According to the PUE, residential properties must be connected to the power supply network through a residual current device (RCD). Any electrician will tell you that this device must be selected very carefully. How exactly to do this and why it is so important - this is what we will talk about now.
Why should an RCD be of high quality and reliable?
Types of RCD
The high demands placed on the RCD will seem quite justified if you understand the purpose of this device. It has only one function: to turn off the power supply if there is a current leak of a certain (threshold) value. A current leak is not fraught with harmless economic damage, as it might seem at first glance, but with much more serious consequences - it always indicates that one of the following situations is occurring:
- a person or animal has been struck by an electric shock;
- due to insulation damage, contact has appeared between a current-carrying element and some grounded metal structure, which can lead to a fire;
- the grounded body of any device or equipment is energized, as a result of which the user who touches it runs the risk of receiving an electric shock.
Thus, no less than the user’s life depends on whether the RCD works at the right moment. Therefore, you should never skimp on the quality of this device.
What you need to know about RCDs
The main element of the RCD is a differential transformer consisting of three coils. The first is connected to the phase, the second to the neutral conductor. In this case, the currents flowing through these coils generate magnetic fields with oppositely directed lines of force.
If the currents in the phase and neutral conductors are equal, then the geometric sum of the field lines will be equal to zero, that is, they will simply destroy each other. If the currents differ, then a residual field will form in the device, which will excite a current in the 3rd coil, and it, in turn, will force the shutdown relay to operate.
Note. The current that causes the RCD to trip is called differential current; accordingly, the RCD is also called a residual current switch.
Thus, to put it in simple language, the RCD compares the currents at the input and output of the circuit connected to it, and if “debit and credit do not match,” it blocks the supply of electricity. Two important conclusions can be drawn from this:
- The RCD does not protect against overcurrents (short circuit) or overloads, since in such situations the currents at the input and output of the circuit remain equal (there is no leakage). Thus, this device cannot be considered as an alternative to a fuse or circuit breaker - at least one of these devices must be installed at the entrance to the apartment or house. Instead of separate RCDs and circuit breakers, you can use a so-called differential circuit breaker, in which both of these devices are combined.
- The RCD will not turn off if a person touches the live element and the neutral wire at the same time. In this case, there will be an electric shock, but there will be no leakage - all the current will remain in the circuit.
Therefore, even with an RCD, one must not lose vigilance: live parts must be protected by casings, potentially dangerous places must be fenced off and marked with warning symbols and inscriptions.
Device characteristics
The most important characteristic of an RCD is the differential current setting, that is, the minimum value of the leakage current at which the device will disconnect the circuit. It is most often displayed in milliamps (mA) and can be 6, 10, 30, 100, 300 and 500 mA. This parameter is also called the sensitivity of the RCD: the lower it is, the more sensitive the switch is.
Another important characteristic is the response time of the device, that is, the duration of the period between the appearance of a leak and the shutdown of the RCD. Obviously, this period should be as short as possible, but there are special RCDs that are triggered with a time delay. They will be discussed below.
The third parameter is the rated current of the RCD, that is, the maximum current that the device can withstand without failure.
Calculation of RCD parameters
When calculating RCD parameters, manufacturers take into account the following data:
- A current of 50 mA is considered dangerous to humans. Therefore, all RCDs designed for protection against electric shock have a differential current setting of no more than 30 mA. Switches with a higher setting are fire protection.
- The response time is ensured so that in the event of an electric shock, fibrillation of the heart muscle does not occur. A period of 20–40 ms is considered safe in this regard.
- Each current strength corresponds to its own heat generation power. For example, with a current leak of 500 mA, 100 W of heat is generated. Based on this, the differential settings. current of fire-fighting RCDs does not exceed 500 mA.
The lower the ignition temperature of the building material of the building, the lower the leakage current setting of the fire protection RCD should be.
Criterias of choice
Now let's see how to select an RCD depending on its operating conditions.
Rated current
The rated current of the RCD must be one step higher than the circuit breaker installed in front of it. Thus, after a 16 A machine it is necessary to install an RCD with a rated current of 25 A, and after a 40 A machine - with a rated current of 50 A.
It would be a mistake to install an RCD with the same rated current as the machine: the circuit breaker, although triggered quickly, is still not instantaneous. So in the event of an overload, during its operation time, a current higher than the rated current will flow through the RCD. This time may be quite enough for it to fail.
Differential current (leakage)
Selecting the differential setting current, you should first of all take into account the amount of rated current flowing in the circuit. The point here is this: if you install an overly sensitive RCD at a high current, frequent false alarms will occur. Acceptable values of the leakage current setting for rated currents of different values are shown in the table:
However, as mentioned above, protection against electric shock can only be provided by an RCD with a leakage current setting of up to 30 mA inclusive. More precisely, for dry rooms - 30 mA, for rooms with high humidity (this includes bathrooms) - 10 mA.
To be able to install such RCDs, a section of the network with a large rated current is divided into several subsections (all consumers are divided into several groups) and each of them is equipped with its own differential switch. Current with sufficient sensitivity.
Let us draw the reader's attention to one exception: in networks with a “TT” type grounding system, it is mandatory to install an RCD with a leakage current setting of 30 mA, regardless of the rated current.
