RCD(Residual Current Device) is a switching device designed to protect the electrical circuit from leakage currents, that is, currents flowing through unwanted, under normal operating conditions, conductive paths, which in turn provides protection against fires (ignition of electrical wiring) and from electric shock to a person. current.
The definition of "switching" means that this device can turn on and off electrical circuits, in other words, switch them.
The RCD also has other name options, for example: differential switch, differential current switch, (abbreviated current differential switch), etc.
The device and principle of operation of the RCD
And so, for clarity, we present the simplest scheme for connecting a light bulb through an RCD:
It can be seen from the diagram that during the normal operation of the RCD, when its moving contacts are closed, the current I 1 of, for example, 5 Amperes from the phase wire passes through the magnetic circuit of the RCD, then through the light bulb, and returns to the network through the neutral conductor, also through the magnetic circuit of the RCD, while the value of the current I 2 is equal to the value of the current I 1 and is 5 amperes.
In such a situation, part of the current of the electrical circuit coming from the phase wire will not return to the network, but passing through the human body will go to the ground, therefore the current I 2 that will return to the network through the RCD magnetic circuit along the neutral wire will be less than the current I 1 entering the network, accordingly, the value of the magnetic flux Ф 1 will become greater than the value of the magnetic flux Ф 2, as a result of which the total magnetic flux in the RCD magnetic circuit will no longer be equal to zero.
For example, current I 1 \u003d 6A, current I 2 \u003d 5.5A, i.e. 0.5 Ampere flows through the human body into the ground (i.e. 0.5 Ampere - leakage current), then the magnetic flux Ф 1 will be equal to 6 conventional units, and the magnetic flux Ф 2 - 5.5 conventional units then the total magnetic flux will be equal to:
F sums \u003d F 1 + F 2 =6+(-5.5)=0.5 arb. units
The resulting total magnetic flux induces an electric current in the secondary winding, which, passing through the magnetoelectric relay, puts it into operation, and it, in turn, opens the moving contacts, turning off the electrical circuit.
Checking the performance of the RCD is carried out by pressing the "TEST" button. Pressing this button artificially creates a leakage current in the RCD, which should lead to the tripping of the RCD.
RCD connection diagram.
IMPORTANT! Since there is no protection against overcurrents in the RCD, for any scheme of its connection, an installation must also be provided to protect the RCD from overload currents and short circuits.
RCD connection is carried out according to one of the following schemes, depending on the type of network:
RCD connection without grounding:
RCD connection diagram in the mains(when the zero working and zero protective conductors are separated):
IMPORTANT! In the coverage area of the RCD, it is impossible to combine zero protective (ground wire) and zero working conductors! In other words, it is impossible in the circuit, after the installed RCD, to connect the working zero (blue wire in the diagram) and the ground wire (green wire in the diagram) to each other.
Errors in the connection diagrams due to which the RCD is knocked out.
As mentioned above, the RCD is triggered by leakage currents, i.e. if the RCD has tripped, it means that a person has become energized or, for some reason, the insulation of the electrical wiring or electrical equipment has been damaged.
But what if the RCD spontaneously trips and there is no damage anywhere, and the connected electrical equipment is working properly? Perhaps the whole point is one of the following errors in the network diagram of the protected RCD.
One of the most common mistakes is to combine the zero protective and zero working conductor in the RCD coverage area:
In this case, the amount of current leaving the network through the RCD through the phase wire will be greater than the amount of current returning to the network through the neutral conductor. part of the current will flow past the RCD along the ground conductor, which will cause the RCD to trip.
Also, there are often cases of using a ground conductor or a third-party conductive grounded part as a zero working conductor (for example, building fittings, a heating system, a water pipe). Such a connection usually occurs when the zero working conductor is damaged:
Both of these cases lead to the fact that the RCD knocks out, because. the current leaving the network through the phase wire, the current through the RCD does not return back to the network.
How to choose an RCD? Types and characteristics of RCD.
To choose the right RCD and eliminate the possibility of error, use ours.
RCD is selected according to its main characteristics. These include: 
- Rated current- the maximum current at which the RCD is able to work for a long time without losing its performance;
- Residual current- the minimum leakage current at which the RCD will turn off the electrical circuit;
- Rated voltage- the voltage at which the RCD is able to work for a long time without losing its performance
- Current type- constant (indicated by "-") or variable (indicated by "~");
- Conditional short-circuit current- the current that the RCD can withstand for a short time until the protective equipment (fuse or circuit breaker) trips.
