For most people, an RCD and a differential machine, and just a circuit breaker, are indistinguishable and they do not see the difference. Outwardly, they are very similar, the inscriptions on the case are almost the same, there is a test and start button, but still these are different devices and let's see how the RCD differs from the difavtomat. In the material, we will consider the purpose of both devices and their fundamental differences in important parameters.
Understanding the purpose of these devices and how RCDs differ from differential automaton will help you make the right choice when designing the electrical network of a private house or apartment.
Content
Purpose of the residual current device (RCD)
The devices are similar in appearance, but there is a difference, since they perform different tasks. The residual current device monitors the current passing through it and breaks the circuit (works) in the event of any leakage to earth after it. The maximum leakage current, above which the RCD will trip, is indicated on its case (10 mA to 500 mA).
The occurrence of differential current (difference at the input and output of the RCD), can occur for various reasons, for example, a malfunction of household appliances or damage to the cable insulation, in which part of it begins to drain to the ground.
Note! In the place where the leakage of electric current occurs when the insulation of the electrical wiring is damaged, the temperature of the wire rises, which can lead to fire and fire.
How to check the quality of insulation, read our article: How to use a megger to measure cable insulation resistance?
Note that in buildings with old electrical wiring, fires due to ignition of the wiring occur quite often.
For a person, the value of the current passing through it, exceeding 30 mA, is considered deadly. Therefore, in electrical panels to protect socket groups, an RCD with a current cutoff is installed 10 mA or 30 mA. RCD with a large rating of this parameter (e.g. 100 or 300 mA) is called fire-fighting and it is needed not to protect a person, but in order to prevent a fire in the place of damaged cable insulation.
It is important to understand that the RCD does not protect the network from overcurrents, this is the key difference between it and the difavtomat. In case of occurrence short circuit, it can burn out, but not work out (because during a short circuit there is no leakage of current to the ground). Therefore, it does not apply on its own, but is established in series with circuit breaker.
Thus, the main purpose of the RCD is to protect against electric shock to a person (if it will flow through the human body to the ground) and timely de-energization of the network section with damaged electrical wiring insulation.
The purpose of the differential machine
The differential machine is a universal device that combines the functions of an automatic switch and a residual current device. This means that the difavtomat is able to provide protection against short circuit, overload and current leakage.
The size of a difavtomat for a single-phase 220 V network is equal to the size of an RCD or a two-pole circuit breaker (two modules). Thus in shield they occupy the same place, but the differential machine has, in addition to the functions of tracking current leaks, also a trip for thermal protection and exceeding the current limit. Therefore, in the absence of space in the electrical panel, you should install difavtomat instead of a bunch RCD + circuit breaker.
Difavtomat has two protections (two types of release):
- electromagnetic;
- thermal.
The electromagnetic release will trip when the current exceeds the rated current by a certain number of times. This number depends on the type of differential machine.
Reference! For type "A", the excess of the nominal value will be 2-3 times, "B" - from 3 to 5, "C" - 5-10 times more than the nominal value, "D" - 10-20 times more.
This is the instantaneous value of the current, for example, during a short circuit or with a large starting current of powerful electrical equipment.
Thermal protection is triggered when a current passes through the machine that exceeds the nominal value for a certain time. This time must be viewed according to the time-current characteristic of a particular machine.The greater the excess, the faster the machine will turn off.
It is also worth noting that the cost of a difavtomat is significantly higher than that of an RCD.
The difference between an RCD and a differential machine
Let's take a closer look at the individual technical characteristics, how the RCD differs from the difavtomat and how you can use the advantages of each of them.
Note the main difference that RCD does not provide network protection against overload and short circuits. That is, it acts solely as an indicator by which current leakage is controlled.
If all electrical appliances are connected to the network at the same time and an intentional overload is created, the protection device will not work, and the differential circuit breaker will instantly de-energize the network, preventing ignition and melting of the insulation.
Let's take a closer look at the devices themselves and then it will become clear how to distinguish an RCD from a difavtomat externally:
- marking of the rated operating current of the electromagnetic release - one of the key differences between an RCD and a difavtomat (only difavtomat has it). The case must indicate the operating current (with the letter - C16, C32) and the leakage current. If only one parameter is indicated or without a letter, then this is an RCD - it indicates the magnitude of the leakage current and the switching capacity of the contacts.
- wiring diagram on the device - similar circuit diagrams are shown on the case, on the RCD diagram it is an oval denoting a differential transformer and an electromechanical relay. On the diagram of the second device, thermal and electromagnetic releases are additionally applied.
- name on the instrument case on the side – not applied on all devices;
- abbreviation on the device - on the devices of domestic manufacturers, the HP is indicated (differential switch) or RCBO (residual current circuit breaker).
It is important to note that the reliability of operation differs little, the main differences are in the operation time and the operation of two types of special releases in the difavtomat. The disadvantage of the latter is the impossibility of determining what caused the operation: network overload, short circuit or leakage.
The advantage of AVDT is the combination of two devices in its case. In the switchboard there is an additional place for a single-pole machine. However, in the event of a breakdown, a full replacement will be required. The residual current device occupies two places, since it must be connected complete with the machine. This kit simplifies the repair process in case of failure - only one element to be replaced.
Which device is better to choose
In general, it doesn’t matter what to install - a difavtomat or a separate RCD with a circuit breaker, the question will only be in the free space in the shield. The main thing is right choose the denomination and the value of the leakage current based on the cross section and material of the cable, as well as selectivity the entire system as a whole.
In the selection process, we recommend paying attention to foreign manufacturers, as they are characterized by the best response time, reliability of elements and cases.
Here are some models that have proven themselves well among users:
- Legrand in electronic-mechanical or electronic modifications;
- - have many advantages, are universal;
- ABB – immediate shutdown in case of short circuit;
- IEK AD 12 – maintains operability when the voltage of the electrical network drops to 50 V;
- EKF AD 32 - often used to connect boilers in the kitchen and bathroom.
So, there really are differences between the two devices, both technically and externally. You can assemble a working circuit with both options, but the choice remains with the designer of the electrical network of a house or apartment.
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