Note. Imported RCDs with a leakage current setting of 6 mA are produced to US standards - according to the requirements of local standards, the sensitivity of an RCD that provides protection against electric shock must be within the range of 4 - 6 mA.
Please also note that there are RCD models with an adjustable differential setting. Current, and it can be adjusted both discretely and smoothly.
Device type
RCDs are divided into several types according to two criteria. The first sign is a type of leakage current:
The second sign is the response time. Conventional RCDs, as mentioned, turn off 20 - 40 ms after the leak appears. But there are varieties that operate with a time delay:
These “inhibited” switches are installed to protect the usual ones. The connection is organized according to a cascade scheme: an RCD with a delay is installed on a common line, then the line branches into several groups and a regular RCD is installed on each of them. If a failure occurs with one of the conventional devices and it does not react to the leak, after a split second the common RCD will trip.
RCD design
According to their design, differential switches. Currents are divided into two types:
- Electromechanical. They consist only of a differential transformer (see above) and a shutdown relay.
- Electronic. Additionally, they include an electronic amplifier, which amplifies the current arising in the third (control) coil of the differential transformer in the presence of a leak. By equipping the RCD with an amplifier, the manufacturer has the opportunity to use a less powerful differential. Transformers and electronic switches are more compact and less expensive than electromechanical ones.
It would seem that the choice should definitely be made in favor of an electronic RCD. But you should know that not all of them are reliable enough. The point is this: an amplifier, like any electrical device, needs power and in the absence of it the switch cannot work. The power, of course, is taken from the circuit being serviced, that is, the RCD amplifier is connected to it at the very beginning in parallel with other loads.
Let us now imagine that the neutral wire breaks somewhere above the RCD (most often, the conductor is disconnected from the neutral bus). The integrity of the phase was not violated, therefore, all current-carrying elements remained energized, but the circuit was open, which means that all consumers, including the RCD amplifier, are inoperative. That is, the user will not be able to turn on any electrical appliance.
But if he touches, for example, a bare wire or a housing that has suffered a breakdown, he will receive an electric shock.
By the way, the likelihood that the user will touch a current-carrying element is quite high: seeing that the device is not working, 9 out of 10 citizens will think that this is happening precisely because of the lack of voltage, and will lose their vigilance.
So, in such a situation, if a person is struck by electric current, the electromechanical RCD will work, but the electronic one will not, since its amplifier will be inoperative due to lack of power. To increase the reliability of electronic RCDs, they began to be equipped with an additional disconnecting mechanism that is triggered when the amplifier's supply circuit is opened - this is the model you need to look for.
At the same time, it makes sense to find an option that “can” automatically turn on when the power is restored, otherwise the RCD will have to be turned on manually every time after a power outage.
We should also talk about how to recognize which RCD you are holding in your hands. There is no direct indication of its type in the form of the inscription “electronic” or “electromechanical”, and even sellers often cannot clarify the situation. Here's what to do:
- First of all, look at the diagram of the device shown on it itself. Electronic RCDs must have an amplifier in its composition - some kind of symbol to which power is supplied. In the vast majority of cases, the amplifier is indicated by a triangle. Nothing like this will be shown on the electromechanical RCD diagram.
- If, due to lack of experience, you are unsure about the correct interpretation of the circuit, pass a current through one of the poles of the RCD, connecting it to the most ordinary battery. Before this, of course, you need to remember to turn the device to the “on” position. If it turns off, then you have an electromechanical model. If not, reverse the polarity of the battery to allow current to flow in the opposite direction. If this time the switch does not work, then it is definitely electronic.
If there is a permanent magnet, bring it to the front panel of the RCD and move it slightly. The electromechanical switch will turn off during these manipulations, but the electronic switch will not.
Manufacturers
Today, the highest quality products are offered by manufacturers from Europe and the USA. First of all, these are companies:
- "ABB" (Sweden + Switzerland);
- Legrand, Schneider Electric (France);
- Moeller (German company, but recently acquired by Americans);
- General Electric (USA).
Of course, RCDs from these manufacturers are relatively expensive.
Devices from Siemens (Germany) are somewhat cheaper, but in terms of quality they are slightly inferior to RCDs from leading companies.
Products from domestic manufacturers may differ greatly, since some factories are owned by foreign investors. For example, a plant that produces RCDs under the Kontaktor brand is owned by the French company Legrand. This determines both the quality - it is much higher than that of many other domestic brands, and the price - it is comparable to the cost of a European-made RCD.
The following settled in the middle class:
- Kursk plant "KEAZ";
- DEKraft company.
The latter is notable for the fact that there are almost no negative reviews of its products online.
But IEK devices, on the contrary, are criticized quite often. Users report that they hum even under moderate loads and have a flimsy body that is easily deformed when the clamping screws are tightened. However, IEK brand equipment is quite popular because it has a very attractive price.
Devices from the well-known Chinese manufacturer EKF Electrotechnica can be included in the same category as the products of the IEK brand, both in price and in quality. But at the same time, the “Chinese” have a fairly long warranty period, for example, for circuit breakers it is 5 years. For comparison, the warranty for similar products from the KEAZ plant is 2 years.
Let us remind the reader once again that the quality of the RCD is often a matter of life and death, so it is not worth purchasing switches from dubious and little-known manufacturers for the sake of saving money. We should also not forget that products from well-known brands are actively counterfeited, so it is better to purchase RCDs in large stores that work directly with the manufacturer, or from official dealers.