RCD selection is based on the following criteria:
- By rated voltage and type of network: The rated voltage of the RCD must be greater than or equal to the rated voltage of the circuit it protects:
Unom. RCD⩾ Unom. networks
At single-phase network required bipolar RCD, at three-phase network — four-pole.
- By rated current: in accordance with paragraph 7.1.76. PUE the use of RCDs in group lines that do not have protection against, without an additional device that provides this protection, is not allowed, while a design check of the RCD in overcurrent modes is necessary, taking into account the protective characteristics of the higher-level device that provides protection against overcurrent.
From the foregoing, it follows that the RCD must be preceded by a protection device (or) it is for the current of this higher protection device that it is necessary to select the rated current of the RCD based on the condition that the rated current of the RCD must be greater than or equal to the rated current of the protection device installed before it:
I nom. RCD ⩾ I nom. protection apparatus
At the same time, it is recommended that the rated current of the RCD be one step higher than the rated current of the higher protection device (for example, if a 25 Amp automatic device is installed in front of the RCD, it is recommended to set the RCD with a rated current of 32 Amperes)
For reference - the standard values of the rated currents of the RCD: 4A, 5A, 6A, 8A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, etc.,
— By differential current:
The differential current is one of the main characteristics of the RCD, which shows at what value of the leakage current the RCD will turn off the circuit.
In accordance with paragraph 7.1.83. PUE: The total leakage current of the network, taking into account the connected stationary and portable power receivers in normal operation, should not exceed 1/3 of the rated current of the RCD. In the absence of data, the leakage current of electrical receivers should be taken at the rate of 0.4 mA per 1 A of load current, and the network leakage current at the rate of 10 μA per 1 m of the length of the phase conductor. Those. The network differential current can be calculated using the following formula:
Δ I network \u003d ((0.4 * I network) + (0.01 * L wires)) * 3, milliamps
Where: Inetworks- network current (calculated according to the formula above), in Amperes; Lwires— the total length of the wiring of the protected electrical network in meters.
Calculating ΔI networks accept the nearest higher standard value of the RCD differential current ΔI RCD:
Δ I RCD ⩾ ΔI networks
The standard RCD residual current values are: 6, 10, 30, 100, 300, 500mA
Differential currents: 100, 300 and 500mA are used to protect against fires, and currents: 6, 10, 30mA - to protect against electric shock. In this case, currents of 6 and 10mA are used, as a rule, for the protection of individual consumers and, and a differential current of 30mA is suitable for general protection of the power supply.
If the RCD is necessary to protect against electric shock, and according to the calculation, the leakage current was more than 30mA, it is necessary to provide for the installation of several RCDs on different groups of lines, for example, one RCD to protect sockets in rooms, and the second to protect sockets in the kitchen, reducing the by the very power passing through each RCD and, as a result, reducing the leakage current of the network, i.e. in this case, the calculation will need to be made for two or more RCDs that will be installed on different lines.
- By type of RCD:
RCDs are of two types: electromechanical And electronic. We considered the principle of operation of an electromechanical RCD above, its main working body is a differential transformer (a magnetic circuit with a winding) that compares the values of the current flowing into the network and the current returning from the network, and in electronic this function is performed by an electronic board for which voltage is needed.
Read also:The classification of RCDs, residual current devices controlled by differential current, is carried out according to the installation method, according to the response time delay, according to the type of execution, according to sensitivity, according to the current rating, according to the response time.
The industrial classification of RCDs is made according to the following characteristics:
- According to the method of installation;
- Depending on the supply voltage;
- By the design of the shutdown mechanism;
- By delay time off;
- By type of performance;
- By parameters;
- According to the application current.
We will analyze each type of classification separately.
RCD classification by installation method
- stationary execution,
- for installation in and
- portable RCD,
- adapters for installation in sockets.
Classification according to supply voltage
According to the interaction with power sources, RCDs are divided into:
- Functionally independent from power - F1;
- Functionally dependent on power - D1;
- Functionally conditionally dependent - HF1.
Classification according to the design of the trip mechanism
According to this characteristic, RCDs are:
- With direct shutdown mechanism. This shutdown mechanism is an integral part of the device.
