What is ouzo and how does it work? RCD - residual current device (description, purpose, marking, connection diagram)
You can hear an opinion that disputes the need to install residual current devices (hereinafter referred to as RCDs). To refute or confirm it, it is necessary to understand the functional purpose of these devices, their principle of operation, design features and connection diagram. Also an important factor is the correct connection, depending on the specific task. We will try to answer as broadly as possible all questions regarding this topic.
Functional purpose
According to the official definition, this type of device plays the role of a high-speed protective switch that reacts to current leakage. That is, it works when a circuit is formed between the phase and the "ground" (PE conductor).
To give a classic example, an electric water heater is installed in the bathroom. It works without problems for a warranty period and even more, then there comes a moment when the body of one of the heating elements cracks and a phase breaks down into water.
If in this case a circuit is formed: phase - person - earth, the load current will not be enough to trigger the electromagnetic protection, it is designed for short circuit. As for thermal protection, its response time is much longer than the resistance of the human body to the destructive effects of electric current. The result can not be described, the worst thing is that in an apartment building such a boiler can pose a threat to neighbors.
In such cases, the presented device is the only effective way to provide reliable protection. It's time to consider its concept, design and principle of operation.
Device diagram
First of all, we present a schematic diagram of the device, indicating its main elements.
Designation:
- A - Relay that controls the contact group.
- B - Differential CT (current transformer).
- C - Phase winding on DTT.
- D - Zero winding on DTT.
- E - Contact group.
- F - Load resistance.
- G - Button that starts testing the device.
- 1 – Phase input.
- 2 - Phase output.
- N - Contacts of the neutral wire.
Now let's explain how it works.
Principle of operation
Let's say that a certain device with internal resistance R n is powered from our protective device, while the case of the connected device is grounded. In this case, during normal operation, currents equal in value, but different in direction, will flow through the windings I and II of the DTT.
Thus, the total value of i 0 and i 1 will be zero. Accordingly, the magnetic fluxes caused by the currents in the DCT will also be counter-current, therefore their total value will also be zero. Given the above conditions, no current will be generated in the secondary winding of the DDT, so the relay that controls the contact group is not initiated. That is, the safety device will remain on.
Now consider the situation in which a breakdown occurred on the body of the connected equipment.
As a result of the appearance of a leakage current (i y) to the "ground", the balance of currents flowing through the primary windings I and II will be disturbed. This will lead to the fact that the value of the magnetic flux will also become different from zero, which will cause the formation of a current (i 2) on the secondary winding of the DTT (III), to which the relay that controls the contact group is connected. It will work and the connected equipment will be de-energized.
The test button on the device simulates current leakage through the resistor R t, which makes it possible to verify that the device is working. This check must be carried out at least once a month.
Design
The figure below shows a typical protective device with the top cover removed, which allows you to see the main components of the design.
Designations:
- A - The mechanism of the button that starts testing the device.
- B - Contact pads for connecting the phase input and neutral wire.
- C - Differential CT.
- D - The electronic board of the current amplifier coming from the secondary winding, to the level necessary to operate the relay.
- E - The lower part of the plastic housing with a standard DIN-rail mount.
- F - Arc chutes on a breaking group of contacts.
- G - Contact pads for connecting the phase output and neutral wire.
- H - Release mechanism (operated by relay or manually).
List of main characteristics
Having dealt with the design of the devices and their principle of operation, let's move on to the main parameters. These include:
- The type of electrical wiring to be protected, it can be single-phase or three-phase. This parameter affects the number of poles (2 or 4).
- The value of the rated voltage, for two-pole devices is 220-240 Volts, for four-pole devices - 380-400 Volts.
- The value of the rated current load, this parameter corresponds to that of circuit breakers (hereinafter AB), but has a slightly different purpose (will be described in detail below), measured in Amperes.
- Rated differential (breaking) current, typical values: 10, 30, 100 and 300 mA.
- Type of breaking current, accepted designations:
- AC - Corresponds to sinusoidal alternating current. Both its slow increase and sudden manifestation are allowed.
- A - In addition to the previous characteristics (AC), the ability to monitor the leakage of the rectified pulsating current is added.
- S - Designation of selective devices, they are distinguished by a relatively high response delay.
- G - Similar to the previous type (S), but with less delay.
Now it is necessary to explain the meaning of the rated current parameter, since some questions arise with it. This value indicates the maximum allowable current for this protective electromechanical device.
When choosing this parameter, it is necessary to take into account that it must be one step higher than that of the AB on this line. For example, if the AB is designed for 25 A, then it is necessary to install protective devices with a rated current of 32 A.
Please note that this type of device is not designed to operate from short circuit and overload. If such an accident occurs, then all the wiring will burn out and a fire will occur, but the device will remain on. That is why such protective devices must be used in conjunction with AB. Alternatively, you can install a diffuser, in fact it is also a residual current device, but equipped with a short circuit and overload protection mechanism.
Marking
The marking is applied to the front panel of the device, we will tell you what it means using the example of a two-pole device.
Designations:
- A - Abbreviation or logo of the manufacturer.
- B is the designation of the series.
- C - The value of the rated voltage.
- D - Rated current parameter.
- E - The value of the breaking current.