Rated voltage
When choosing an RCD, be sure to look at whether it is 1-phase or 3-phase. In the first case, the rated voltage of 230 V will be marked on the case, in the second - 400 V.
Installation method
It is useful to know that, along with stationary RCDs intended for installation in a switchboard on a DIN rail, portable RCDs are produced. They are similar to a regular extension cord - they are plugged into an outlet and at the same time they themselves have several outlets for connecting electrical appliances.
It should be taken into account that the cost of such switches is significantly higher than that of stationary models.
Functionality check
All modern RCDs have a button labeled “TEST” (test). When you press it, current is supplied to a special test wire, as a result of which the RCD, if it is operational, should turn off. But there are two important points to consider:
TEST button
- Turning off the RCD when you press the “TEST” button only indicates the integrity of the internal circuits, but this fact does not guarantee that the characteristics of the device (breaking leakage current and response time) comply with regulatory requirements. Therefore, do not lose your vigilance and if you purchase an RCD in a small store or market, ask to see the certificate.
- In the same way, the operation of a switch that has already been installed in place when this button is pressed does not mean that it is connected correctly. It is likely that when the “TEST” button is pressed, the device will turn off, and will ignore the real current leakage due to a connection error.
If you want to truly test the performance of the RCD, you need to invite a professional electrician and ask him to perform a current leak test. We specifically draw the reader’s attention to the fact that this operation should be performed by a specialist.
Features of operation: Once a month, it is recommended to check the RCD using the " buttonTEST".
Connecting RCDs and circuit breakers
If electrical consumers in a house or apartment are divided into several groups, each of which is protected by its own circuit breaker, then in order to save money, you can install one RCD for 2 - 3 such groups. Today, it is possible to organize a connection in this way in almost any household network: among modern RCDs with a differential setting. current 30 mA, there are models designed for fairly high rated currents - up to 100 A.
When selecting an RCD for a group of machines, you should take into account the rated current of not only the higher-level machine, but also the lower ones. Let's explain with examples.
Example 1
Scheme for example 1
Let us recall that in the general case it is recommended to install an RCD with a rated current that is one step higher than the rated current above the installed circuit breaker. But in this case, as you can see, each of the two RCDs does not exceed the rated current, but, on the contrary, is inferior to the input circuit breaker: its rated current is 50 A, while the rated current of each of the RCDs is only 40 A.
However, the diff switches current are reliably protected from overload: the total rated current of the machines connected to each of them is only 32 A (2x16 A), which is 20% less than the rated current of the RCD of 40 A.
Example 2
The following scheme is not so reliable:
Scheme for example 2
The rated current of the 1st RCD is 25 A and the input circuit breaker with a rated current of 40 A does not protect it. But there is no risk of overloading this device, since the current passing through it cannot exceed 22 A (6 and 16 A circuit breakers are connected to the RCD). But the 2nd RCD, designed for a rated current of 40 A, can burn out: it is not protected by the circuit breakers connected to it, since their total rated current is 58 A (3x16 + 10), and the input circuit breaker is protected, so to speak, end-to-end .
If there is an overload, then before the input circuit breaker is triggered, a current higher than the rated one will flow through RCD No. 2, as a result of which it may fail. It is recommended to either install an RCD with a higher rated current (the next level is 50 A), or protect it with an additional circuit breaker with a rated current one step lower (32 A).
Example 3
But this scheme is clearly incorrect:
Scheme for example 3
Both RCDs with a rated current of 40 A are not protected by either the upstream circuit breaker (50 A) or the downstream ones (total rated currents are 57 and 48 A).
The optimal option for connecting an RCD
If there are several RCDs with their own group of circuit breakers, then it is very important not to mix wires from different groups. It is better to provide each group with its own zero bus - when all consumers are connected to a common zero bus, false alarms of the RCD are possible. The connection with individual buses is shown in the following diagram. Also shown here is the connection of a selective RCD.
RCD connection diagram
Red indicates phase (L), blue indicates neutral conductor (N), yellow-green indicates ground (PE).
As you can see, a selective RCD with a leakage current setting of 300 mA (position 3) protects RCDs 7 and 14 with a leakage current setting of 30 mA and at the same time protects the lighting circuits (circuit breakers positions 5, 6, 12). Protect the lighting wiring of the RCD with the differential setting. a current of 30 mA does not make sense, since here the probability of electric shock is practically zero.
It is understood that the differential machine 13 serves a dedicated line intended for connecting, for example, a computer or a washing machine, so the neutral conductor from it is routed directly to the load, and not to the neutral bus.
Additional zero buses are indicated by pos. 11 and 18. Groups of sockets 2, 3, 4 are connected to the first and a wire is laid from it to RCD 7; to the second - groups of sockets 5, 6, 7, the bus itself is connected to RCD 14.
Note that this circuit has the same drawback as that shown in example No. 2: the rated current of the input circuit breaker (item 1) is the same as the RCD pos. 7 and 14 - 40 A, while the total rated current circuit breakers connected to each of these RCDs is 3x16 = 48 A. For greater reliability, an RCD designed for a higher rated current should be installed.
