3. RCD - effective fire and electrical protective means

3.3. TYPES OF RCD

According to the operating conditions, the RCDs are divided into the following types: AC, A, B, S, G.

  • RCD type AC is a protective shutdown device that reacts to an alternating sinusoidal differential current that occurs suddenly or slowly increases.
  • RCD type A is a protective shutdown device that responds to an alternating sinusoidal differential current and a pulsating DC differential current that occurs suddenly or slowly increases.
  • RCD type B is a residual current device that reacts to alternating, constant and rectified differential currents.
  • RCD type S - protective shutdown device, selective (with the delay of the tripping time).
  • RCD type G is the same as type S, but with a shorter time delay.

Of fundamental importance in considering the design of the RCD is the separation of devices by the method of technical implementation into the following two types:

RCDs, functionally independent of supply voltage (electromechanical). The source of the energy necessary for functioning - the performance of protective functions, including the shutdown operation, is for the device itself a signal - the differential current to which it reacts;

RCDs, functionally dependent on supply voltage (electronic). Their mechanism for performing a shutdown operation requires energy received either from the monitored network or from an external source. The use of devices functionally dependent on the supply voltage is more limited due to their lower reliability, exposure to external factors, etc. However, the main reason for the smaller spread of such devices is their inoperability with the often frequent and the most dangerous electrical failure condition of the electrical installation, namely, When the neutral conductor in the circuit breaks to the RCD in the direction of the power supply. In this case, the "electronic" RCD, without power, does not function, and the electric installation on the phase conductor carries a life-threatening potential.

Standard IEC 364-5-53 "Electrical installations of buildings - Part 5. Selection and installation of electrical equipment - Switchgear and control equipment" defines the following requirements for RCDs, functionally dependent on the supply voltage:

  • 531.2.2. Selection of devices (RCD), taking into account their functional dependence on supply voltage.
  • 531.2.2.1. Protection devices (RCDs) controlled by residual current may or may not have an auxiliary power source, taking into account the requirements of paragraph 531.2.2.2.
  • 531.2.2.2. The use of residual current-operated protection devices with an auxiliary power supply that does not disconnect the automatically protected circuit in the event of an auxiliary source failure is only permitted if one of the following two conditions is fulfilled:
  • Protection against indirect contact according to 413.1 is ensured even in the event of an auxiliary source failure;
  • Devices are installed in facilities controlled, tested and verified by trained (BA4) or highly qualified (BA5) personnel.

"Electronic" RCD with mains disconnection function

Fig. 3.3. "Electronic" RCD with mains disconnection function:

1 - differential current transformer; 2 - electronic amplifier; 3 - test chain; 4 - holding relay; 5 - control unit; H is the load; Т - button "Test"

The design of "electronic" RCDs in the United States, Japan, South Korea and in some European countries (Figure 3.3), as a rule, includes the function of disconnecting the protected electrical installation from the network in the event of a power failure. This function is implemented constructively by means of an electromagnetic relay operating in the self-holding mode. The relay's power contacts are in the on position only when the current flows through its winding (similar to a magnetic starter).

When the voltage at the input terminals of the device disappears, the relay arm disappears, while the power contacts are opened, the protected electrical installation is de-energized. A similar design of the RCD provides guaranteed protection against human injury in the electrical installation and in the event of a break in the neutral conductor.

In the US, mostly RCDs are used, built into the socket blocks. On one site, for example, a small apartment is installed on 10-15 devices. Sockets that are not equipped with an RCD are necessarily fed with a loop from the outlet blocks with an RCD.

Unfortunately, in our country, unlike the generally accepted in the world practice concept, a number of enterprises produce electronic RCDs based on a typical circuit breaker.

These devices function as follows.

When a differential current occurs from the residual current module containing a differential transformer and an electronic amplifier, an electrical signal is sent to the circuit breaker configured with the module, either from an electrical signal (to a modified current-cut coil) or from an anchor of an intermediate relay through a lead, a mechanical effect is applied to the mechanism for free disconnection of the switch. As a result, the circuit breaker operates and disconnects the protected circuit from the network. In the absence of voltage at the input terminals of such a device (for example, when the neutral conductor breaks to the RCD), firstly, due to a lack of power, the electronic amplifier does not function, and secondly, there is no energy necessary to operate the circuit breaker.

Thus, in the event that a zero conductor breaks in the power supply network, the device is inoperative and does not protect the monitored circuit. At the same time, in this emergency mode (with a break of the zero conductor), the risk of electric shock to a person is exacerbated, as through the phase conductor through unconnected contacts of the circuit breaker, the potential is transferred to the electrical installation. The user, believing that there is no voltage in the network, loses the usual vigilance with respect to the electrical voltage and often makes attempts to repair the malfunction and restore power supply - opens an electric board, checks contacts, - thereby exposing his life to a mortal danger.

In European countries - Germany, Austria, France, the electrical norms allow the use of only the first type of RCD - independent of the supply voltage. RCDs of the second type are allowed to be used in circuits protected by electromechanical RCDs only as additional protection for end users, for example, for power tools, non-stationary receivers, etc.

Electromechanical UZOs are manufactured by leading European companies - Siemens, ABB, GE Power, ABL Sursum, Hager, Kopp, AEG, Baco, Legrand, Merlin-Gerin, Circutor, etc.

In Russia, electromechanical devices - ASTRO * UZO - are widely used. More than 30 modifications of ASTRO * RCDs are serially produced by the state enterprise - MPE IPF.

As a note, it should be noted that, unfortunately, in the domestic market there has appeared a huge number of a variety of falsifications of RCDs and devices of unidentified origin, which often have an attractive appearance, but technical parameters that do not even withstand acceptance tests.

The use of such devices, taking into account the special purpose of the RCD - protection of human life and property, is completely unacceptable. Therefore, when acquiring an RCD, it is necessary to pay special attention to the availability of accompanying technical documentation, including two certificates - a certificate of compliance and a fire safety certificate.

RCD device with built-in overcurrent protection

Fig. 3.4. Device RCD with built-in protection against overcurrents:

1 - coil of current cutoff; 2 - bimetallic plate; 3 - differential current transformer; 4 - magnetoelectric release, which reacts to the differential current; 5 - test resistor; 6 - power contacts; H is the load; Т - button "Test"

There is a class of devices - RCDs with built-in overcurrent protection (RCBO), the so-called "combined" RCDs (Figure 3.4).

Practically all RCD manufacturers have in their production program a RCD with built-in protection against overcurrents. As a rule, their share in the total volume of manufactured protective shutdown devices does not exceed one to two percent. This is explained by the rather limited scope of their application - a small, unchangeable load, an autonomous power receiver, and so on.

An illustrative example is the illumination of billboards installed on street pavilions of public transport stops where two or three fluorescent lamps are powered by a combined RCD with a rated operating current of 6 A and a rated breaking current of 30 mA.

The structural feature of the RCD with built-in overcurrent protection is that the opening mechanism of the power contacts is triggered by the action of any of the three elements - a coil with a current cutoff core reacting to the short-circuit current, a bimetallic plate reacting to the overload currents and a magnetoelectric release reacting On the differential current.

The use of RCDs with built-in protection against overcurrents is advisable only in justified cases, for example, for single consumers of electricity.