- F - Graphic designation of the type of breaking current, can be duplicated by letters (in our case, a sinusoid is shown, which indicates the type of AC).
- G - Graphic designation of the device on circuit diagrams.
- H - The value of the conditional short circuit current.
- I - Device diagram.
- J - The minimum value of the operating temperature (in our case: - 25 ° C).
We have given a typical marking, which is used in most devices of this class.
Connection options
Before moving on to typical connection diagrams, it is necessary to talk about a few general rules:
- Devices of this type must be paired with AB, as we mentioned above, this is due to the fact that protective devices are not equipped with short circuit protection.
- The value of the rated current of the protective device, it must be one step higher than that of the AB standing with it in a pair.
- Do not confuse input and output contacts. That is, the input marked, as a rule, “1” should be supplied with a phase, and “N” should be zero. Accordingly, "2" is the phase output, and "N" is zero.
- Zero after the apparatus must not be connected to zero before it.
Now let's consider the simplest circuit in which protection against short circuit and leakage current is installed on each line.
In this case, everything is simple, an AB is installed at the input (A in Fig. 7) with a rated current of 40 A. After it there is a common device (B), it is also called a fire-fighting device. This device must have a leakage current of at least 100 mA, a rated current of at least 50 A (see paragraph 2 of the general rules above). Next are two bundles of RCD-AB (C-E and D-F). The rated current parameter for "C" and "D" is 16 A. For "E" and "F" this parameter should be a step higher, in our case it is 20 A. As for the magnitude of the breaking current, for wet rooms this the indicator should be 10 mA, for other consumer groups - 30 mA.
This connection option is the simplest and most reliable, but at the same time more expensive. It can still be used for two internal lines, but when their number is 4 or more, it makes sense to install one protection device per group AB. An example of such a scheme is shown below.
As you can see in this scheme, we have one common (fire) protective device and four group ones for lighting, kitchen, sockets and bathroom. This connection option allows you to significantly reduce costs, compared with the scheme, where a bunch of RCD-AV is connected to each line. In addition, the necessary level of protection is provided.
In conclusion, a few words about the need for protective grounding. For the normal functioning of the RCD, it is necessary. On the Internet, you can find a switching circuit without PE (in fact, it is no different from the usual one), but it should be noted that the operation will only occur when there is contact with batteries, cold or hot water pipes, etc.
Oleg Udaltsov
Eaton Power Distribution Components Product Specialist.
What is a residual current device
A residual current device, also known as an RCD, is a device installed in an electrical panel in an apartment or house to automatically turn off the power supply to the network in the event of a ground fault current.
Ground fault current occurs in wiring and / or electrical appliances when, for some reason, the insulation is broken in them, or when the bare parts of the wires that must be fixed in the terminals, for example, inside household electrical appliances, touch the housing of the devices - and the current begins to "leak" in the wrong direction.
This can lead to a fire due to overheating (first the wiring or device, and then everything around) or to the fact that a person or a pet will suffer from the current - the consequences can be extremely unpleasant, even death. But this will happen only if you touch the conductor or the body of the equipment, which is energized.
The main difference between an RCD and a conventional circuit breaker is that it is designed specifically to break the earth fault current that the circuit breaker cannot detect. The RCD is able to turn it off in a fraction of a second, until the moment when it becomes dangerous to a person or property.
Where and how much to install
For one- and two-room apartments - in the general electrical panel of the apartment. If the housing area is large, then in several local electrical panels distributed throughout the house.
An RCD will be required for the entire system to protect against, as well as for separate lines that feed groups of electrical appliances with a metal case (washing and dishwasher, electric stove, refrigerator, and so on) - to protect against electric shock. If a malfunction occurs or an accident occurs, not the entire apartment will be de-energized, but only one line, so it will be easy to determine the culprit of the RCD trip.
However, it must be borne in mind: neither RCDs nor conventional automata save from an electric arc, or an arc breakdown.
An electric arc can occur when, for example, the wire from an electric lamp is often pinched by a slamming door and the metal part of the wire inside is damaged. At the site of damage, sparks hidden from the eyes will occur, accompanied by an increase in ambient temperature and, as a result, ignition of flammable objects nearby: first the wire sheath, and then wood, fabric or plastic.
To protect against such hidden threats, it is better to choose solutions that combine the functions of an automaton, RCD and arc fault protection. In English, such a device is called an arc fault detection device (AFDD), in Russia the name “arc fault protection device” (AFDD) is used.
An electrician can include the installation of such a device in the circuit if you tell him that you need an increased degree of protection. For example, for a children's room, where a child can handle wires inaccurately, or for groups of sockets for powerful electrical appliances with flexible wires prone to breakage.
It is equally important to install protection devices where the wiring is laid in an open way and it can be damaged. And also when planned, in order to avoid risks in case of accidental damage to hidden electrical wiring while drilling walls.
How to choose
A good electrician will recommend the manufacturer of the RCD and calculate the load, but you need to be sure that the recommendations are correct. And if you buy everything for repairs yourself, then all the more you need to understand what to look for when choosing a device.
Price
Do not purchase a device in the lower price range. The logic is simple: the better the components inside, the higher the price. For example, in some cheap devices there is no protection against burnout, and this can lead to ignition.
