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 the differential relay has the same range as circuit breakers th ( 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 big side – 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 tripping 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 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 apply electronic type Not recommended.
Despite all the advantages that electrical energy has, it also has disadvantages. The main one is the danger of electric shock. To protect people from the effects of electricity, many devices have been created, and one of them is called RCD - Residual Current Device. But to organize effective protection using these devices, you need to have a good understanding of how to choose an RCD and how to connect it.
Purpose and design of RCD
The device is an automatic device that turns off the voltage when a differential current (differential current, leakage current) occurs. It is precisely this, which occurs during one or another malfunction of electrical equipment, that in most cases is a consequence of accidents. The worst thing is that the fault causing the leak does not manifest itself in any way. Washing machine washes, the computer calculates the task, the boiler diligently heats the water. But as soon as a person touches the casing of such a device or gets under the shower, something irreparable will happen.
The task of the differential switch is precisely to detect leaks and emergency shutdown of faulty equipment when it appears. How does he do this?
Electrical equipment, as you know, receives energy through wires. Household appliances use two conductors - phase and neutral. If the equipment is industrial or simply powerful, then it can use three phases, but this does not change the essence of the problem. So, roughly speaking, the current runs through the phase(s), powers electrical equipment and runs through zero*. In this case, the incoming and outgoing current values are always equal - as much energy entered the device through the phase wire, the same amount came out through the zero wire.
*Since the voltage in the network is variable, the direction of the current changes 100 times per second, but this is not significant for understanding the basic principle.
Suppose an emergency situation occurs - the seal in the washing machine leaked and the control board flooded, water got inside the electric meat grinder, the wire insulation frayed due to vibration, or a thermal breakdown of the electric motor occurred. For this reason, tension appeared on the metal parts of the unit. If the device is grounded, this voltage from the chassis or casing flows into the ground loop, causing a leak. There will be no leakage in ungrounded equipment, but it will appear if a person standing on a conductive surface (concrete floor, metal or wooden, but wet flooring, etc.) touches the casing.
The occurrence of diphthk through the ground loop or the human body
In this situation, the current entering through the phase conductor will no longer be equal to the current exiting through the zero one. The value of the leakage flowing into the ground will be added to it: Iphase = Izero + Ileakage.
The task of the RCD is precisely to determine this imbalance. During operation, the device constantly compares the value of the currents passing through the phase and zero. As long as they are equal, everything is fine. As soon as the phase current exceeds zero, the device will emergency turn off the voltage.
Contrary to popular belief, the occurrence of leakage current is not that uncommon. Almost every housewife has encountered a situation where an electrical appliance begins to “pinch.”
Characteristics of the residual current device
Your health and, possibly, your life will depend on how correctly you choose a protection device. That is why this issue must be approached with full responsibility. What are the characteristics of RCDs?, which have another name - differential switches (not to be confused with automatic machines)?
- Differential current. Main parameter of the device. Essentially, this is the sensitivity of the protection system. For example, the differential switch shown below will trip if there is a leak of 30 mA (position 3 in the photo).
- Operating current or power. Current load in amperes that the device can withstand for a long time without damage or overheating (position 1).
- Operating voltage. The maximum voltage of the network into which the RCD will be built (position 2).
- Type of tension. The type of network voltage into which the protection device will be built. Can be variable, constant or variable + constant (position 5).
How to choose an RCD
Obviously, each of the above parameters is equally important when choosing an RCD. A device with low operating voltage and power will simply burn out, and the wrong choice of current value and voltage type will make the device absolutely useless - it will either trigger falsely or not work at all.
Selection by diftok
This is the main and most important selection criterion. Let's see, how to calculate an RCD for a specific object. According to the PEU, the permissible leakage value in electrical installations is taken at the rate of 0.4 mA per 1 A of load current. Additionally, you need to know the length of the electrical circuit and add another 0.01 mA to the result obtained for each meter of the supply wire. Agree, such a calculation of an RCD for, say, house wiring is quite complex and labor-intensive. But you can do everything much simpler, and the level of protection will not suffer.
If the device is planned to be installed not for fire safety purposes, but to protect people (usually this is the main task in home networks), then it should confidently respond to a leak of no more than 50-80 mA. This value is considered the maximum permissible for the human body. On the other hand, if the home network is very long and branched, and even with damp rooms (for example, a bathroom), then the amount of natural leakage distributed along the entire line may be higher than the same 50 mA.
How can we be here? Install a more powerful device, as some “experts” recommend? In no case! After all, if, God forbid, you get under voltage, powerful automation will not be able to save you - it will either work after the deadly current has passed through the body, or will not turn off at all. In any case, the person himself will no longer care.
The way out of the situation is not to choose a rougher RCD, but to install several sensitive devices, each of which will monitor a separate section of the circuit. For example, one device for the bathroom and kitchen, another for sockets, and a third for lighting.
Power and voltage
Using these criteria, choosing an RCD is much easier. You know the voltage in the networks: for a single-phase line it is 220 V, for a three-phase line it is 380 or 660 V. The type of voltage in both cases is variable. If your network is single-phase, then you can choose a single-phase device. If there are three phases, then a three-phase differential switch will be needed.
