Protective device. RCD: operating principle and why you need to install it
The abbreviation RCD is created from the phrase “Residual current device”, which defines the purpose of the device, which is to remove voltage from the circuit connected to it in the event of random insulation breakdowns and the formation of leakage currents through them.
Principle of operation
To operate the RCD, the principle of comparing currents entering and leaving the controlled part of the circuit is used based on a differential transformer, which converts the primary values of each vector into secondary values strictly proportional in angle and direction for geometric addition.
The comparison method can be represented by ordinary scales or a balancer.
When balance is maintained, everything works normally, but when it is disturbed, the qualitative state of the entire system changes.
In a single-phase circuit, the phase current vector approaching the measuring element and the zero current vector leaving it are compared. Under normal operating conditions with reliable intact insulation, they are equal and balance each other. When a fault occurs in the circuit and a leakage current appears, the balance between the vectors under consideration is upset by its value, which is measured by one of the transformer windings and transmitted to the logic block.
Comparison of currents in a three-phase circuit is carried out according to the same principle, only the currents of all three phases are passed through a differential transformer, and an unbalance is created based on their comparison. In normal operation, the currents of the three phases are balanced during geometric addition, and if the insulation of any phase is damaged, a leakage current occurs in it. Its value is determined by the summation of vectors in the transformer.
Structural scheme
The operation of a residual current device can be simplified block by block using a block diagram.
The imbalance of currents from the measuring element is sent to the logical part, which operates on the relay principle:
1. electromechanical;
2. or electronic.
It is important to understand the difference between them. Electronic systems are now rapidly developing and are becoming increasingly popular for many reasons. They have wide functionality and great capabilities, but require electrical power for the operation of logic and the executive body, which is provided by a special unit connected to the main circuit. If the electricity goes out for various reasons, then such an RCD, as a rule, will not work. The exception is rare electronic models equipped with this function.
Electromechanical relays use the mechanical energy of a charged spring, which is similar in principle to an ordinary mousetrap. For the relay to operate, a minimum mechanical force on the sensed actuator is sufficient.
Just as a mouse touches the bait of a prepared mousetrap, the leakage current that occurs when there is an imbalance in the differential transformer leads to the operation of the actuator and the disconnection of voltage from the circuit. For this purpose, the relay has built-in power contacts in each phase and a tester preparation contact.
Any type of relay has certain advantages and disadvantages. Electromechanical structures have been working reliably for many decades and have proven themselves well. They do not require external power, and electronic models are completely dependent on it.
It is now generally accepted that the most effective measure of protection against electric shock in electrical installations with voltages up to 1000 V is a residual current device (RCD).
Without objecting to the importance of this protection measure, most experts have been arguing for many years regarding the values of the main parameters of the RCD - installation current, response time and reliability.This is explained by the fact that the parameters of the RCD are closely related to its cost and operating conditions.
Indeed, the lower the setting current and the shorter the response time, the higher the reliability of the RCD, the more expensive its cost.
In addition, the lower the setting current and the shorter the response time of the RCD, the stricter the requirements for the insulation of the protected area, since even a slight deterioration in it under operating conditions can lead to frequent, and in some cases long-term, false shutdowns of the electrical installation, thereby making it impossible to normal operation.
On the other hand, the higher the RCD setting current and the longer its response time, the worse its protective properties.
RCD design
The layout of a single-phase RCD is shown in the picture below.
In it, voltage is supplied to the input terminals, and a controlled circuit is connected to the output terminals.
The three-phase residual current device is made in the same way, but it controls the currents of all phases.
The picture shown shows a four-wire RCD, although three-wire designs are commercially available.
How to check the RCD
Any design model has a built-in functionality check function. For this purpose, the “Tester” block is used, which is an open contact - a button with a spring self-reset and a current-limiting resistor R. Its value is selected to create a minimum sufficient current, artificially simulating a leak.
When you press the “Test” button, the RCD connected to operation should turn off. If this does not happen, then it should be rejected, look for a breakdown and repair or replace it with a working one. Monthly testing of the residual current device increases the reliability of its operation.
By the way, the serviceability of electromechanical and individual electronic structures can be easily checked in the store before purchase. For this purpose, it is enough, with the relay turned on, to briefly supply current to the phase or zero circuit from a battery with any connection polarity according to options 1 and 2.
A working RCD with an electromechanical relay will work, but in the vast majority of cases electronic products cannot be checked this way. They need power for their logic to work.
