Without knowledge of what an RCD (residual current device is), it is quite difficult to ensure the safe operation of electrical appliances. You can often hear stories about how a working washing machine is shocking when touched during operation. The same thing happens if you simultaneously touch the case of a working computer and heating radiators. Such a phenomenon does exist, and is not a breakdown, but rather a feature of the operation of electrical equipment without grounding.
Significantly increase electrical safety allow protective shutdown devices.
What is an RCD? This device is a high-speed circuit breaker, the operation of which is based on a comparison of currents that are suitable for the load and go back to the network, performing an open circuit in case of inequality (the appearance of a differential current). That is, if somewhere in the protected section of the line there is a leak to the ground, the device detects it and de-energizes the line.
Consider the principle of operation 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 in the phase and neutral wires, and the third (L3) is connected to the starting mechanism. Depending on the design, the mechanism can be represented by a relay with high sensitivity (electromechanical type of RCD) or electronic components, usually a comparator and amplifier (electronic type). This trigger mechanism is mechanically connected to the actuator and power opening contacts.
When the load is connected to the RCD, current flows through the windings L1 and L2. Since in normal operation the value of the current supplied to the consumer is equal to the output, the total magnetic flux created by them is neutralized and equal to zero (the windings are nearby, the current is directed counter-current). Therefore, the L3 winding is not affected. The operating mode changes dramatically while a person touches the ground and the case of a damaged device under voltage. In this case, an additional current appears in the phase conductor (winding L1) in excess of the winding current L2. In fact, a difference arises, because part of the current going to the circuit through the phase wire will go into the ground through a person, without ever returning to the RCD. The magnetic flux of the winding L1 will exceed the compensating effect of the flux L2, respectively, the generated magnetic flux will create a small current in the winding L3, sufficient to operate the relay or electronic circuit and actuate the actuator to break the power contacts.
A person who knows what an RCD is, for sure, knows how to perform a simple test of its performance. There is a “Test” button on the case of any RCD, by clicking on which you can check the operability of the device. Pressing the button briefly shunts one of the windings, removing the compensating magnetic flux created by it from the circuit, so the device works.
The fair question is how to properly connect an RCD? The device must be mounted together 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 no leakage occurs in this operating mode, but the current value rises to too high values. By the way, that is why it is important to choose an RCD with a margin of rated current when buying. For example, if the machine is designed for 16 Amps, then the RCD should be at least 25 Amps. In simple terms, the rated current of an RCD is the permissible current for its constituent conductors.
Obviously, in our time it is useful not only to understand the question of what an RCD is, but also to be able to use it in your own home. After all, this is an additional safety for all family members.