Single-phase and three-phase differential switches
Now about how to choose an RCD and a machine based on power. Why automatic? The fact is that the differential switch does not operate due to overload or short circuit, but reacts only to differential current. If a short circuit occurs in the house, the difavtomat will safely burn out from overload along with the wiring. Therefore, the installation of an RCD in conjunction with a machine is mandatory.
As for the operating current of the differential switch, it must be no less than that for which the input circuit breaker is designed. If you already have a circuit breaker, just look at what current it is rated for. If it’s not worth it, then you’ll have to install it. For an ordinary apartment without particularly powerful consumers, an automatic circuit breaker with a power of up to 32 A, which can withstand a load of 7 kW, is usually sufficient. It should be noted here that the voltage and rated current indicated on the body of the switch and circuit breaker may be more than necessary, but never less.
In order for the differential switch to operate reliably and for a long time, it must be installed correctly in the house circuit. Here are a few mandatory conditions that must be performed when installing an RCD:
Option for connecting an RCD to a home network
Checking the correct connection
After the protection device is installed, it is necessary to check the correct operation of it, so as not to find out about problems in the connection after the fact - once under voltage. The test is carried out by two methods - a test button and the artificial creation of a leakage current. The first one is extremely simple - press the button located on the RCD body and labeled accordingly. The circuit built into the device will simulate a leakage current, and the protection will operate, de-energizing the line.
If you don’t trust this button (you never know what it imitates), then you can check the device by artificially creating a leakage current. Connect some electrical device between the phase contact of the socket and its grounding terminal - table lamp, soldering iron, etc.
Diagram for checking the correct operation of the differential switch
The RCD will immediately notice the leak and immediately disconnect the circuit. Everything is fine. If your sockets are not connected to a ground loop, which is very, very bad, then you can use a half-meter pin stuck into the ground as a ground contact. This, of course, is not a full grounding, but the current through it is more than enough for the RCD to detect a leak.
Difavtomat as a double protection option
There is another device that can be triggered by leakage current. It's called a differential automaton. Its difference from a conventional RCD is that the device additionally includes an automatic switch that responds to a short circuit. Installing a difautomat makes sense if you are doing the installation from scratch or your wiring does not have an automaton at all. When buying a difavtomat, you get two devices in one case - an RCD and an automatic machine. It is both cheaper and easier to install.
The difavtomat is similar in appearance to a residual current device, but has a built-in circuit breaker
Choice differential machine produced according to the following criteria:
- Differential current.
- Rated current.
- Operating voltage.
- Type of current.
- Cut-off current.
Almost this entire list is probably familiar to you - you used it to choose an RCD. It is also suitable for choosing a automatic machine. The only additional criterion is the cutoff current - the current at which the automatic device built into the device is triggered. Usually it is equal to triple the operating current, so if you have chosen the operating current correctly, then the cutoff current will also be optimal.
Content:One of the devices of great importance in electrical engineering is the residual current device. Its main purpose is to disconnect the entire electrical network or its individual section from power by opening the contacts. This ensures protection against and prevention of fires. In modern electrical engineering, the use of these devices in many cases becomes mandatory, therefore, the question often arises of how to choose the right RCD. These protective devices are used not only in single-phase, but also in three-phase networks under various loads, therefore, their selection is made depending on the specific operating conditions.
Purpose of RCD and principle of operation
The main task of the RCD is to neutralize currents when various damage occurs in electrical installations. The residual current device is the most effective protective device. Unlike fuses or circuit breakers, RCDs are capable of breaking a circuit in a split second and saving a human life.
The danger is not only the possibility of direct electric shock. Sometimes simply touching the parts of live devices and devices is enough. Therefore, protective devices must operate in a timely manner. In order to correctly solve the problem of how to choose an RCD for a home, the conditions in which it will operate must be taken into account.
The operation of protective devices uses the phenomenon of electromagnetism. In this regard, the design of the RCD includes coils with a magnetic core connected to current-carrying wires that transmit electricity to the consumer. At the same time, a magnetic flux occurs, which is the arithmetic sum of the currents flowing through these conductors. In this case, the incoming currents have a positive value, and the outgoing currents have a negative value. In the absence of leaks and short circuits, they will be equal and add up to zero. This state of the circuit indicates the serviceability of the installed equipment.
In the event of a leak, a partial reverse flow of current occurs through the grounding conductors, which leads to an imbalance. The difference in differential currents causes the excitation of magnetic flux in the core. Its value will be proportional to the difference in electric current. When a certain threshold is reached, the device is triggered and turns off the power supply to consumers.
How to choose the right RCD
In order to select the best option for a residual current device, you need to know its basic parameters. Devices with different characteristics used in specific conditions that must be taken into account when choosing. The nature of leakage currents allows us to divide them into different types. This division depends on a smooth or sudden increase in current. RCDs with such characteristics are most widespread as they are most suitable for the widest operating conditions.