When connecting an RCD to a group of machines, identifying the location of the leak is quite simple. For example, the RCD, pos. 7. It is necessary to turn off the automatic machines pos. 8, 9 and 10, then turn on the RCD and turn on the mentioned machines one by one. As soon as the circuit breaker with a leak is turned on, the RCD will immediately turn off.
Whether it will be able to save your life in the event of an emergency depends on whether you have chosen and installed the RCD correctly. Therefore, this issue should be approached with all due care. The recommendations outlined in our article will help you avoid mistakes that could be fatal.
How many RCDs do you need? Today, the most common option for protection against current leakage to ground is the installation and connection of one (common) RCD for all electrical wiring. Indeed, the cost of repairs is not the least important, especially since with the correct choice of technical parameters of the device, the latter is quite capable of providing a sufficient level of protection.
In some cases - when the electrical wiring consists of one electrical group, connecting more than one RCD is simply not required. Installing several residual current devices may be justified if there is electrical wiring “divided” into several electrical groups and, of course, has its own certain advantages and disadvantages.
As written at the beginning, one device per apartment (private house) is quite enough. Speaking of safety, by installing several protective devices, you can implement more precise protection against current leakage, individually selecting protection devices with optimal operating parameters for specific electrical appliances.
Thus, installing a separate, more sensitive RCD with a leakage current rating of usually 10 mA, say, for a washing machine, will be justified if the input (general) protection device has a differential shutdown current of 100-300 mA. An introductory, serviceable 30 mA RCD, in the event of a malfunction in the insulation of a household electrical appliance and the occurrence of leakage currents, can operate quite quickly, ensuring the proper level of electrical safety.
Another thing is the ease of operation of electrical wiring with several protection devices; this is undoubtedly an obvious advantage. So, if the insulation of any household electrical appliance is broken, the RCD installed specifically to protect it will trip, de-energizing the dangerous area, and not the entire home.
The main disadvantage of using multiple protection devices is, of course, the higher cost of implementing such a protection option. If you choose it, you should definitely take into account the availability of free space for their installation in the distribution board.
In addition, we must not forget that the RCD is protection against earth leakage currents, and not against current overloads and short circuits.
Therefore, after it, a circuit breaker of the required rating must be included in the protected circuit, protecting not only the circuit, but also the protective shutdown device itself when overcurrents occur in this circuit - i.e., space in the panel must be provided for the placement of these circuit breakers.
If there is a shortage of free space in the panel - when the number of RCDs and circuit breakers to protect the planned electrical groups is too large, it is recommended to use differential circuit breakers that combine the functionality of RCDs and circuit breakers - protection against short circuits, long-term current overloads and current leaks to the ground.
Which circuits should be protected using separate residual current devices? These are, first of all, electrical groups that supply sockets in wet rooms: bathrooms, kitchens, balconies, etc. It makes sense to group sockets in living rooms in several units, for example, one RCD per room (2-5 sockets).
Do lighting circuits need to be protected from leakage currents? Many consider this completely unnecessary, citing the fact that in everyday life, users have practically no contact with lighting fixtures, controlling the light using switches or switches.
It’s hard to argue with this; indeed, our contact with lighting devices is limited, however, it still occurs; From time to time you have to replace burnt out light bulbs or wipe off dust from them.
In addition, manufacturers of all kinds of chandeliers and lamps (including those with bodies made entirely of non-conducting materials) provide for the connection of a grounding conductor. To protect lighting circuits, it is advisable to install one RCD with a leakage current rating of 30 mA that eliminates false alarms.
Assembling a panel for an apartment. How to assemble a shield. Almost a master class
What is an RCD - residual current device. Why do you need an RCD, how to install an RCD
Selecting an RCD (residual current device) is a non-trivial task that requires a person to immerse himself in some knowledge about electrical installation and the principles of operation of electrical equipment. Many, frightened by possible errors in calculations, shift this responsibility to a professional electrician, but such a service costs money. In addition, it is not always possible to be sure that a third party will make the optimal choice in terms of price and required characteristics. Therefore, it is useful to know how to choose an RCD for a house or apartment yourself, especially since anyone can cope with this.
Types of devices
The principle of operation of residual current devices fully corresponds to their definition - when a current leak or short circuit occurs in the network that is powered through this device, the circuit breaker immediately opens the circuit within itself, stopping the current supply. This allows you to avoid fires, electric shocks to people from the body of electrical equipment and other consequences of varying severity. The system works so quickly that a person who touches the live body will not even have time to take the blow (if the RCD is adjusted for instant response). Similar actions occur when there is a short circuit or a maximum increase in the temperature of the wiring.
RCD disassembled
Before choosing an RCD, you should know that these devices are divided into two categories according to the type of leakage current:
- AC are simple RCDs that respond to changes in parameters in the AC circuit section. Often used to protect low-power household appliances or a branch of an apartment's electrical network.
- A - more advanced devices that work with leakages of not only alternating current, but also rectified pulsating direct current. In modern apartments they are necessary to protect washing machines, electric stoves, boilers and other powerful appliances.
RCDs of the second type have a more complex design, therefore, they are more expensive to purchase. In addition to these two types, there are several less commonly used versions of devices:
- B – automatic machines for working with direct and alternating current, intended for use in production and industrial facilities;
- S – protective shutdown device with a specified time setting for shutdown. The main purpose is to prevent fire due to cable fire. For this reason, S RCDs are installed in the apartment's distribution panel to protect all wiring.
