1 "Test" button, the RCD does not work, this indicates a malfunction and the need to replace it.

RCD is characterized by two current parameters, for example, RCD 25 A, 30 mA. The larger value is the maximum load current, the smaller one is the rated differential operating current, often called leakage current for short.

Detailed information about RCDs and their scope is given in the book "RCD - residual current device." Study guide. - M: CJSC "Energoservice", 2003, as well as on the site of the device-protective-shutdown.rf The appearance of the RCD is shown in fig. 2.

Remote switch based on RCD
Fig. 2

Forced operation of the RCD is possible by artificially creating a sufficient leakage current, which allows the RCD to be used as a remote switch by adding the SB1 button and the R1 resistor shown in fig. 3. RCD F372 from ABB with the above parameters was used. For him, the resistance of the resistor R1 should be no more than 220 V / 30 mA = 7,3 kOhm. It is chosen equal to 5,6 kOhm, so that when the mains voltage decreases to 170 V, a current flows through it, guaranteeing a shutdown. Resistor R1 is selected from the MLT series with a rated power of 2 watts. In the time before shutdown (tens of milliseconds), it does not have time to heat up.

Installation of the remote switch is not difficult: the SB1 button is used from an electric bell for a voltage of 220 V. It is located in a convenient place and is connected to other elements with a well-insulated wire. The cross section of the wire does not matter, since the current through the button flows small and for a short time. Resistor R1 can be installed both next to the RCD and in the socket for connecting the load, or even in the SB1 button case if there is enough space in it.

Remote switch based on RCD
Fig. 3

Similarly, a remote switch can be used to disconnect a load from a three-phase network if the load is connected through a three-phase RCD.

If instead of the SB1 button you use the normally open contacts of the emergency mode sensor that occurs during the operation of the object and in which it is necessary to turn off the power supply, then the remote switch will cope with this task.

An example is the shutdown of a water pump when a storage tank is filled with water and a float switch is used as a sensor.

Author: B. Popov

See other articles Section Protection of equipment from emergency operation of the network, uninterruptible power supplies.

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