Triggering technology allows RCDs to be divided into electromechanical and electronic. In the first case, high-precision mechanisms are triggered as a result of leakage currents. These are the most reliable and expensive devices capable of operating under any conditions. Electronic devices are cheaper, however, for normal operation of the electronics, the use of external power is required. Their efficiency is significantly reduced when voltage surges occur. The response speed of RCDs allows their use in multi-level protection systems. This allows you to turn off all emergency sections individually.
There are other parameters that require knowledge of electrical engineering. Therefore, when choosing an RCD, it is best to seek help from qualified specialists. However, if the exact characteristics of the electrical network are known in advance, you can independently select the most suitable protective device. Among them, the most important are the following:
- Voltage. The RCD can be designed for a single-phase network with a voltage of 220 V or a three-phase network with a voltage of 380 V. The first option is usually used in apartments, and the second in private houses, dachas and cottages. If there are sections with one phase in three-phase wiring, then protective devices designed for 220 volts are used for them.
- Number of poles. In single-phase networks, two-pole RCDs are used, designed for one phase and zero, and in three-phase networks, four-pole devices are used, to which three phases and zero are connected.
- Rated current. This is also the throughput current of the RCD, which depends on the number and power of connected electrical appliances and equipment. Therefore, this indicator for a general (input) protective device must be calculated for all installed consumers. For linear RCDs, the total power is calculated based on the number of devices on a particular line. The RCD ratings set by the manufacturers are 16, 20, 25, 32, 40, 63, 80, 100 A.
- RCD leakage current. The value upon reaching which it turns off. It also varies in ratings of 10, 30, 100, 300 and 500 mA. For ordinary apartments, a 30 mA device is best suited. With a lower current rating, the device will constantly respond to even minor fluctuations in the network and turn off the power.
- Type of leakage current. The symbols AC, A, B, S and G are marked on the body of the device. For example, AC reacts only to alternating leakage current, and B - to direct and alternating currents. The rest of the markings also correspond to certain parameters, including the device shutdown time delay.
What types of RCDs are there?
The main classification of residual current devices is based on their operating current. For example, devices respond to currents of 100, 300 and 500 mA. They protect wiring from fire in the event of insulation failure and short circuit. Typically, an introductory RCD is installed behind the electric meter and provides protection for the entire facility. For people electricity becomes dangerous at 50 mA. Therefore, devices that protect against fire are not able to protect a person from electric shock. For these purposes, devices are used that turn off the network when the current reaches a value of 10 or 30 mA.
Protective devices differ in the number of poles and can be used in single- or three-phase networks. Each type of device has a different way of functioning. The markings on the body of the device must be correctly deciphered and you need to know exactly what it means:
- AC - RCD category used only in networks alternating current. Accordingly, the device responds only to alternating current.
- A - protective devices of this category operate not only with alternating current, but also with direct current.
- B - has more advanced functions and responds to three types of current. In addition to direct and alternating current, the device turns off with rectified differential current.
- S - devices with the ability to delay time when switched off.
- G - are also selective devices, but with a shorter time delay.
RCDs are also classified according to their technical design. This allows you to select a RCD of better quality. Most often, electromechanical devices are used that do not have their own power supply. They operate and shut down when a differential current appears.
Another type refers to electronic protective devices that require connection to an external power source. In this regard, the reliability of protection decreases, so such RCDs are used less frequently. When the auxiliary power is turned off, they turn off the network automatically, and when the power is restored, the network also turns on automatically. Separate device designs do not provide automatic switching on circuit when power is restored.
How to choose an RCD based on power
Unlike circuit breakers that protect against overloads and short circuits, residual current devices are designed to protect against current leakage. The reason is faulty insulation of electrical appliances or contact of live parts with the housing. In these cases, the RCD is instantly turned off, the line is de-energized, and consumers are protected from electric shock.
In order to calculate the RCD by power, you need to know the total number of consumers connected to a given line. In the case when the question of how to select an RCD and a machine based on power is being decided, both protective devices must have corresponding values ensuring their normal operation. If the installation of circuit breakers is not provided for by the project, in this case the total power consumed by electrical appliances is calculated. As a rule, this value in a standard apartment in a multi-storey building does not exceed 25A.
When installing RCDs in private houses, it is recommended to divide all consumers into groups connected to separate lines extended to each floor, outbuildings, external lighting, etc. If the RCD has less power than the existing consumers, it will constantly turn off due to overloads. That is, the device will actually not work normally and will not be able to protect the line. Designed for a current consumption of 5A will help partially solve this problem.
How to calculate RCD
In order to calculate the protective device and solve the problem of how to choose an RCD based on power, the table of parameters will help you do this as quickly and accurately as possible. You need to use two technical characteristics- leakage current and maximum current to obtain the desired result. The calculations use a mains voltage of 220 V, with a frequency of 50 Hz.
Calculation and selection of the maximum current rating of the RCD is quite simple. It is necessary to establish the value of the total electrical power of devices and equipment switched on simultaneously. For example, if this indicator is 6000 watts, then the calculated current value will be equal to: I = P/U. Substituting the required values into the formula, we get the result: 6000W/220V = 27A. The closest RCD from the standard range of rated currents will be 32A.