- G – automatic devices for fire protection of individual devices, which, as a rule, have a shorter response delay.
Selecting a device based on network parameters
This is perhaps the most important section in the article. He will help you choose the right protective equipment.
The selection of a residual current device must be carried out in accordance with the operating parameters of the network in which it will be built, for example, in terms of power. During repairs, this work falls on the electrician, who can correctly route the wiring and install individual branches for powerful electrical equipment. However, in a situation where the repair is carried out independently or the laying of a separate wiring line arises due to the purchase of a washing machine, dishwasher or boiler, you will have to select the RCD yourself.
The main operating parameters of the RCD are the rated residual current and the rated load current. The first value should be no higher than a third relative to the sum of the leakage currents of all devices connected and being connected during operation of the network of devices. This feature is due to the fact that the machine operates in a fairly wide range: 50-100% of the rated current. This is necessary for the reason that connecting to any network can “cut off” 17% (one third of 50%) of the differential current and the RCD will stop supplying.
If it is not possible to determine the amount of leakage currents, an approximate calculation is used in which the load leakage current is taken equal to 0.4 mA per 1A of power consumption, and the network leakage current is 10 μA per 1 m of phase conductor.
Calculation
An example of a calculation for an electric stove with a power consumption of 5 kW and a wire laying distance to the distribution panel of 11 meters. Based on the conditional data presented above, the estimated leakage current is 11 mA. The approximate consumption of an electric stove at full power is 22.7 A, and the estimated leakage current is 9.1 mA. The sum is correspondingly equal to 9.21 mA. To protect against leakage currents, in this case it is proposed to use a device with the closest nominal value in terms of differential. current, that is, a 30 mA RCD.
After this, you need to determine the rated current of the RCD. To do this, you need to take the maximum current consumption and select the appropriate protective device. In the example, this maximum is 22.7 A, which means it is necessary to take a 25A or 32A RCD. Thus, a disconnect device suitable for protecting the above electric stove must have a rating of 25A 30mA or 32A 30mA. The differential circuit breaker for RCD protection must have the appropriate parameters - 25A for the first and 25-32A for the second case.
Protection in apartments and houses
It should be said that the RCD and the machine must be selected correctly so that their operating parameters allow the current supply to be turned off at the right time. In situations where an automatic-RCD combination is installed to protect wiring from fire, devices with a very high leakage current rating are taken - from 500 mA or 300 mA. This reserve prevents constant false shutdowns, but has a certain feature.
The fact is that the current consumption of an incandescent light bulb with a power of 60 W is no more than 0.3A - the machine does not work, because this value is below the nominal value even in a situation where the current goes to the ground and not to the neutral core. It turns out that fire protection was done correctly, but contradicts the requirements for protecting people from electric shock.
Today, certain standards have been adopted on how to choose the right residual current device for an apartment or private house. To begin with, it must be said that nowadays for both cases it is recommended to install only AC type residual current devices that support the operation of electrical appliances with pulsating direct current.
So, most modern apartments have a single-phase electrical network with an alternating voltage of 220V. Therefore, for them it is correct to install an RCD and a circuit breaker with a rated current of 32 A. This indicator is optimal - the device does not trip too often due to overload, provides reliable protection of people from electric shock and does not allow the wires to catch fire. The machine must be shared if the total load does not exceed this figure (taking into account all possible connections). But separate devices are installed for washing machines, dishwashers and similar equipment.
Number of RCDs in the apartment
In an average one-room apartment with modern technical equipment (TV, microwave, computer, washing machine, iron), the following number of RCDs will be optimal:
- One device with a leakage current of 30 mA is in the kitchen.
- One device with a leakage current of 10 mA - per branch supplying the bathroom.
- One device with a leakage current of 30 mA - for the remaining rooms.
As a rule, private houses are now supplied with a three-phase main line with alternating current, so here a four-pole RCD and the same differential circuit breaker should be installed in the distribution panel. Due to the presence of a large number of high-power electrical appliances, in the distribution boards of private houses, not one automatic machine is installed, but several - along the line of lighting, sockets and powerful consumers. Often a cascade power supply circuit is obtained.
Under these conditions, it is recommended to install a disconnecting device with a leakage current of at least 100 mA in compatibility with a type S circuit breaker (selective shutdown delay). AC type RCDs with a rating of 30 mA in this circuit are suitable for powering individual rooms and groups of rooms.
Technical requirements and features of RCD selection
- To get rid of possible difficulties, it is advisable to choose a protective device that opens the circuit of not only the phase conductors, but also the neutral one. This is done so that you don’t have to think about overcurrent protection at “zero”.
- The working “zero”, located in a circuit protected by an RCD, should not come into contact with the protective “zero” or grounded elements - this always leads to a disconnection of the network.
- Regardless of the scope of application, the protective device must be designed to withstand possible overloads in the operating area. By default, this is ensured by a 30% margin when selecting nominal values. So, with a total leak of 20 A, it will be safer to install an RCD of 32 A rather than 25 A.