A cheap device can be made of brittle materials and break easily when you lift up the lever that has fallen when triggered. According to the RCD standard, it should be designed for 4,000 operations. This means that you will have to be puzzled by the choice only once, but only if you have purchased a quality product. By purchasing a low-quality device, you put yourself and loved ones at risk, not to mention material losses in case of fire.
Case quality
Pay attention to how tightly all parts of the device fit together. The front panel should be monolithic, and not consist of two halves. The preferred material is heat-resistant plastic.
Device weight
Give preference to heavier devices. If the RCD is light, then the manufacturer has saved on the quality of internal components.
Conclusion
To resolve issues related to electricians in the house, it is advisable to involve professionals. However, the entire responsibility should not be shifted to their shoulders. It is better to be guided by the proverb "Trust, but verify." With even basic knowledge of the subject and an understanding of the scenario for the future use of electrical appliances in the house, you can save yourself and loved ones from problems with electricity.
It is a mistake to assume that automatic current breakers are installed on the housing of household appliances to protect a person from injury in case of current leakage. For this purpose, the shields are equipped with a protective device. Having found out the principle of operation of ouzo, you can not be afraid for the life of your loved ones and children.
Protection protects against the effects of current on the body, when touching the body of the devices. The leakage of electricity that has occurred, by the amount of power, the current of which the machine does not react. Another important job of protection is to keep your home safe from fire.
Functional features of protection equipment
The case of the device made of conductive material, as well as individual parts and even pipelines, sometimes turn out to be dangerous to humans. The phase breaks through them, with various wiring breakdowns and other reasons. There is such a dangerous situation, as a rule, in 2 cases:
The main task - the leak must be immediately detected and the supply of electricity to this group of contacts is stopped. And also to switch off when a person touches a bare wire and prevent fires in the building.
Important. The protection is triggered by leaks, but it should be remembered that the case of any household appliance will become deadly if, during installation, you mix up the phase and ground wires at the entrance to the building.
What to look for when choosing an RCD
For the correct purchase and the safety of your home, you need to pay attention to the following indicators:
Important. Regardless of the brand and manufacturer of the protection device and different markings, 2 main characteristics show the value of the operating and leakage current. These values are indicated, regardless of the type of device and its price.
The principle of operation of the protection device
The principle of operation of the protective device is the reaction of the sensors when the incoming value of the differential currents changes. An ordinary transformer can act as a current sensor. According to its design features, it is manufactured as a toroidal core. The magnetoelectric relay has a fairly significant sensitivity to leakage, on it we set a certain value of the device actuation.
Devices in which the ouzo principle of operation is performed with the installation of a control relay are by far the most reliable, trouble-free. Even commercially available electronic devices that control leakage by means of an electronic circuit are inferior to electromechanical devices in some cases.
The principle of turning off electricity to consumers in a device with a relay is based on its operation and the impact on the mechanism for interrupting the electrical circuit. It consists of 2 parts:
- According to the passport of the device, a contact group is selected for the maximum value of the current in the network.
- In the event of an emergency and touching a bare area with a hand, a spring is provided to trigger the device.
The serviceability of the protection can be checked using the “Test” button equipped on the device case. By pressing it, we create an artificial malfunction of the electrical network in the event of an electric current leakage. The value is set sufficient to enable protection.
In this simple way, you can independently examine and check for the serviceability of the RCD without calling a technician and without paying for his visit. This check is carried out at least once a month.
By measuring the values of the current and the operating time of the RCD, a specialist electrician using a special device can conduct a more accurate check.
Correct operation of protection in different modes
How does ouzo work under normal conditions? Without leakage, the operating voltage, up to 12 V, flows towards and in parallel, while magnetic fluxes with the same magnitude are induced on the secondary winding of the transformer. They equalize with each other. Such operation does not trigger the residual current device due to the fact that the value of the current supplied to the secondary winding is zero.
Leakage current occurs when accidentally touching a bare section of wiring or a device case with a phase closed to it. In this case, the correct direction and magnitude of the currents passing through the transformer are violated. On the secondary winding, an unbalance of the current values occurs, from which the relay is triggered. It acts on the spring, the voltage supply to the network stops.
This is a simple explanation of the operation of the RCD, if necessary, there is enough information on the Internet to study this issue in more detail.
It must be remembered that the purpose of a residual current device is an additional measure for the safe use of electrical appliances. This device responds to leakage current. For this reason, it is necessary to install RCDs together with automata to turn off the network in case of a short circuit.
RCD in any electrical circuit is a very important element. The main purpose of the RCD is to protect a person from electric shock when in contact with live parts. In addition, the RCD, the principle of operation of which will be discussed in this article, prevents the likelihood of fires that can be triggered by the ignition of electrical wiring.
In certain situations, the RCD, the principle of operation of which is quite simple, stops supplying voltage to the protected line. This happens if a person touches the current-carrying parts of electrical installations, and non-current-carrying elements, which, as a result of insulation breakdown, are energized. Another reason for opening contacts is the occurrence of current leakage to the electrical installation case or ground.
Consideration of the principle of operation of the RCD in general and on a specific example
When inexpensive apartments are rented from the developer, then all the electrics, including RCDs and diffusers, as well as wiring and circuit breakers, have already been installed. If you are building your own house or want to install an RCD in an apartment with your own hands, then you should know the principle of operation of this device and the rules for its installation.