If the RCD is calculated based on leakage current, in this case a simplified scheme is used, according to which Various types protective devices are selected in accordance with the operating conditions of the objects:
- In ordinary residential premises - at 30mA.
- In bathrooms, kitchens and other rooms with high humidity and higher requirements for electrical safety - at 10 mA.
- At large facilities with electrical networks with a length of over 1000 m or at the input - 100 mA.
Quite often there is a need to select an RCD for a group of machines, the calculation of which is carried out according to certain rules. The installation of these devices in the circuit is carried out sequentially; the machines can be installed both before and after the RCD. The current values of the circuit breakers must be lower than in the RCD, but not less than the actual current consumption. Correct calculation of RCDs and circuit breakers shows that in the event of overloads and short circuits, the circuit breaker will protect not only the line itself, but also the residual current device installed on it.
RCD current ratings
The rated currents of the main types of RCDs are 16, 25, 40 and 63A. This value corresponds to the current value that the device can pass through itself without time limits. Within this line, a selection of RCDs is made for the electrical panel of an apartment or private house.
The rated current value is necessary when deciding how to calculate an RCD for a group of machines. In this case, for protection you need to select a circuit breaker with a rated current less than or equal to the rated current of the differential switch. Experts recommend choosing a rating one step higher than that of the machine, since it can pass through itself a current greater than the rated one for a long time. If the currents are equal, during this period the RCD may simply burn out.
Which RCD should be installed at the entrance to the apartment?
In modern high-rise buildings it is prohibited to use three-phase wiring, so many owners are wondering how to choose an RCD for an apartment. Meanwhile, there is nothing complicated here, since for single-phase wiring a two-pole device marked AC is used. Based on the leakage current, the RCD for the apartment is selected at the rate of 30 mA. Devices with a lower shutdown threshold may cause false alarms.
How many RCDs should I install in an apartment? It all depends on the total power of consumers. If it is too big, it is recommended to break it home network on separate lines and install a protective device on each of them. Additionally, a general RCD is installed at the entrance to the apartment to protect against fire if the insulation is damaged.
Often false alarms occur due to old electrical wiring. If these processes occur systematically, it may require complete replacement.
RCD at the entrance to the house
Unlike a standard apartment, a private house has an individual layout with a different number of rooms. Therefore, the question often arises, what kind of RCD to install in a private house? At such facilities, not only single-phase, but also three-phase electrical networks with a voltage of 220 or 380 volts can be used. Therefore, in the first case, the same RCDs are used as in apartments, and in the second, four-pole ones are used, where terminals are provided for three phases and a neutral wire.
In addition, the choice of RCD for a private house is carried out according to the type of current. However, it should be borne in mind that in private homes, powerful electric motors are often started, consuming a powerful starting current for a short time. It is recommended to determine in advance which RCD and then choose required device, maintaining performance under these conditions.
The question of how to choose an RCD for wooden house to protect not only from leakage currents, but also from fires. For this purpose, a multi-stage protection system is used, in which powerful devices prevent fires, and devices with a lower response threshold protect against current leaks. However, you should not install an RCD with a minimum cut-off current of 10A, especially if the power lines are long. A sensitive device will react to the slightest changes and cause false alarms.
Electricity in the modern world is developing by leaps and bounds. In many apartments and residential buildings you can find elements of automation, which until recently were found only in industrial facilities. For example: voltage control relay, SPD, phase control relay. Still, a widely used electrical protection device, is an RCD. Today we will analyze the algorithm of actions and complete a mission called “How to choose an RCD.”
RCD- a device that analyzes the supply circuit for current leakage. Having detected a malfunction (losses higher than the rated value of the device), the device turns off the electrical circuit, ensuring the safety of consumers.
To explain it more simply, then this device compares current value, in two wires connected to the load. In the “ideal version”, it will be the same, and its difference should tend to zero if the wiring is working properly.
In reality, losses occur under normal and abnormal conditions. In normal situations, losses are minimal.
How to choose an RCD. Criteria.
It is important to choose a protective device wisely. Focus on the main criteria:
- Differential leakage current. Represents a key parameter of the device. Sets the permissible leakage limit.
- Rated operating current. This characteristic reflects the amount of current that the protection device can withstand while performing its tasks in full;
- Device voltage. Most often 230 or 400 V;
- Type of RCD for monitoring the type of leakage.
- Conditional rated short circuit current.
In addition to these basic parameters, there are also additional ones, but we will talk about them in one of our next articles.
How to choose an RCD based on differential leakage current.
As you learned from the material above, there are regulated and non-standardized leaks. Calculation of standardized leaks is carried out according to
clause 7.1.83 (PUE 7) with unknown data:
Analyzing the above, a correctly selected device does not respond to standardized, But must shut down the network as quickly as possible in case of emergency leaks.