- The protective device must continue to operate during short-term voltage drops of up to 50% (no more than 5 seconds) of the nominal value. This is necessary for the operation of automatic machines with a delayed response.
- Due to the increased electrical hazard in bathrooms, kitchens, showers and other rooms with high humidity, it is required to install an RCD with a response current of 10 mA if they are connected to a separate line on the distribution panel. If the kitchen, corridor and bathroom are on the same branch of the electrical network, the rated current of the RCD should be standard - 30 mA.
- To correctly install the RCD while maintaining operability, pay attention to the technical features of the requirement. Many imported models, for example, exclude the possibility of connecting aluminum wires.
- In old houses, where dilapidated, often aluminum wiring is laid, with unreliable insulation, installing protection in a power panel makes little sense. Due to weak wiring with many damages, leaks will occur quite often and lead to regular network outages. In this case, to ensure the safety of equipment and people, it is necessary to use portable RCDs, similar to an adapter for a socket.
Having firmly decided to use an RCD to protect your family from electric current, and your home from fires, you need to correctly calculate the characteristic indicators of protection and consumption in order to select the appropriate rating.
Three-phase and single-phase RCD
First of all, you need to clearly understand and distinguish both the parameters of the protective device itself and the characteristics of the connected electricity consumers.
RCD parameters and examples
The RCD body indicates:
- Ismax – maximum short circuit current (SC) no more than 0.25 s., depends on the cross-section of the conductors and their length, approximately equal to the distance to the supply transformer substation. The closer it is, the greater Iкзmax will be. This parameter is indicated as a number surrounded by a frame;
Explanation: in practice they use: for private residential buildings Ikzmax = 4500A, for multi-apartment buildings Ikzmax = 6000A, for industrial installations Ikzmax = 10000A.
- Un – rated voltage, 220V for single-phase, 380V for three-phase network;
- In – rated (operating) current. This parameter is selected one value greater than that of the circuit breaker. That is, you need to first calculate the network load by summing up the currents consumed by all devices.
Explanation: if the input machine is regulated by technical conditions, then there is no need to count, just select the next value from the series: 16, 20, 25, 32, 40, 63, 80, 100.
For example, if the input is a 25A machine, then the RCD should be selected 32A;
- IΔn – differential leakage current, a distinctive parameter characteristic only of residual current devices and automatic circuit breakers (RCD+automatic). Has a number of values: 10, 30, 100, 300, 500 mA;
Examples:
The type of differential leakage current that triggers the device is indicated by letters or symbols:
RCD close-up. You can consider the parameters
Temperature. For conventional devices it is in the range of -5 +40°C, but in the post-Soviet space special devices have gained particular popularity: -25+40°C;
Electrical diagram. For a non-specialist, it can say little, but you need to pay attention to the presence of a triangle indicating an amplifier, which means that the RCD is an electronic type.
They are cheaper, but less reliable, especially in conditions of unstable network voltage - it powers the electrical amplification circuit, which is prone to failure under these conditions. If there is a break in the zero and a simultaneous leakage of phase voltage, this system will not work.
Description of RCD parameters on the body It is worth recalling once again that RCDs are used only in conjunction with circuit breakers.
Taking into account the characteristics described above, knowing the rating of your input circuit breaker, you can select an RCD for a country house or apartment, using only these data, without delving into the complexity of electrical calculations.
An example of choosing an RCD without calculation
Let's say there is an In=20A machine at the input. A suitable protective device rating would be 25A, type A (this requirement is often found on many household electrical appliances). For the input device IΔn=30 mA, for individual electrical appliances IΔn=10 mA. (in this case it is also necessary to install a circuit breaker in series, the In of which is selected according to the load).
The In value of the RCD should also be one value higher.
In order to select a suitable fire protection RCD for large branched networks, you first need to find out the total current consumption IΣ by all devices.
IΣ = IP1+ IP2+ IP3+…IPn
In the case of power calculations, IΣ can be calculated based on the formula:
where PΣ is the total power.
Then the total leakage current IΔΣ should be calculated. According to the requirement of PUE 7.1.83, if it is impossible to find out the leakage current IΔP for a particular electrical receiver, it is selected equal to 0.4 mA for each load Ampere, and for the conductor the value IΔL = 10 μA = 0.01 mA for each meter of length L of the phase wire is taken.
Having already calculated the value of IΣ, you can calculate IΔΣ =0.4* IΣ +0.01*L. Also, the above-mentioned clause of the PUE requires that the rated differential disconnection current of the device exceed three times the total leakage current.
The final calculation formula takes the form:
IΔn= 3*(0.4* IΣ +0.01*L)=3* IΔΣ
Specific example with calculation
We mean low temperatures (special temperature design, -25°C), lack of gas (heating and cooking only thanks to electrical appliances), the presence of refrigerators, washing machines, boilers, and various household equipment. We assume that calculations for individual user groups have already been made; it is necessary to calculate the general input protection device (type S).
You can find out the current consumption for each device from the passport of the electrical device and make calculations using a calculator. We accept the conditional calculated value IΣ = 52A. The closest value of the circuit breaker is 63A, respectively, In the RCD will be 80A. Using a ruler or tape measure, measure the length of the entire live cable, regardless of whether the load is connected to it.