How to connect an RCD to a load
Residual current devices are designed for use in power supply circuits using the TN-S or TN-C-S system with the connection of a protective zero PE bus in the electrical wiring, to which the housings of all electrical devices are connected.
In this situation, if the insulation fails, the potential arising on the housing immediately flows through the PE conductor to the ground and the comparison body calculates the fault.
In normal power supply mode, the RCD does not disconnect the load, so all electrical appliances operate optimally. The current of each phase in the magnetic circuit of the transformer induces its own magnetic flux F. Since they are equal in magnitude, but opposite in direction, they cancel each other out. There is no total magnetic flux and cannot induce an emf in the relay winding.
When a leak occurs, the dangerous potential flows to the ground through the PE protective busbar. An EMF is induced in the relay winding from the resulting imbalance of magnetic fluxes (currents in phase and zero).
The residual current device instantly calculates the fault in this way and de-energizes the circuit with power contacts in a split second.
Features of operation of an RCD with an electromechanical relay
Using the mechanical energy of a charged spring in some cases may be more profitable than using a special unit for electrical power supply of the logic circuit. Let's consider this using an example when the zero of the supply network is broken, and the phase is supplied.
In such a situation, the static electronic relays will not receive power and therefore will not be able to operate. At the same time, in this situation, the three-phase system experiences a phase imbalance and an increase in voltage.
If an insulation breakdown occurs in a weakened area, the potential will appear on the housing and will flow through the PE conductor.
In an RCD with an electromechanical protection relay, they will operate normally from the energy of a charged spring.
How does an RCD work in a two-wire circuit?
The undeniable advantages of protection against leakage currents in electrical equipment made according to the TN-S system through the use of RCDs have led to their popularity and the desire of individual apartment owners to install RCDs in two-wire systems not equipped with a PE conductor.
In this situation, the body of the electrical appliance is isolated from the ground and does not communicate with it. If an insulation breakdown occurs, the phase potential appears on the housing and does not drain from it. A person who has contact with the ground and accidentally touches the device is exposed to leakage current in the same way as in a situation without an RCD.
However, in a circuit without a residual current device, current can pass through the body for a long time. When the RCD is installed, it will sense a malfunction and turn off the voltage during the setting time within a fraction of a second, thereby reducing the degree of electrical injury.
Thus, the protection facilitates the rescue of a person when energized in buildings equipped according to the TN-C scheme.
Many home craftsmen try to independently install RCDs in old houses awaiting reconstruction to switch to the TN-C-S system. In this case, in the best case, they make a homemade grounding loop or simply connect the housings of electrical appliances to the water supply network, heating radiators, and iron parts of the foundation.
Such connections can create critical fault situations and cause serious damage. Work on creating a grounding loop must be carried out efficiently and controlled by electrical measurements. Therefore, they are performed by trained specialists.
Types of fastening
Most RCDs are made in a stationary design for mounting on a common Din-rail in an electrical panel. However, on sale you can find portable structures that are plugged into a regular electrical outlet, and the protected device is then powered from them. They cost a little more.
Without electricity it is impossible to imagine modern civilization. Progress has given people many electrical appliances that have made life much easier. So, now, when cleaning rooms, you don’t need to wave a broom, raising clouds of dust, but just turn on the vacuum cleaner; to boil a kettle, you do not need to inflate the samovar, but you can use an electrical appliance; ironing clothes is done without a massive charcoal iron, etc.
A feature of modern devices is their high power consumption, which requires modernization of the wiring that residents of houses and apartments have inherited since Soviet times. Anyone who has decided to take this step must have at least a general idea of what an RCD is. The residual current device, although not indispensable, significantly increases electrical safety. Today we will talk about why exactly a protective RCD is needed, and we will also explain in simple language the principle of its operation.
electrical safety
A mandatory element of any home electrical network (we will talk about this case later) is a circuit breaker. This device is mounted near the electric meter or in a special panel, and it is called an input device. Its task is simple: to perform switching, and also to interrupt the power supply without human intervention in the event of a sharp excess of the rated current (electromagnetic protection) or during a prolonged load above the permissible limits (thermal setting). A properly selected circuit breaker can prevent a wiring fire and partially protect a person from possible electrical injury. However, the protective functions are significantly expanded when another device is installed - an RCD. The installation points may coincide with the installation locations of conventional switches.
How does “classic” protection work?