To manufacturing plants according to GOST R 50807 clause 5.4., recommended: if the leakage level is less than half the device rating, the device should not disconnect the circuit. That is, a device with a leakage rating of 30 mA, with losses of less than 15 mA, should not operate. Shutdown is permissible in the range of 15-30 mA - this point should also be taken into account.
According to the same clause 7.1.83 (PUE 7), The differential current of the protection device must be 3 times higher than the calculated normalized losses in the network.
Selection of RCD based on operating current.
It's important to remember that The device is not protected from short circuits. Therefore, the device of this type necessary additionally protect with a standard machine gun. It is worth considering the following:
The operating current of the RCD is selected at least one or two steps higher than the rating of the protective device (automatic machine) protecting this device.
This selection criterion is due to some machines that allow overloads from 13 to 45% in the network in a time interval from a couple of minutes to an hour.
Selection of RCD by voltage.
If the network is single-phase (230 V), then we install a two-pole device, if it is three-phase (400 V), then we install a four-pole device. But it's worth remembering that There are two types of residual current devices:
- Electromechanical (works on a certain amount of leakage, operation does not depend on the presence of voltage in the circuit);
- electronic (they work for a certain amount of leakage if there is voltage in the circuit. If the circuit is without voltage, then the device will not work).
Based on the above, we recommend purchasing electromechanical devices. This is especially true if there are power surges in your electrical network.
Selection of RCD according to the type of differential leakage current.
Divided into three main classes (types):
- AC class— the most common devices operate with variable leakage;
- class A- devices that operate in case of losses of alternating and direct current. The cost of these devices is higher than AC type devices, but the level of security is also higher;
- class B— work on losses of alternating, direct and rectified current. Such devices are not used in residential buildings.
Selecting an RCD based on its ability to withstand UTKZ.
The magnitude of the short circuit is quite large, and the circuit breaker is not capable of switching off the circuit at lightning speed when a short circuit occurs. Therefore, an important parameter of an RCD is its ability to withstand short-circuit current (SC) over a short period of time.
They produce devices with the following parameters of the rated conditional short circuit current (TCSC) Inc:
- 3000 Amps (3.5kA);
- 4500 Amps (4.5kA);
- 6000 Amps (6 kA);
- 10000 Amperes (10 kA).
For household networks quite enough devices that withstand the nominal UTKZ Inc from 4.5 kA to 6 kA.
How to choose an RCD based on response time.
For correct operation of automation, selectivity of protection devices is required. Those. so that in an emergency situation it is the damaged section of the circuit that is turned off, and not the entire circuit. In the case of RCDs, selectivity is usually achieved by choosing devices with different differential current ratings. In addition, it is advisable to use devices with separation by response time, for this You can use devices of the following classes (types):
- Class AC, A, B— there is no time delay. Response time 0.02-0.03 seconds;
- class G— delay when triggered from 0.06 to 0.08 seconds;
- class S— delay when triggered from 0.15 to 0.5 seconds;
How to choose an RCD. Selection example.
So, the task is to select a residual current device and select protection for it for the bathroom. Additional Information The bathroom has an electronically controlled washing machine, an exhaust fan and two sockets for other loads. Initial data:
- The total rated current (I total rated) of installed appliances in the bathroom is 10 A;
- The total length (L) of the cables supplying the lines from the RCD to the consumers is 40 m, made with VVGng cable 3×1.5 mm2;
- There are rare power surges in the house.
Step 1. Calculation of normalized losses and selection of the rated leakage current of the device.
Let's do the calculation:
I standard specification = I sum. nom. x 0.4 + L x 0.01;
I standard value = 10 x 0.4 + 40 x 0.01 = 4.4 mA;
I nom.nom.nom.= I nom.nom.nom. x 3;
I nom.nom.= 4.4 x 3 =13.2 mA.
Next, you need to analyze the calculated data. According to calculations, the normalized loss in the network is 4.4 milliamps. If we follow the guidelines in the regulations, we must use a device with a nominal leak size, at least three times higher than the rated leakage current, that is, at least 13.2 mA. The closest device (greater than 13.2 mA) on sale is a 30 mA device. However, in such borderline situations, when the calculated result is very close to the lower limit and the choice is to install the device at 10 mA or 30 mA, I would still recommend that you use a 10 mA device. Yes, perhaps this is a little incorrect in relation to regulatory documents, but it is much more logical from a security point of view. Select a 10 mA RCD.
Step 2. Select device type.
When analyzing the network parameters, it is better to choose an electromechanical type device, due to the fact that voltage surges are possible in the network. Additionally, increased humidity may occur in the room. This factor increases the risk of electric shock, therefore, it is necessary to choose a device with the maximum degree of reliability, that is electromechanical type.
Step 3. Selecting the rating of the circuit breaker. Selecting the current rating of the RCD.