- With indirect shutdown mechanism. The residual current monitoring device is assembled from a summing current transformer, a trip relay, a trip device in the form of a contactor or a circuit breaker.
- RCD with a direct shutdown mechanism is used in private houses and apartments. The entire RCD device is located in a single housing and, despite its compactness, fully performs.
Pole classification
According to the number of poles for connecting RCDs are divided:
- For bipolar (L,N)
- and four-pole (L1,L2,L3.N).
Time delay classification
According to the tripping delay, RCDs are classified:
- RCD without tripping delay;
- RCD type "G", with tripping delay;
- RCDs type "S", selective RCDs with a long time delay.
Overcurrent protection classification
The RCD may or may not include an overcurrent protection device.
Classification by main parameters
The main characteristics of the RCD are:
- Rated load current - 16A, 20A, 25A, 32A, 40A, 63A, 80A, 100 Amp;
- Rated residual breaking current - 10mA, 30mA, 100mA, 300mA, 500mA (milliamps).
Classification according to the type of differential current in the network
According to the type of differential current in the network, RCDs are classified as follows:
- AC type - alternating sinusoidal current, suddenly arising or slowly increasing. The most common, common option.
- Type A, almost the same as type AC, but in addition a rectified ripple current. This type of device has a more complex design compared to the AC type. It provides high-quality protection and is more expensive than AC type. Type UZO-A is recommended for apartments and cottages.
- Type B - differential current direct and alternating. This type is used in prom. mixed feed installations;
- Type S and G - RCD marking with time delay. RCD trip delay - S is 200 ms - 300 ms. For RCD type G, the delay is defined as 60-80 ms.
This is the whole classification of RCD.
If we open the catalog of any RCD manufacturer, we can read the following there:
- RCD type "AS" protects only against leakage of alternating sinusoidal current;
- RCD type "A" protects against leakage of alternating current and leakage of pulsed (pulsating) current.
We all know that in our network, an alternating sinusoidal current "flows" through the wires and all home consumers work from this network. Therefore, it seems like we can safely install RCDs of the "AC" type everywhere and not think about anything else. But is it?
Let's take a closer look at our modern home appliances, such as the washing machine. It is plugged into a 220-230V AC sinusoidal voltage outlet. If you look further, then the alternating current consumed by it through the power wire reaches the switching power supply. Here, further, the sinusoidal current is converted into a different form. If you look at its graph, then it will no longer be a sinusoid, but, for example, pulsed half-cycles. All this is due to the presence of electronic semiconductor components in modern consumers. In such power supplies and after them, pulsed (pulsating) currents flow. So, if a non-sinusoidal current leaks, then the RCD of the "AC" type may not fix it and, accordingly, not turn off the damaged section of the circuit.
I also note right away that all protective devices are tested at the manufacturer's factories. RCD type "AC" is tested only for sinusoidal alternating current leakage. Manufacturers guarantee the correct operation of their "AC" devices only against leakage of this kind of current. And the correct operation of the RCD is to turn off the faulty section of the circuit when the leakage current reaches the setpoint of a particular RCD for a period of time that is safe for humans. RCD type "AC" may and will operate on the leakage of impulse current, but it may operate with a time delay and from a larger leakage current than the setting of a particular RCD. This can be very dangerous for a person.
Similar switching power supplies are found in almost every modern home consumer. If there is something electronic in the technique (display, control unit, etc.), something in it is regulated (engine speed, time, operating mode, etc.), then we can safely say that it there is a switching power supply. Even if you disassemble fluorescent (energy-saving) lamps, you can find compact switching power supplies in them. That's just such household appliances that need to be protected with the help of an RCD of type "A".
Now let's move on to the evidence for the need to use type "A" RCDs for proper human protection.
The first proof will be GOST R IEC 60755-2012 "General requirements for protective devices controlled by residual (residual) current". It has a very nice B.1 plate. It shows the waveforms of the current depending on the electronic circuit of the consumer.
The left side shows the simplest circuit of the electronic part of most household consumers, and the right side shows the form of the differential leakage current. See the table below.
As you can see, in most cases, the use of RCDs of the "AC" type will be useless, since the differential leakage current will not have a sinusoidal shape.
Here is a screenshot from an ABB webinar showing a similar plate. It shows well that the use of RCDs of the "AS" type in most cases is not permissible. I will post this video later. I recommend everyone to watch it from start to finish.