RCD (the principle of operation is based on the detection of incoming and outgoing currents at the entrance to the system) can respond to minimal leakage and perform its protective function. To measure the leakage, a sensing element such as a differential transformer with three windings is installed in the device.
The principle of operation of the RCD can be easily understood with a specific example. If a person touches the live parts of the installation, or an insulation breakdown occurs on its case, the amount of current flowing through the phase wire will exceed the current in the neutral wire.
The total (final) flux of magnetic induction, in this case, will necessarily change, will differ from zero and will be the cause of current induction in the control winding. The relay to which the winding is connected will operate and the release of the contacts of the power protective device will be set in motion.
As a result of which a dangerous electrical installation is de-energized in a fraction of a second, it ensures the safety of human health.
Connecting an RCD to a single-phase network: basic rules
The RCD diagram is indicated on the body of the device and allows you to understand the principle of its operation, correctly connect the device to the protection circuit of the electrical circuit, avoiding incorrect operation of the device or its failure.
The RCD circuit, according to which it is connected to the power supply system, depends on various parameters and factors. In residential premises, as a rule, a single-phase version of electrical wiring with a rated voltage of 220 V is used.
Before installation, you need not only to understand the principle of operation of the RCD in a single-phase network, but also to familiarize yourself with the safety rules.
The principle of operation of the RCD and the wiring diagram imply the use of two wires of wiring connected to the input terminals, and two wires to the output of the device, connected to the corresponding output terminals. Install the device only when the power is off. Before installation, you need to make sure that there is enough space in the shield for the selected device.
And the wiring diagram is quite simple. There are several options for installing this device, but the principle, in general, remains unchanged.
The most common and affordable is the option in which the device is at the entrance to the house / apartment. The disadvantage of this option is that when the device is triggered, the entire living space is de-energized, and it is difficult to determine the cause of what is happening.
More expensive, however, very convenient is the connection option with the installation of several RCDs - in this case, each device will be responsible for a separate group of sockets or lighting.
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Connecting an RCD (residual current device) is a generally accepted measure in world practice to improve the electrical safety of consumers. The number of human lives saved by RCDs goes into the millions, and the use of RCDs in the power supply networks of multi-apartment and private residential buildings, residential areas and industrial facilities prevents billions of damage from fires and accidents.
But Galen's rule: "Everything is poison and everything is medicine" is true not only in medicine.. Outwardly simple, RCD with thoughtless or careless application can not only prevent nothing, but also become a source of trouble. By analogy: someone built Kizhi with one ax, someone can build some kind of hut with them, but you can’t give someone an ax in their hands, they will chop off something for themselves. So let's get acquainted with the RCD in more detail.
First of all
Any serious conversation about electricity is sure to touch on electrical safety rules, and for good reason. Electric current does not carry visible signs of danger, its effect on the human body develops instantly, and the consequences can be long and severe.
But in this case, we will not talk about the general rules for the production of electrical work, which are already well known, but about something else: RCDs fit very poorly into the old Soviet TN-C power supply system, in which the protective conductor is combined with neutral. For a long time it was not clear whether it fits at all.
All editions of the PUE unequivocally require: it is prohibited to install switching devices in the circuits of protective conductors. The wording and numbering of the paragraphs changed from edition to edition, but the essence is clear, as they say, even to the marabou bird. But what about the recommendations for the use of residual current devices? They are switching devices, and at the same time they are included in the gap of both the phase and ZERO, which is also a protective conductor?
Finally, in (PUE-7A; Electrical Installation Rules (PUE), 7th edition, with additions and changes, M. 2012), paragraph 7.1.80 still dotted i: “It is not allowed to use RCDs that respond to differential current , in four-wire three-phase circuits (TN-C system)”. Such a tightening was caused, contrary to previous recommendations, by recorded cases of electrical injuries WHEN ACTIVATED RCD.
Let's explain with an example: The hostess was washing, in the car it hit the heater body, as shown in the figure with a yellow arrow. Since the current distributes 220 V along the entire length of the heating element, something around 50 V will appear on the case.
This is where the following factor comes into play: the electrical resistance of the human body, like any ionic conductor, depends on the applied voltage. With its increase, the resistance of a person falls, and vice versa. Say, the PTB provides an absolutely reasonable calculated value of 1000 ohms (1 kOhm), with sweaty steamed skin or in a state of intoxication. But then, at 12 V, the current should be 12 mA, and this is more than the non-releasing (convulsive) current of 10 mA. Has anyone ever been hit by 12 volts? Even drunk in a saltwater jacuzzi? On the contrary, according to the same PTB 12 V is an absolutely safe voltage.
At 50-60 V on wet steamed skin, the current will not exceed 7-8 mA. This is a strong, painful blow, but the current is less than convulsive. You may need treatment for the consequences, but it will not come to resuscitation with defibrillation.
And now let's "defend" the RCD, not understanding the essence of the matter. Its contacts do not open instantly, but within 0.02 s (20 ms), and not absolutely synchronously. With a probability of 0.5, the ZERO contact will open first. Then, figuratively speaking, the potential reservoir of the heating element at the speed of light (literally) will be filled up to 220 V along its entire length, and 220 V will appear on the body, and the current through the body will pass 220 mA (red arrow in the figure). Less than 20ms, but 220mA is more than two instant killing 100mA values.