We know that the line is made with VVGng 3×1.5 cable. The limit that this cable can withstand without overheating is about 23 A. It is also known that circuit breakers have certain performance characteristics. These characteristics allow for a certain level of network congestion. In addition, the total rated current load in the room is known - 10 A. Based on the known data:
- We protect cable networks and RCDs with a single-pole or double-pole circuit breaker of 13 A - C 13. This machine completely protects the circuit. At maximum overload, the maximum current in the circuit that the machine will allow is 18.85 A. This is below 23 A, therefore, the cable will not heat up and will last a long time. If the current is above 18.85 A, the circuit breaker will turn off the network. In extreme cases, you can use a C 16 circuit breaker, but in case of large overloads, this circuit breaker can allow slight heating of the cable.
- We select an RCD at least one rating higher, that is, in our case, it is advisable to select a rating of at least 20 A.
Final choice of residual current device.
We choose an RCD of electromechanical type. The rated current of the device is at least 20 A. The rated differential current is 10 mA. The device class is A. This is due to the fact that the washing machine has electronic components powered by direct voltage. In an emergency situation, the device must work as quickly as possible, since the room has high humidity. Therefore, we don’t even consider devices with a time delay. The nominal UTKZ that the device can withstand is 6 kA. For protection we use a single-pole or double-pole circuit breaker C13. As a last resort, C16.
That's all, I hope the article was useful. In one of our next articles, we will look at connection diagrams of this device. Therefore, subscribe to the blog, join our social media so as not to miss new interesting articles.
If your apartment or house has a large number household appliances, then it is advisable to take care of additional protection. This is due to the standards and possible damage during operation of the conductor insulation. After all, if a damaged wire gets on the body of the device, you touch it, this can have serious consequences. A circuit breaker installed in the electrical panel of an apartment or house only protects the circuit from short circuits and high current. To protect against current leakage, it works in tandem with it. As an alternative to protective shutdown, it is possible to use difavtomats (differential protection circuit breakers), in the housing of which both fuses are located, but this is a separate topic. In this review we will “focus” on how to correctly connect RCDs and automatic machines in an apartment or private house. At the same time, we will consider the electrical parameters of the wiring and the total current strength of household appliances, basic diagrams, supplemented by video reviews.
This topic is very relevant and difficult for unprepared readers to understand. Therefore, we will try to decompose all useful information step by step, concretize and build a kind of logical chain.
When electrifying an apartment or private house, three stages can be roughly distinguished:
- Electrical supply to the distribution board.
- Installation and completion of an electrical panel.
- Final wiring from the switchboard.
All these stages are interconnected. After all, without knowing the specific final parameters, it will be impossible to complete the electrical panel and select required device protection. Therefore, before learning how to properly connect RCDs and automatic machines, let’s consider specific example houses or apartments.
Selection of automatic fuses
Let's take the layout of a private house as a starting point. In apartments, especially those built recently, the issue of connecting residual current devices is not so acute, and everything in the switchboard is equipped according to the design. But with a private house, things are a little different - the design and layout falls on our shoulders (with the involvement of specialists).
For clarity, consider the following layout of a private house(program used):
Having considered the house project, we can highlight the following rooms:
- Living room (1st floor).
- Room (1st floor).
- Kitchen (1st floor).
- Corridor (1st floor).
- Bathroom (1st floor).
- Three rooms (2nd floor).
- Bathroom (2nd floor).
Based on this, we will form certain consumer groups:
Sockets 1st floor:
- Living room
- Room— automatic switch C 16, wire cross-section (copper) 3 × 2.5 mm², estimated power consumption 1600 W.
- Corridor + bathroom— automatic switch C 16, wire cross-section (copper) 3 × 2.5 mm², estimated power consumption 1600 W.
- Kitchen
You can combine the sockets of the living room, room and corridor into one group. In this case, we use a C 25 machine, wire cross-section (copper) 3 × 2.5 mm². Approximate power consumption 4800 W.
Switches 1st floor:
- Living room.
- Room.
- Corridor.
- Kitchen.
- Bathroom.
- Outdoor Lighting.
You can combine the switches of the living room, room, corridor, kitchen and local area into one group. In this case, we use a B 10 machine, wire cross-section (copper) 3 × 1.5 mm². Approximate power consumption 1600 W.
Sockets 2nd floor:
- Room 1— automatic switch C 16, wire cross-section (copper) 3 × 2.5 mm², estimated power consumption 1600 W.
- Room 2— automatic switch C 16, wire cross-section (copper) 3 × 2.5 mm², estimated power consumption 1600 W.
- Room 3— automatic switch C 16, wire cross-section (copper) 3 × 2.5 mm², estimated power consumption 1600 W.
- Bathroom + washing machine— automatic switch C 16, wire cross-section (copper) 3 × 2.5 mm², estimated power consumption 3000 W.
You can combine the sockets of rooms 1, 2 and 3 into one group. In this case, we use a C 25 machine, wire cross-section (copper) 3 × 2.5 mm². Approximate power consumption 4800 W.
Switches 2nd floor:
- Room 1.
- Room 2.
- Room 3.
- Bathroom.
You can combine the switches of rooms 1, 2, 3 and the bathroom into one group. In this case, we use a B 10 machine, wire cross-section (copper) 3 × 1.5 mm². Approximate power consumption 800 W.