There is also a good wording in the ABB catalog that RCDs of type "A" are intended for ...
And in our modern home appliances, the physical quantity is necessarily regulated. These are the drum rotation speed in the washing machine, the fan speed and temperature in the air conditioner, the operating mode and temperature of the microwave oven, etc.
The second proof of the use of RCD type "A" is the passport (instruction) for the household appliances themselves. To verify this, take and open it, for example, from your washing machine, dishwasher, microwave oven, etc. Open the section "Connecting to the mains" in it and read what is written there. It is strictly written there that this technique must be protected only with the help of RCD type "A". These are the recommendations of designers, engineers, developers of these devices, by which they were produced. These people know better than we how their device works, what currents flow in it, and therefore their demand must be unquestioningly followed.
Here is a clipping from the passport for the Bocsh washing machine. This pictogram indicates RCD type "A".
Of course, not in every passport you will find this recommendation. For some reason, some manufacturers of home appliances neglect this requirement and do not specify it. But, all eminent European brands always pay special attention to human safety and highlight this moment in the "Connecting to the power grid" section.
Below I propose to watch a webinar by a representative of the ABB concern, which talks about choosing the type of RCD "AC" or "A". True, at the beginning it talks about the TN-C grounding system, but starting from the 54th minute, a conversation begins about the choice of RCD types. I still recommend not to be lazy and watch the whole video, as it contains a lot of useful information.
Who should not be listened to when choosing the type of RCD?
This is primarily managers and sellers of electrical goods stores. They always try to sell the goods that they have in stock, and RCD type "A" is not a warehouse item, especially in the regions of the country and is made to order. Also, many managers do not know what is the difference between RCD types "A" and "AC". With these words, I do not want to offend all the sellers of electrical goods. Perhaps somewhere people who understand the types of RCDs work, but I have not seen such people in Samara)))
Do not always rely on the recommendations of electricians. Unfortunately, many also do not know the difference in this case. Very often I met a phrase from electricians that an RCD does not need to be installed at all, since it constantly works. Do not listen to relatives and a neighbor who have had two automatic machines for 20 years and everything works. Even today, YouTube has become very dangerous, since everyone who feels like it uploads videos and, unfortunately, most of the videos do not carry the correct information.
Who should be listened to when choosing the type of RCD?
Be sure to follow the instructions in the instructions for the equipment. Watch webinars hosted by major corporations such as ABB, Legrand, IEK, etc. Their videos contain a lot of useful and competent information. Webinars are conducted by leading engineers and equipment developers who know what they are talking about. On the official websites of large concerns, you can find the schedule of webinars and their recordings. These are the ones I recommend to watch.
Summing up all of the above, we can conclude that RCDs of the "AS" type can be installed to protect circuits to which resistive loads are connected, such as incandescent lamps, conventional hobs and ovens, conventional heaters, simple electric kettles. For all other equipment with electronic components, it is imperative to install an RCD of type "A".
That's exactly why I recommend to everyone who assembles electrical panels to always choose RCD type "A". If an RCD is installed in the switchboard, to which several circuit breakers are planned to be connected, then type "A" must definitely be selected here, since there is a high probability of connecting electronic equipment to the network.
IMPORTANT!!! The same applies to the choice of difavtomatov (AVDT). They also come in "AC" and "A" types.
In Europe, for a long time in the residential sector only type "A" RCDs have been used, since only it can provide the necessary level of human safety. By clicking on this link you can see an example of an electrical panel from Germany. All RCDs of type "A" are installed in it.
Unfortunately, in the budget series of protective devices there is no type "A" RCD with leakage currents of 10-30mA. They are only in expensive and more professional series, for example, the F202 series from ABB or the DX3 from Legrand. But if we compare different types of RCDs from the same series, then the difference in cost between "A" and "AC" is approximately 500 rubles.
Yes, type "A" RCDs have become very expensive today, but still a person's life is more expensive !!!
Perhaps I am wrong in my conclusions. If so, then correct me. It will also be useful for me to present the whole real picture with the choice of RCDs of type "AC" or "A". But, I made my data conclusions on the study of relevant regulatory documents and recommendations of specialists from specialized companies.
There are various types of residual current devices (RCD) according to their technical design. Below is an approximate classification of RCDs.