So, why not install RCDs in old houses? Still, it is possible, but carefully, with a full understanding of the matter. You need to choose the right RCD and connect it correctly. How? This will be discussed further in the relevant sections.
RCD - what and how
RCDs in electrics appeared simultaneously with the first power lines in the form of relay protection. The purpose of all RCDs remains unchanged to this day: to turn off the power supply in the event of an emergency. As an indicator of an accident, the vast majority of RCDs (and all household RCDs) use leakage current - when it rises above a predetermined limit, the RCD trips and opens the power supply circuit.
Then RCDs began to be used to protect against breakdown and fire of individual electrical installations. For the time being, for the time being, the RCDs remained “fireproof”, they responded to a current that excluded the ignition of an arc between the wires, less than 1 A. “Fire” RCDs are produced and used to this day.
Video: what is RCD?
RCD-E (capacitive)
With the development of semiconductor electronics, attempts began to create household RCDs designed to protect a person from electric shock. They worked on the principle of a capacitive relay reacting to a reactive (capacitive) bias current; while the person works as an antenna. The well-known indicator-phase indicator with neon is built on the same principle.
RCDs-E have exceptionally high sensitivity (fractions of µA), can be made almost instantly triggered and are absolutely indifferent to grounding: a child standing on an insulating floor and reaching out with his finger to the phase in the outlet will not feel anything, and RCD-E will “smell” him and turn off the power until he removes his finger.
But RCDs-E have a fundamental drawback: in them, the flow of leakage current electrons (conduction current) is a consequence of the occurrence of an electromagnetic field, and not its cause, therefore they are extremely sensitive to interference. There is no theoretical possibility to “teach” UZO-E to distinguish between a little hustler who has picked up an “interesting little thing” from a tram that has sparkled on the street. Therefore, UZO-E are used only occasionally to protect special equipment, combining their direct duties with a touch indication.
UZO-D (differential)
Having “turned” the RCD-E “on the contrary”, it was possible to find the principle of operation of the “smart” RCD: you need to go directly from the primary electron flow, and determine the leakage by the unbalance (difference) of the total currents in the POWER conductors. If exactly the same amount flows from the consumer as it went to him, everything is in order. If there is an imbalance, it is leaking somewhere, you need to turn it off.
The difference in Latin is differentia, in English difference, therefore such RCDs were called differential, RCD-D. In a single-phase network, it is enough to compare the magnitudes (modules) of currents in the phase wire and neutral, and when an RCD is connected in a three-phase network, the full vectors of currents of all three phases and neutral. An essential feature of the RCD-D is that in any power supply circuit, the protective and other conductors that do not transmit power to the consumer must pass by the RCD, otherwise false alarms are inevitable.
It took quite a long time to create household RCDs. Firstly, it was necessary to accurately determine the value of the unbalance current, which is safe for a person with an exposure time equal to the RCD operation time. RCDs tuned to an imperceptible or smaller non-letting current turned out to be large, complex, expensive, and pickups “caught” only slightly worse than RCDs.
Secondly, it was necessary to develop high-coercivity ferromagnetic materials for differential transformers, see below. The radio ferrite was not suitable at all, it did not hold the working induction, and the UZO-D with iron transformers turned out to be too slow: the own time constant of even a small iron transformer can reach 0.5-1 s.
UZO-DM
By the 80s, the research was successfully completed: the current, according to experiments on volunteers, was chosen to be 30 mA, and high-speed differential transformers on ferrite with a saturation induction of 0.5 T (Tesla) made it possible to remove power from the secondary winding, sufficient to directly drive the breaker electromagnet. Differential electromechanical UZO-DM appeared in everyday life. Currently, this is the most common type of household RCD, so DM is omitted, and they simply say or write RCD.
Differential electromechanical RCD works like this, see the figure on the right:
The appearance with explanations of the designations on the case of a three-phase and single-phase RCD is shown in the figure above.
Note: using the “Test” button, the RCD is supposed to be checked monthly and every time it is turned on again.
An electromechanical RCD only protects against leakage, but its simplicity and "oak" reliability made it possible to combine an RCD and a current circuit breaker in one case. To do this, it was only necessary to make the breaker latch rod double and bring it into the current and RCD electromagnets. So there was a differential machine that provides complete consumer protection.
However, the difavtomat is not an RCD and an automatic machine separately, this should be clearly remembered. External differences (power lever, instead of a flag or a re-enable button), as shown in the figure, are only appearance. An important difference between an RCD and a differential machine affects when installing an RCD in power supply systems without protective grounding (TN-C, independent power supply), see the section below on connecting an RCD without ground.
Important: a separate RCD is designed to protect against leakage ONLY. Its rated current shows to what extent the RCD remains operational. RCDs for ratings of 6.3 and 160 A with the same unbalance of 30 mA give the same degree of protection. In difautomatic machines, the cutoff current of the machine is always less than the rated current of the RCD, so that the RCD does not burn out when the network is overloaded.