So on at this stage we have decided on consumer groups, circuit breakers and power cable cross-sections. Let's display the resulting data in table form:
Table 1. Selection of circuit breakers (automatic fuses) for consumer groups:
| GROUPS | MACHINE TYPE | WIRE TYPE | |
| Sockets 1st floor | C25 1P | VVG 3 × 2.5 mm² | |
| Sockets kitchen 1st floor | C16 1P | VVG 3 × 2.5 mm² | |
| Sockets 2nd floor | C25 1P | VVG 3 × 2.5 mm² | |
| Sockets bathroom 2nd floor | C16 1P | VVG 3 × 2.5 mm² | |
| Lighting 1st floor | B10 1P | VVG 3 × 1.5 mm² | |
| Lighting 2nd floor | B10 1P | VVG 3 × 1.5 mm² | |
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The circuit breaker is a single-pole 10 amp automatic fuse. It protects the phase conductor connected to the machine of single-phase two-wire or single-phase three-wire electrical wiring from melting of the insulation and conductive core of the wire due to overheating from short circuit currents and prolonged heating by the flow of current exceeding 10 A. The machine ensures the cessation of the flow of electricity into the single-phase electrical system with load up to 2.2 kW in the event of an emergency, the power supply is turned off. The characteristic curve determines the use of the B10 circuit breaker to protect wiring with switching currents (inrush currents) up to 30 - 50 amps. | ||
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The circuit breaker is a single-pole 10 amp automatic fuse. It protects the phase conductor connected to the machine of single-phase two-wire or single-phase three-wire electrical wiring from melting of the insulation and conductive core of the wire due to overheating from short circuit currents and prolonged heating by the flow of current exceeding 16 A. The machine ensures that the flow of electricity into the single-phase electrical system is stopped with load up to 3.52 kW in the event of an emergency, the power supply is turned off. The characteristic curve determines the use of the C 16 circuit breaker to protect wiring with switching currents (inrush currents) up to 80 - 160 amps. | ||
 |
The circuit breaker is a single-pole 10 amp circuit breaker. It protects the phase conductor connected to the machine of single-phase two-wire or single-phase three-wire electrical wiring from melting of the insulation and conductive core of the wire due to overheating from short circuit currents and prolonged heating by the flow of current exceeding 25 A. The machine ensures that the flow of electricity into the single-phase electrical system is stopped with load up to 5.5 kW in the event of an emergency, the power supply is turned off. The characteristic curve determines the use of the C 25 circuit breaker to protect wiring with switching currents (inrush currents) up to 125 - 250 amps. | ||
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- a type of VVG copper power cable whose insulated cores are located parallel in one plane. This power cable has a sheath and insulation made of polyvinyl chloride plastic (PVC), which ensures that fire does not spread when installed alone. The cable is suitable for use in dry and damp rooms and on outdoors, but are not recommended for laying in the ground. Withstands low (up to -50°C) and high (up to +50°C) temperatures. Resistant to humidity up to 98% and various chemicals. VVG-P power cables have different cross-sectional areas of current-carrying conductors, which depend on the object of use. Copper cable with core cross-sectional area 1.5 mm² designed for a current of 19 A and a power of 4100 W, and with a core cross-sectional area 2.5 mm²- current 27 A and power 5900 W. | ||
How to choose an RCD. Calculation for the considered example
In the previous paragraph, we looked at what automatic fuses will be needed for a particular home. Now, based on these data, using calculations, we will determine the necessary automatic devices for this configuration. For clarity and convenience, we will select a residual current device according to the groups formed in the table above. But first, let’s define the recommendations and already known methods of selection and calculation:
- In order to ensure better electrical safety and at the same time maximum uninterrupted power supply, it is advisable to install a separate automatic device for each group of consumers. For these purposes, devices with a leakage current value (set) are used. 10 mA and 30 mA, at which the protection is triggered.
- For wet groups, made as a separate line, an RCD with a setting of 10 mA is installed. In our example, we will include the bathroom on the second floor, in which the washing machine will be located, as a wet group.
- Rated current of residual current device is selected equal to or one step higher than the rated current of the circuit breaker that protects this section of the circuit.
Based on the first and second points, you can decide on the following: in all groups except for the “2nd floor bathroom outlet” we install devices with a setting of 30 mA, and in the wet group of the second floor bathroom - 10 mA.
How to choose ouzo
Based on the third point, you can first determine the rating of the residual current device for a specific circuit breaker of a consumer group.
Table 2. Selection of RCD for consumer groups:
You can supplement the answer to the question of how to choose an ouzo for an apartment or house by checking the types of devices we have previously selected by truncation. They must perform protective functions in accordance with the standards. To do this, it is necessary to calculate the leakage current of the electrical installation IΔ:
- IΔ = IΔep + IΔnetwork, Where IΔep— leakage current of the electrical receiver, mA; IΔnetwork— network leakage current, mA.
When calculating the leakage current in an electrical installation, the PUE is prescribed to take the leakage current of electrical receivers at the rate of 0.4 mA per 1 A of load current, and the circuit leakage current at the rate of 10 μA per 1 m of phase conductor length. Respectively:
- IΔep = 0.4 X Icalc, Where Icalc— calculated current in the circuit load, A.