1. RCD classification by purpose:
RCD without built-in overcurrent protection (residual current switches, see Fig. 1, a, b),
RCD with built-in overcurrent protection (differential circuit breakers, Fig. 2, a),
have thermal and electromagnetic releases and protect against overload currents and short circuits.
2. According to the control method: RCDs that are functionally independent of voltage, RCDs that are functionally dependent on voltage (Fig. 2, b).
Residual current devices that are functionally dependent on voltage, in turn, are divided into: devices that automatically open power contacts in the event of a voltage failure with or without a time delay. When the voltage is restored, some models of these devices automatically re-close the contacts of their main circuit, while others remain in the off state, to devices that do not open the power contacts when the voltage disappears.
There are also two versions of devices in this group. In one embodiment, the device does not open its contacts when the voltage fails, but retains the ability to open the power circuit when a differential current occurs. In the second option, in the absence of voltage, the devices are unable to trip when a differential current occurs.
RCDs that are functionally independent of the supply voltage (electromechanical). The source of energy necessary for functioning - performing protective functions, including the shutdown operation, is the signal itself for the device - the differential current to which it responds, RCDs that are functionally dependent on the supply voltage (electronic). Their mechanism to carry out the shutdown operation requires energy, either from a controlled network or from an external source.
The reason for the lesser distribution of electronic RCDs is their inoperability when the neutral conductor that feeds them breaks. In this case, the body of the power receiver connected to the network through an RCD that does not open its contacts when the voltage disappears will be energized. In addition, despite the lower cost, their use is limited due to the lower reliability of electronic components.
Rice. 1. Electrical circuits of residual current devices: a - two-pole RCD, b - four-pole RCD, I - differential current transformer, II - comparison unit, III - disconnection unit, 1-6 - phase conductors, N- neutral conductor, I d> - designation of the differential current comparison unit with setting
Rice. 2. RCD electrical circuits: a - with overcurrent protection (TP - thermal release, EMR - electromagnetic release), b - with an electronic comparison unit (II) powered by the mains, I - differential current transformer, II - comparison unit, III - shutdown block
3. According to the installation method:
RCDs used for fixed installation,
RCDs of a portable type, including those connected with a cord. This, for example, is an RCD type A plug, plugged into a socket with a grounding contact, having a "Test" button with rated currents: working - 16 A, differential - 30 mA.
4. According to the number of poles and current paths, the most common are:
two-pole RCDs with two protected poles,
four-pole RCDs with four protected poles.
A number of manufacturers also produce three-pole RCDs with overcurrent protection.
5. According to the conditions of regulation of the breaking differential current:
RCD with one value of rated residual breaking current,
RCD with several fixed values of residual operating current.
6. According to the operating conditions in the presence of a DC component:
RCDs of AC type, reacting to a sinusoidal alternating differential current, slowly increasing or occurring abruptly,
RCD type A, responding to both sinusoidal alternating differential current and pulsating direct differential current, slowly rising or occurring abruptly,
U30 type B, responding both to sinusoidal alternating differential current and to pulsating direct differential current, slowly rising or appearing abruptly, and also responding to direct current.
7. By the presence of a time delay:
RCD without time delay - type of general application,
RCD with time delay - type S (selective).
In branched power supply systems, RCDs are used with different values of rated differential currents and shutdown times. At the beginning of the network, a selective RCD (type S) is installed with a differential current of 300 or 500 mA. Selective RCDs for currents of 1000 and 1500 mA are also produced.
In order to exclude false alarms during short-term increases in leakage current, as well as to ensure earlier tripping of RCDs at subsequent levels of power supply, selective RCDs have a trip time of 130 - 500 ms.
Residual current devices with a residual current of 30 mA provide protection against electric shock, and selective RCDs with a current of 300 mA provide fire protection.
In the event of damage to the insulation and the flow of a differential current of 300 mA or more, the RCD of the lower level of protection with a current of 30 mA will first trip. The selective RCD, which has a longer trip time, will not work in this case and the power supply of undamaged electrical receivers will remain.
8. According to the method of protection from external influences:
RCDs of a protected design that do not require a protective sheath for their operation,
RCDs of unprotected design, for the operation of which a protective sheath is required.
9. According to the method of installation:
RCD surface mount,
flush mounted RCD,
UZO panel-panel board installation.
10. According to the instantaneous trip characteristic (for RCDs with built-in overcurrent protection):
RCD type B,
RCD type C,
RCD type D.