UZO-DE
In this case, "E" does not mean capacity, but electronics. UZO-DE are built directly into or in the electrical installation. The difference in currents in them is captured by a semiconductor magnetically sensitive sensor (Hall sensor or magnetodiode), its signal is processed by a microprocessor, and the circuit opens the thyristor. UZO-DE, in addition to compactness, have the following advantages:
- High sensitivity, comparable to UZO-E, combined with the noise immunity of UZO-DM.
- As a result of high sensitivity, the ability to respond to bias current, i.e., RCD-DE proactive, will turn off the voltage before it hits someone, regardless of the presence of grounding.
- High speed: for the "buildup" of the RCD-DM, at least one half-cycle of 50 Hz is required, i.e. 20 ms, and at least one dangerous half-wave must pass through the body for the RCD-DM to work. RCD-DE is able to operate at a “breakdown” half-wave voltage of 6-30 V and cut it off in the bud.
The disadvantages of RCD-DE are primarily high cost, own power consumption (negligible, but when the mains voltage drops, RCD-DE may not work) and a tendency to failure - electronics after all. Abroad, chipped sockets were widely distributed back in the 80s; in some countries their use in children's rooms and institutions is required by law.
We UZO-DE are still little known, but in vain. The bickering between mom and dad about the cost of a socket with "fool protection" is not comparable to the price of a child's life, even if an incorrigible vermin and troublemaker run amok in the apartment.
UZO-D indices
Depending on the device and purpose, main and additional indices can be added to the name of the RCD. According to the indices, you can make a preliminary selection of the RCD for the apartment. Main indices:
- AC - are triggered by the unbalance of the variable component of the current. As a rule, they are fire-fighting, for an unbalance of 100 mA, because cannot protect against short-term impulse leakage. Inexpensive and very reliable.
- A - react to the unbalance of both alternating and pulsating currents. The main version is protective for 30 mA unbalance. False trips / failures are possible in the TN-C system in any case, and in the TN-C-S with poor grounding and / or the presence of powerful consumers with significant intrinsic reactivity and / or switching power supplies (UPS): washing machine, air conditioner, hob, electric oven, food processor; to a lesser extent - dishwasher, computer, home theater.
- B - react to leakage current of any kind. These are either industrial RCDs of the "fire" type for 100 mA of unbalance, or built-in RCDs-DE.
Additional indices give an idea of the additional functionality of the RCD:
- S - selective in response time, it is adjustable within 0.005-1 s. The main area of application is in the power supply of objects powered by two beams (feeders) with an automatic transfer switch (ATS). Adjustment of the response time is necessary so that when the main beam fails, the AVR has time to work. In everyday life, they are sometimes used in elite cottage settlements or mansions. All selective RCDs are fire, for an unbalance of 100 mA, and require the installation of protective 30 mA RCDs after them for a lower current, see below.
- G - high-speed and ultra-high-speed RCDs with a response time of 0.005 s or less. They are used in children's, educational, medical institutions and in other cases when the "overshoot" of at least one striking half-wave is unacceptable. Exclusively electronic.
Note: household RCDs are most often not indexed, but differ in design and unbalance current: electromechanical for 100 mA - AC, they are also for 30 mA - A, built-in electronic - B.
PATTERN
Almost unknown to non-specialists, a type of RCD is not differential, triggered by current in a protective conductor (P, PE). They are used in industry, in military equipment and in other cases when the consumer creates strong interference and / or has its own reactivity that can “confuse” even UZO-DM. They can be both electromechanical and electronic. Sensitivity and speed for domestic conditions are unsatisfactory. A high quality serviced ground is required.
RCD selection
To choose the right RCD, the index is not enough. You also need to find out the following:
- Buy separately RCD with automatic or difavtomat?
- Select or calculate the cutoff value for extra current (overload);
- Determine the rated (working) current of the RCD;
- Determine the required leakage current - 30 or 100 mA;
- If it turned out that for general protection you need a “fire” RCD for 100 mA, determine how many, where and what kind of secondary “life” RCDs for 30 mA are required.
Separately or together?
In an apartment with TN-C wiring, you can forget about the difavtomat: the PUE prohibits, but ignore it, so the electricity itself will soon remind you. In the TN-C-S system, the difavtomat will cost less than two separate devices if the wiring is planned to be reconstructed. If the current machine is already standing, then a separate RCD that is coordinated with it in terms of operating current will be cheaper. Scriptures on the topic: RCD is incompatible with a conventional machine gun - an amateurish nonsense.
What overload to expect?
The cutoff current of the machine (extractors) is equal to the maximum allowable current consumption of the apartment (house), multiplied by 1.25 and added to the nearest higher value from the standard range of currents 1, 2, 3, 4, 5, 6.3, 8, 10, 13, 16 , 20, 25, 32, 35, 40, 50, 63, 80, 100, 125, 160, 250, 400, 630, 1000, 1600, 2500, 4000 and 6300 A.
The maximum current consumption of the apartment must be recorded in its data sheet. If not, you can find out in the organization operating the building (obliged to report by law). In old houses and new budget ones, the maximum allowable current is usually 16 A; in new ordinary (family) - 25 A, in business class - 32 or 50 A, and in suites - 63 or 100 A.
