Consumer Electronics and TipsElectronics

Why do we need a residual-current device (RCD) in case we have a fuse and what is it actually?

For those readers who are not tempted by working with electrical installations the term “residual-current device” probably doesn’t mean anything or creates vague associations connected with protection of something from some residual current, but what does residual current mean? You may not believe it but this clever machine could save your life or protect your home from fire, moreover it is mandatory for residential installations and industrial production plants. Apropos, does your block of flats have RCD?
Let’s start from the beginning.
Our life is impossible without electricity, but it has its dark side as well. Just a few milliamps of current through the human body are sufficient to trigger dangerous consequences. For example, the threshold of the respiratory paralysis is reached at 30 mA, i.e. we stop breathing.
How can one “get an electric shock”?
This can happen basically in two ways: by direct contact with live wires or by touching the metal casing of appliances and facilities that should have current insulation, but as a result of insulation damages they have become live. On the other hand, one of the most frequent causes of fires, cited in the news programs, is damage in the electrical installation, due to damages in the insulation of the cables, either because they were gnawed by mice or because they are very old or mechanically damaged.  How can we protect ourselves and our property from accidental direct contact or leakage currents that are too small to activate the fuse, and at the same time large enough to cause a fire or cause health effects or even death?
The solution is the residual-current device
How does the residual-current device work?
It operates through an electrical transformer, electromechanical relay and a mechanism for opening the contacts, which respectively detect, compare the detected (induced) current with a preliminary set limit (the so called threshold actuating the residual-current device or sensitivity), and when the leakage current is higher than the threshold actuating the RCD, the RCD is activated – it disconnects the damaged circuit. It is important to note that the operation of this system is independent of the supply voltage. This is important because:

Ordinance № 3 of the Ministry of Economy and Energy doesn’t allow the use of RCD in residential buildings, when its work is affected by the supply voltage, such as electronic security.

Where must the RCD be installed and what should be the threshold of actuation (deactivating current)?

DTZresidual-current device

The RCD can be installed at home, in the office and at industrial places.

1. If a person touches a current-carrying line in his home, a few dozens of milliamps of current can go through his body, which is not enough to activate the automatic fuses because they are adjusted for currents from 10 to 35 or 50 A, but this current level (30 mA) can be health and life endangering. Therefore RCD with 30 mA sensitivity can protect people from an electric shock. In order to clarify the difference between these two numbers, we shall add that the difference between 30 mA and 10 A is exactly 333 times.

“Ordinance № 3 for the structure of electrical installations and current-carrying lines” requires that all circuits with electrical outlets must use RCDs with a rated current of actuation not exceeding 30 mA.

2. At the touch of a casing that has become live due to a damage of an appliance, a hazardous discharge can go through the human body to the ground, so the speed of response of the fuse is vital. If the actuation time of the fuse at 220V voltage is above 0.4 s, the use of RCD is imperative because it will ensure your safety. Besides the safety fuse will be actuated at a much higher value of the current, so the use of residual current protection is necessary in all cases.
According to article 1790 of Ordinance № 3 the use of RCD in this case is also obligatory .

3. At small leakage currents when there is broken cable insulation, the current reaches values of approximately 500 mA, which can cause a fire. These current values cannot switch off the fuse, so it is obligatory to use RCD with 300 mA actuating current for disconnecting the damaged part of the circuit.

Ordinance № 3 recommends RCD with actuating current of 300 mA for the purpose of fire protection.

IMPORTANT! For proper operation of the residual current device there must be a separate neutral conductor and a protective conductor, i.e. three-wire execution for a single-phase circuit or a five-wire execution for a three-phase circuit. After the RCD, the neutral and the protective conductor must not be connected anywhere between themselves.

Where is the place of the RCD in the circuit diagram?

According to Ordinance № 3 RCDs are placed in the floor switchboards or in apartments.

False actuation of the RCD

RCDs have false actuation for three reasons:

1. When there is overvoltage due to lightning or switching of heavy consumers.
2. When there is a very large group of consumers, such as computers, audio and TV equipment connected to the same circuit, as this type of consumers bring permanent small leakage currents which, when added together, can reach the threshold of actuating the RCD.
3. When installation cables are mechanical damaged, when laying a bridge cable directly under the plaster as well as in the presence of many circuits protected by one RCD, the sum of the leakage currents can reach again the threshold of actuating the RCD.

To avoid false actuation of the RCD, the selection of high-quality protection with high precision of actuation is recommended as well as the distribution of consumers in more circuits protected by a greater number of RCDs.

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