What is an RCD

Without knowing what RCDs are (protective cut-out devices), it is quite difficult to ensure safe operation of electrical appliances. Often you can hear stories about how a functioning automatic washing machine beats current when you touch it while working. The same happens if you simultaneously touch the housing of a working computer and radiators. Such a phenomenon, indeed, exists, and is not a breakdown, but rather a feature of the operation of electrical equipment without grounding.

The safety shutoff devices can significantly increase the electrical safety.

What is an RCD? This device is a high-speed switch, whose operation is based on comparing currents that are suitable for the load and retreating to the network, performing disconnection of the circuit with their inequality (the appearance of a differential current). That is, if somewhere in the protected section of the line there is a leak into the ground, the device detects it and de-energizes the line.

Let's consider the operation principle of the RCD. The heart of the device is a current transformer, also called differential. It consists of three windings: primary (L1) and secondary (L2), respectively included in the breaks of phase and zero wires, and the third (L3) is connected to the trigger. Depending on the version, the mechanism can be represented by a relay with high sensitivity (electromechanical type RCD) or electronic components, usually a comparator and an amplifier (electronic type). This trigger is mechanically connected to the actuator and power break contacts.

When the load is connected to the RCD, current flows through the windings L1 and L2. Since in the normal mode of operation the value of the current flowing to the consumer is equal to the output current, the common magnetic flux created by them is neutralized and equal to zero (the windings are near, the current is directed counter to). Consequently, no influence is exerted on the winding L3. The operating mode changes radically with simultaneous touch of the person to the ground and the case of the damaged device, which is under voltage. In this case, an additional current appears in the phase conductor (winding L1), which exceeds the current of the winding L2. In fact, there is a difference, because some of the current going to the circuit through the phase conductor will go to the ground through the person, never returning to the RCD. The magnetic flux of the winding L1 exceeds the compensating effect of the flow L2, accordingly, the generated magnetic flux will create in the coil L3 a small current sufficient to trigger the relay or electronic circuit and actuate the actuator to break the power contacts.

A person who knows what RCD is, probably knows how to perform a simple test of its performance. On the case of any RCD there is a button "Test", by clicking on which it is possible to check the operation of the device. Pressing the button briefly shuns one of the windings, removing the compensating magnetic flux created by it from the circuit, so the device is triggered.

The fair question is how to connect the RCD correctly? The device must necessarily be installed in conjunction with a circuit breaker, because it does not contain thermal and electromagnetic releases, respectively, does not protect against overload (thermal) and high short-circuit currents (short circuit). The short-circuit current can damage the RCD, since there is no leakage in this mode of operation, but the current value rises to too high values. By the way, that's why it's important to choose the RCD with a margin on the rated current. For example, if the machine is rated at 16 amperes, then the RCD must be at least 25 amperes. In simple terms, the rated current of the RCD is the permissible current for the conductors that make up it.

Obviously, in our time it is useful not only to understand the issue of what an RCD is, but also to be able to use it in your own home. After all, this is an additional security for all members of the family.