Let us assume that the total length of the wires is 280 m. Substitute the data into the formula:
IΔn= 3*(0.4* IΣ +0.01*L)=3*(0.4* 52 +0.01*280)= 70.8 (mA).
The nearest value IΔn=100mA will be sufficient to provide reliable protection without false alarms.
Final RCD:
80A, type S, IΔn=100mA, t -25°C.
A modern electrical network includes an RCD. What kind of RCD is installed at the entrance to the apartment? What 2 parameters determine the choice? How many devices are enough?
TEST:
- Which electrical appliance do we protect from leakage current?
A) Air conditioner, power 1950 W
B) Boiler with a power of 4200 W
- What current should a differential relay be selected for?
- Which room is the consumer installed in?
A) Bedroom
- Trip current value?
Answer options.
- For the air conditioner, we select an RCD with a current of 3 A (2 is enough for the machine; we select one step higher) for placement in the Bedroom and a permissible leakage of 10 mA.
- In front of the boiler we install a 6 A differential relay (3 for the machine). Installation - in the Kitchen with a differential current of 6 mA.
- The air conditioner can be protected by a 6 A differential relay (2 A for an automatic) when installed in the Bedroom and a leakage value of 10 mA.
We select an RCD based on 2 main parameters.
A residual current device is a device that protects people and property from two dangers - fire and electric shock.
There are two types of residual current devices– differential automatic or differential relay. Both of these devices are very similar and interchangeable. In the future, we will consider a differential relay (together with a machine) as a protective device as the most economical and flexible option.
By what parameters do you choose the right RCD? Differential protection devices differ in operating current and leakage current (or differential current).
For complete leakage protection, a joint circuit breaker is required.
The first parameter of differential protection is operating current.
The nominal range of differential relays has the same range as that of circuit breakers ( machine guns) – first column in Table 1. The current is selected according to the load power and for a single-phase network is indicated in second table column. Differential relays are the most expensive devices in the electrical network and therefore the value for the machine is chosen smaller.
TABLE 1. Selection of power rating.
2 more nuances about the choice of the operating current of the machine and the RCD.
Fig.1. Class of a two-pole circuit breaker on the front side (in the red oval).
- The machines have an additional class marking in the form of a Latin letter (Figure 1). It means how quickly the machine will open the load circuit when consumption is exceeded. Passes through the machine when overloaded a lot of electricity. Excess current leads to failure of the difrel, despite the fact that the machine operates indefinitely with a slight overload.
- The wiring is calculated based on the load current consumed (Table 2).
Example 1: How to choose an RCD according to power for a water heater ( boiler) with a power of 2 kW? The core cross-section should be 1.5 sq.mm. We select the machine according to table 1 from the standard row in the larger direction - 2 A. The differential relay must have a rated operating current of 3 A.
How to choose 380V? The rating of the machine can be simply calculated as twice the load power. For our water heater – 2kW x 2 = 4 A.
| Core cross-section, sq. mm. | Current, A | power, kWt |
| 1,5 | 19 | 4,1 |
| 2,5 | 27 | 5,9 |
| 4 | 38 | 8,3 |
| 6 | 46 | 10,1 |
| 10 | 70 | 15,4 |
| 16 | 85 | 18,7 |
| 25 | 115 | 25,3 |
| 35 | 135 | 29,7 |
| 50 | 175 | 38,5 |
| 70 | 215 | 47,3 |
| 95 | 260 | 57,2 |
| 120 | 300 | 66,0 |
TABLE 2. Maximum current and power for cable cross-section.
- What to do if the consumption of several loads is very large? It is required to supplement 3 loads with a consumption of 16A each with a differential relay. The obvious way is to choose a 50 A rating ( rounding up from the value 3x16 = 48 A). Such differential relays are found, but more often they are made in China and are of poor quality. For a number of European companies, the maximum range is limited to 25-30A. Each load must be protected by a separate protective device.
The second parameter is the shutdown current.
The shutdown current is selected from the series in Table 3. Electrical receivers and wiring, due to imperfect insulation or internal structure, contain their own, natural leak. Before installing a protective shutdown, it is advisable to have it measured by a competent energy supply organization. The leakage current can be calculated based on the natural leakage of 0.4 mA per 1 A consumption for the load and 10 μA ( 0.01 mA) per 1 meter of wiring. According to the rules ( PUE) the nominal value should not be less than three times the natural value.
Let's continue with example 1. Let's take the consumption current equal to the value of the machine (in practice, the value is indicated in the device passport) - 2A, the wiring will be 20 m long. Based on these data:
Iest=2x0.4 + 20*0.01 = 0.8 + 0.2 = 1 mA. We select differential protection with a minimum value of 6 mA.
Let's look at typical schemes. Example 1. Which RCD should I install in my apartment?
Rice. 2. Typical apartment wiring. The grounding and zero lines are not shownOn Fig.2 After the meter, a differential protection is installed as fire protection. The lighting in each room is switched to a separate machine. Differential relays are installed for the most “demanding” rooms - children's rooms, living quarters and bathrooms ( SNIP 31-110-2003 clause 14.40). The kitchen electric stove is separated only automatically. This was chosen with the expectation that there are no moving parts in the electric stove and the risk of damage to the insulation in it is minimal. The lighting is also not protected ( recommendations PUE 7.1.79), because Current leaks for lamps are insignificant and do not pose a danger, and, if necessary, can be easily localized.