For private households, the maximum current is calculated according to the power consumption limit from the data sheet (the authorities will not miss it) at the rate of 5 A per kilowatt, with a factor of 1.25 and the addition to the nearest higher standard value. If the value of the maximum current consumption is directly stated in the data sheet, it is taken as the basis for the calculation. Conscientious designers on the wiring plan directly indicate the cut-off current of the main machine, so there is no need to count.
Rated current RCD
The rated (working) current of the RCD is taken one step higher than the cut-off current. If a difavtomat is installed, it is chosen BY THE CUT-OFF CURRENT, and the current rating of the RCD is inherent in it constructively.
Video: RCD or difavtomat?
Leakage current and general protection circuit
For an apartment with TN-C-S wiring, it will not be a mistake to take an RCD for an unbalance of 30 mA without much thought. A separate section will be further devoted to the TN-C apartment system, but clear and final recommendations cannot be immediately given for private houses.
According to paragraph 7.1.83 of the PUE, the operating (natural) leakage current should not exceed 1/3 of the RCD unbalance current. But in a house with electric underfloor heating in the hallway, courtyard lighting and electric heating of the garage in winter, the operating leakage current can reach 20-25 mA with a living area of both 60 and 300 squares.
In general, if there is no greenhouse with electric heating of the soil, a heated water well, and the yard is illuminated by housekeepers, at the input after the meter it is enough to put a fire RCD with a rated current one step higher than the cut-off current of the machine, and for each consumer group - a protective RCD with the same rated current. But an accurate calculation can only be made by a specialist based on the results of electrical measurements of already finished wiring.
Calculation examples
The first is a new apartment with TN-C-S wiring ; according to the data sheet, the power consumption limit is 6 kW (30 A) . We check the machine - it costs 40 A, everything is OK. We take the RCD a step or two higher in terms of rated current - 50 or 63 A, it doesn’t matter - and for an unbalance current of 30 mA. We don’t think about leakage current: builders should provide it within the normal range, but if not, then let them fix it themselves for free. However, contractors do not allow such punctures - they know what smells like under the guarantee.
Second. Khrushchev, plugs for 16 A. We put the washing machine on 3 kW; the current consumption is about 15 A. To protect it (and protect it from it), you need an RCD with a rating of 20 or 25 A for 30 mA of unbalance, but 20 A RCDs are rarely on sale. We take an RCD for 25 A, but in any case, it is MANDATORY to remove the plugs, and put a 32 A machine in their place, otherwise the situation described at the beginning is possible. If the wiring clearly cannot withstand a short-term surge of 32 A, nothing can be done, you need to change it.
In any case, you need to submit an application to the energy service for the replacement of the meter and the reconstruction of the electrical wiring, with or without replacement. This procedure is not very complicated and troublesome, and a new meter with an indication of the status of the wiring will serve you well in the future, see the section on trips and malfunctions. And the RCD registered during the reconstruction will then allow free-of-charge calls for electricians for measurements, which is also very good for the future.
Third. A cottage with a consumption limit of 10 kW, which gives 50 A. The total leakage according to the measurement results is 22 mA, and the house gives 2 mA, the garage - 7, and the yard - 13. We put a common difavtomat at 63 A cut-off and 100 mA imbalance, we power the house with the garage separately through the RCD for 80 A nominal and 30 mA imbalance. In this case, it is better to leave the yard without its own RCD at all, but take the lamps for it in waterproof cases with a ground terminal (industrial type), and bring their earths directly to the ground loop, it will be more reliable.
RCD connection in the apartment
A typical RCD connection diagram in an apartment is shown in the figure. It can be seen that the general RCD turns on as close as possible to the input, but after the meter and the main (access) machine. The inset there also shows that in the TN-C system, the general RCD cannot be turned on.
If separate RCDs are needed for groups of consumers, they are turned on immediately AFTER the corresponding machines, highlighted in yellow in the figure. The rated current of the secondary RCDs is taken a step or two higher than that of “their own” machine: for VA-101-1 / 16 - 20 or 25 A; VA-101-1/32 - 40 or 50 A.
But this is in new houses, but in old ones, where protection is most needed: there is no land, the wiring is awful? Someone there promised to enlighten on the subject of connecting an RCD without earth. That's right, that's exactly what it came down to.
RCD without earth
Cited at the beginning of paragraph 7.1.80 exists in the PUE not in splendid isolation. It is supplemented with points explaining how, after all (well, there are no ground loops in our houses, no!) “Push” the RCD into the TN-C system. Their essence is as follows:
- It is unacceptable to install a common RCD or difavtomat on an apartment with TN-C wiring.
- Potentially dangerous consumers must be protected by separate RCDs.
- The protective conductors of sockets or socket groups intended for connecting such consumers must be brought to the INPUT zero terminal of the RCD in the shortest way, see the diagram on the right.
- RCD cascade connection is allowed, provided that the upper ones (closest to the RCD input) are less sensitive than the terminal ones.
A smart person, but unfamiliar with the intricacies of electrodynamics (which, by the way, many certified security electricians also sin) may object: “Wait a minute, what's the problem? We put a common RCD, start all PE at its input zero - and you're done, the protective conductor is not switched, grounded without ground! Yes, not so.