- Icalc = Inom = Pnom / (Unom X cosφnom).
Power factor cos φ characterizes the amount of reactive energy consumed by the device. Most household and office equipment have an active load (they have no or little reactance), for them cos φ=1.
Rated power Pnom(W) in our case, we take it from consumption groups, where each was given an approximate consumption power. To make it clearer, let’s take the “1st floor outlets” group. It includes a living room, a room, a corridor and a bathroom. We set the estimated power consumption for individual rooms to 1600 W. In total for the group this indicator will be 4800 W.
Rated voltage for single-phase network Unom = 220 V.
- IΔnetwork = 0.01 X Lwires, Where Lwires— length of phase conductor, m.
According to the requirements of the PUE, the total leakage current of the network, taking into account the connected stationary and portable electrical receivers in normal operation, should not exceed 1/3 of the rated disconnecting differential current IΔn RCD. That is, the rated differential breaking current of the device (which is marked on the housing) must be at least three times greater than the total leakage current of the protected circuit of the electrical installation IΔ:
- IΔn > = 3 IΔ.
Example of calculating device compliance in a consumer group
Consider the group "sockets 1st floor":
- IΔn = 30 mA(from table 2).
- Unom = 220 V.
- Pnom = 4800 W.
- Lwires- in this case it is unknown, and we will determine its optimal length.
By condition IΔn > = 3 IΔ , IΔ<= 30/3 <= 10 мА.
Icalc = 4800 / 220 = 21.81 A.
IΔep = 0.4 X 21.81 = 8.73 A.
According to the basic formula IΔ = IΔep + IΔnetwork:
10 = 8.73 + (0.01 X Lwires);
(10-8.73) / 0.01 = Lwires;
Lwires = 127 meters.
Conclusion: installed on the considered group RCD 25 A 30 mA paired with a machine gun C25 1P acceptable, taking into account that the length of the phase conductor will not exceed 127 meters. Naturally, at the design and installation stage you will know the length of the wire involved in each specific group. And to check compliance using the formula it is calculated IΔ. We multiply the resulting current by three and compare it with the marking of the residual current device used. Condition must be met IΔn > = 3 IΔ.
Let's say Lwires in a group = 250 meters; then, based on the previous calculation, IΔ=8.73+(0.01 X 250) = 11.23 mA; 3 IΔ = 33.69 > IΔn. The condition was not met. As a way out, you can divide the group into two.
Connecting RCDs and circuit breakers in the electrical panel - basic diagrams
We looked at the example of a specific house or apartment and identified groups of electricity consumers, selected automation for them and considered the methodology for calculating compliance with the requirements. Let’s supplement what we learned above with a clear example and a diagram of how to properly connect RCDs and machines. We will complete the electrical panel:
Connection diagram for RCDs and automatic machines
When considering this diagram, several questions may arise:
- Installation of an introductory machine.
- Installation of fire extinguishing device.
- Optimization of the number of residual current devices.
Let's start with the last point. Naturally, the condition of using a separate RCD for each group of electricity consumers is justified. However, there is always an alternative and you can refuse something in this scheme. For example, you can:
- Remove the fire safety device.
- Do not use RCDs for lighting the first and second floors.
- Install one common protection device on the first and second floor sockets. In this case, the rated current of the group RCD is selected so that it is equal to or greater than the sum of the ratings of the group circuit breakers. If the sum of the ratings of group circuit breakers exceeds the rating of the input circuit breaker, then the rated current of the residual current device is selected equal to the rated current of the input RCD, and if the input fire protection device is not installed, then equal to or greater than the rating of the input circuit breaker.
In any case, these are just options that need to be calculated and analyzed. We will devote a separate review to options and diagrams for connecting RCDs. And now, to complete the topic, we will briefly consider the installation of an introductory machine and a fire extinguishing device.
Introductory machine- This is an automatic switch for supplying electricity from the mains to the object if an overload occurs in the circuit or a short circuit occurs. It differs from conventional circuit breakers used in the circuit in its larger rated current. With a small number of loads, it can be single-pole and connected through a phase wire. In our example, we used a two-pole sample. A two-pole circuit breaker is a block with two poles. They are equipped with an integrated lever and have a common lock between the shutdown mechanisms. This design feature is important, since the PUE prohibits breaking the neutral wire.
Switchboards can be equipped with fire protection RCDs. They differ from conventional devices by a higher leakage current setting - 100 mA or 300 mA. In order to achieve selectivity, it is advisable to use devices with the letter S on the front panel. If you install a regular non-selective one, then when one of the group RCDs is triggered, the fire protection device will almost always be triggered.
As a supplement - video: RCD connection diagrams
The topic under consideration is quite extensive, and we tried to figure out how to properly connect RCDs and circuit breakers in an apartment (house) according to a given diagram and layout. The choice of a particular protective device must be justified by calculations and comply with standards. For a better understanding, it is recommended to study additional materials on this issue.
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