Calculation of RCD for an apartment. 1st differential relay.
Let's make a calculation for first circuit serving consumers connected through sockets. We will assume that the following electrical appliances are working simultaneously:
Refrigerator, with a power of 3500 W with a wiring length of 15 m
Air conditioner, 3450 W, length 10m
Living room sockets 2400 W, 20m
Children's sockets 1500 W, 15m
Other sockets 2500 W, 25 m.
We select the values of the automata. From Table 1 it follows that for the refrigerator and air conditioner you need to choose 6 A machines, for children - 2 A, for others - 3 A. Class of automatic machines is C, as the most common in household electrical wiring. Let's calculate the natural leakage for consumers - ( 6+2+3x2) x 0.4 = 14 x 0.4 = 5.6 mA. We select the operating current from Table 1 based on a value greater than the total load - 16A.
The total length of all wiring lines is 15 + 10 + 20 +15 + 25 = 85 meters, which corresponds to a current in 85 * 0.01 = 0.9 mA (round up).
The total leakage was 6.4 mA. We choose an RCD three times larger at 30 mA.
2nd difrel in the apartment.
Calculation for second The protection circuit is quite simple. Let's take into account that the washing machine has a power of no more than 2 kW. The leakage current will not exceed 1-2 mA ( taking into account the length of the wiring) and the room has a humid climate, passing sewer pipes, etc. For such a case, the shutdown value has the smallest rating - 6 mA.
Possible improvements for the apartment electrical network.
The considered scheme has disadvantages. The first circuit of the protective shutdown accounts for a large number of loads. A leak in one will de-energize all devices. The most powerful consumers (refrigerator, air conditioner and others) can be connected through portable protective devices. Such devices are plugged into a socket (Fig. 3).
Rice. 3. “Mobile” RCD. Example 2. Safety shutdown for a garage.
The garage is characterized by the presence of a lighting line and sockets. Electrical tools with a consumption of no more than 16A are connected to the sockets. The differential relay in this case has a rating of 25A ( Rice. 4).
Rice. 4. Safety shutdown in the garage is installed on sockets (according to the recommendation of PUE 7.1.71). A common circuit breaker at the entrance protects all garage wiring. Automatic machines (AV) for lighting are separate (zero is not additionally protected - appendix to SNIP 31-110-2003 A.1.4). 1 differential protection or more?
Video 1.
Video 1 discusses savings on leakage current protection. How relevant is it? 1 one at the entrance to an apartment (private house) often causes false (and other) alarms with a complete shutdown of all devices and lighting. Calling and waiting in the dark for electricians is quite an unpleasant experience.
Photo 1. False alarms are caused by an incorrectly selected protection trigger value. The longer the wiring, the more loads with the conversion of alternating voltage to direct voltage (TVs, computers, media set-top boxes, etc.), the more reasons for the operation of a single installed RCD. 
Photo 2. The obvious solution is to divide consumers into groups. The trip current for normal premises is 30 mA. In this case, the consumer is always switched off at half of this value - 15 mA. 
Photo 3. For electrical appliances for wet rooms - boilers, washing machines, a differential relay with a value of 10 mA (and lower) is installed. A small value of protection operation requires the installation of a relay for each consumer. Video 1 does not discuss the installation of fire protection. When several differential relays with the same leakage value are turned on, it is possible for the device to operate both on a separate load and on the entire group as a whole.
Answers to 5 frequently asked questions:
- Is it necessary to install an RCD in houses where protective grounding is installed?
When the phase conductor is shorted to ground, electricity will “flow” bypassing the neutral wire. In this case, the metal casings of electrical appliances may become energized and will not cause danger as long as the human resistance is much higher than that of the “ground”. However, such a device will be fraught with danger and if there is no significant overload, the circuit breaker will not work. Uncontrolled energy leakage poses a risk of heating and fire. The use of a differential relay will protect against these cases.
- Is it possible to install one automatic circuit breaker after the meter instead of a machine before the meter and an RCD after it?
Yes. It is also possible to install a circuit breaker after the meter if it is necessary to install a disconnector (switch, etc. - PUE requirements) before the meter.
- There are two differential relays connected in series in the circuit. When the protection closest to the consumer is triggered, the entire group of loads is switched off. Which protection device should be removed from the wiring?
It is enough to use a differential relay with selectivity (often marked with the letter S) for a group of consumers. The sequential activation of differential relays is regulated by the requirements of PUE 7.1.73.
- Is it necessary to install an RCD at the input for a wooden private house? After all, the house is “isolated” enough?
Private houses often have input from an overhead line through a long cable. When the consumption value is small, protective shutdown does not occur automatically and quick shutdown is not provided. In addition, according to Technical Circular No. 31.2012, the installation of differential protection is a mandatory fire safety measure.
- Will the differential relay protect in case of a break in the neutral wire?
In the absence of leakage current, the protection will not operate, but when it appears, a shutdown will occur. Thus, even if the zero burns out, it will protect against leakage. The differential relay does not protect against the very fact of a break in the neutral wire. This remark is true for electromechanical protection; for electronic protection, which requires both zero and phase for normal operation, a dangerous condition appears when the protected load can lead to an electric shock. Therefore, the use of the electronic type is not recommended.