The electromagnetic field of the installation and the cord to it are also excluded from consideration. The first is concentrated inside the device, otherwise it will not pass certification and will not go on sale. In the cord, the wires pass close to each other, and their field is concentrated between them, regardless of frequency, this is the so-called. T-wave.
In an apartment with an increased fire hazard, it is permissible, with the obligatory presence of individual consumer RCDs connected according to the recommended circuit, to install a common FIRE RCD for 100 mA of unbalance and with a rated current one step higher than that of protective ones, regardless of the cutoff current of the machine. In the example described above, for Khrushchev, you need to connect an RCD and an automatic machine, but not a difautomatic! When the machine is knocked out, the RCD must remain in operation, otherwise the likelihood of an accident increases sharply. Therefore, the RCD at face value must be taken two steps higher than the machine (63 A for the disassembled example), and by unbalance - one step higher than the final 30 mA (100 mA). Once again: in difautomats, the RCD rating is made a step higher than the cut-off current, so they are not suitable for wiring without ground.
Video: RCD connection
Well, it's knocked out...
Why does the RCD work? Not how, it has already been described, but why? And what if it worked? Once knocked out, then something is wrong?
Right. You can’t just turn it on after a trip until its cause is found and eliminated. And you can find where something is “wrong” yourself without any special knowledge, tools and devices. A regular apartment electric meter will be of great help in this, unless it is completely antique.
How to find the culprit?
First, turn off all the switches, remove everything from the sockets. In the evening, you will have to use a flashlight for this; it is better to immediately attach a hook to the wall when installing next to the RCD and hang a cheap LED flashlight on it.
We turn off the access or main apartment machine. Doesn't turn on? Blame the electromechanics of the RCD; need to be sent in for repair. You can’t dig yourself - the device is vital, and after repair you need to check it on special equipment.
It turned on, but when the voltage was applied, it knocked out again with empty wiring? In the RCD, either the internal imbalance of the differential transformer, or the “Test” button is stuck, or the wiring is faulty.
We try to turn it on under voltage, looking at the counter. If the “Earth” indicator flashed even for a moment (see fig.), or earlier it was noticed that it winks, there is a leak in the wiring. You need to take measurements. If the RCD is installed in the order of reconstruction of the wiring and is registered with the energy service, you need to call the municipal electricians, they are required to check. If the RCD is "self-propelled" - pay a specialized company. The service, however, is not expensive: modern equipment allows for 15 minutes. find a leak in the wall with an accuracy of 10 cm.
But before calling the company, you need to open and inspect the sockets. Insect excrement gives excellent leakage from the phase to the ground.
The wiring does not inspire fear, they even turned it off section by section with automatic machines, but does the RCD knock out “on empty”? Fault inside it. Both the imbalance and sticking of the “Test” most often cause not condensation or intensive use, but all the same “cockroach poop”. In Rostov-on-Don, a case was noted when in a perfectly well-groomed apartment in the RCD, a nesting place was discovered ... Turkestan earwigs, who knows how they got there. Hefty, with huge powerful cerci (tweezers on the tail), terribly angry and biting. In the apartment, they did not show themselves in any way.
The RCD trips when consumers are connected, but there are no signs of a short circuit? We turn on everything, especially potentially dangerous ones (see the section on the classification of RCDs by indices), we try to turn on the RCD, again looking at the meter. This time, in addition to the "Earth", the glow of the "Reverse" indicator is possible; sometimes it is denoted "Return", next. rice. This indicates the presence of high reactivity, capacitance or inductance in the circuit.
You need to look for a defective consumer in reverse order; by itself, it may not reach the RCD before tripping. Therefore, we turn on everything, then turn off suspicious ones in turn, and try to turn them on. Turned on, finally? This is what it is, "reversible". For repairs, but not to electricians, but to "home appliances".
In apartments with TN-C-S wiring, it is possible that it is not possible to clearly determine the source of RCD operation. Then the likely cause is bad ground. Still retaining its protective properties, grounding no longer removes the higher components of the interference spectrum, and the protective conductors work as an antenna, similar to a TN-C apartment with a common RCD. Most often, this phenomenon is observed during periods of greatest drying and freezing of the soil. So what to do? It is obligatory to strain the building operator, let him bring the circuit to the norm.
About filters
One of the main sources of RCD malfunctions is interference from household appliances, and absorbing ferrite filters are an effective way to deal with them. Have you seen knobs - "bumps" on computer cords? This is what they are. Ferrite rings for filters can be bought at the radio store.
But for power ferrite absorbers, the magnetic permeability of the ferrite and the saturation magnetic induction in it are of decisive importance. The first should be at least 4000, and better - 10,000, and the second - at least 0.25 Tl.
The filter on one ring (at the top of the figure) can be built in with a "noisy" installation, if it is not under warranty, as close as possible to the network inlet. This work is for an experienced specialist, so the exact scheme is not given.
Several rings can simply be put on the power cord (in the figure below): from the point of view of electrodynamics, it doesn’t matter if the conductor is wrapped around the magnetic circuit or vice versa. In order not to cut the branded molded cord, you need to buy a plug, a socket block and a piece of a three-core cable. Ready-made power cords with ferrite noise absorbers are also sold, but it costs more than home-made prefabricated parts.