In order to understand the purpose of a residual current device, let's present a simple example from life. The home electrical network is equipped with an automatic switch at the input, selected in accordance with the PUE. In any operating electrical appliance, insulation damage and a short circuit occur, as a result of which the current consumption increases to a value determined by the wiring features, and the electromagnetic release in the input switch registers this and breaks the circuit. It would seem, why do we need another RCD? But let’s imagine that due to damage in the iron, its metal parts are at dangerous potential. A person who is unlucky enough to touch such a device and a cast-iron heating radiator (bathtub, sink) at the same time will receive an electric shock flowing through the body to the “ground”.
Features of slot machines
Only specialists know that the protection of a class “C” switch will operate at 10 times the rated value; for “B” the situation is slightly better, and the response threshold will be half as large; Well, for class “A” the shutdown will occur when the nominal value is doubled. These are quite high values, and under certain circumstances, the “lucky” person has the risk of staying with the above-mentioned iron forever. If you consider that most apartments and houses are “protected” by C-class switches, then there is reason to think about your own safety. The result will be completely different if there is an RCD switch in the circuit.
Additional feature
Let's imagine the same situation, but we will supplement the machine with a residual current device (RCD). A person touches a conductive surface, and a current begins to flow through the body, which goes into the “ground”.
Its peculiarity is that, although the meter takes into account the ampere-hours consumed, and an electromagnetic field is created in the release coil, nothing is returned back to the network. The RCD machine just registers this and breaks the circuit. As a result, a person will feel an electric shock (the magnitude depends on the parameters of the device), but there will be no death.
For those people who are accustomed to using electric boilers to heat water, we recommend not only learning what an RCD is, but also installing this device as soon as possible. It is important to understand that although a residual current device makes the operation of equipment safer, it is not a panacea for all problems. And it cannot replace the need to use a protective grounding loop.
What is an RCD
A residual current device is an electromechanical device designed to improve electrical safety when using electrical equipment. Various designs are possible, but the most famous are solutions for mounting on a DIN strip, like modern single-pole circuit breakers. A plastic case, a shut-off tab and a button for checking the operation of the circuit - that’s all the RCD looks like. The heads of the clamping bolts are recessed in such a way that accidental contact with them is almost impossible. Installation of RCDs can be carried out in two ways: in the input panels, which protects the entire home electrical network, as well as on each line. In the second case, the protection is more effective. If funds are available, it is recommended to combine these two methods.
Physically, the connection is very simple: there are four bolt clamps on the case (for a single-phase network), the first two of which are connected to the input wires, and the second ones are screwed to the outgoing lines. That is, the RCD is installed in a circuit break. The only caveat: the contacts on the inlet are marked for zero and phase, which must be observed during installation for further correct operation. The simplest indicator allows you to identify a phase wire in a few seconds.
Operation
When studying what an RCD is, one cannot ignore the principle of its operation. Two lines pass through the entire device (zero and phase), which can be broken at any time by a trip electromagnet (the same system as the release in conventional switches). The current flowing through the lines induces an emf in the coil. Since its values in the phase and neutral wires are equal, there is potential in the coil, but there is no current - it is balanced. This is in the normal state of the protected circuit. Any leakage from a closed circuit causes the appearance of an induced current (tens of milliamps) and the activation of the shutdown electromagnet.
Looking at a real life example
Let's imagine that a person is taking a bath, the water for which is heated by an electric boiler. The socket for the heater is protected by an RCD. For some reason, the heating element breaks down the spiral onto the housing. Because of this, the entire mass of accumulated water is under dangerous potential, and voltage enters the bath through metal parts. If it is not dielectric and is installed on a conductive floor (most often this is the case), then current begins to flow to the “ground” through the heating element - water - bath circuit. A person, touching metal objects, is somehow included in the chain, falling under the influence of EMF.
As long as there was no damage to the heating element, the magnitudes of the current flowing through the phase and neutral wires through the RCD were equal. That is, in simple terms, as much has come, so much has gone. After all, the circuit is closed. But as soon as a breakdown occurred and a third-party current flow path was formed, the equality ceased to hold, and more was supplied to the boiler than returned. The magnetic field that appears in the RCD coil activates the shutdown mechanism - and the circuit breaks. Everything is very simple. If protection was carried out only by the electromagnetic release of the circuit breaker, then the circuit would break when the rated current exceeded 2-3 times (for class A) or even 10 times (for class C). Needless to say, all this flow of electrons could come to a person if he was holding a shower hose in his hands and standing barefoot on a conductive floor?
There is also a three-phase RCD. In this device, not two wires pass through the coil, but four: one for each phase and zero. In this case, it does not matter how much load falls on each phase, the main thing is that the total incoming current is equal to the returning one.
Peculiarity
Earlier we said that an RCD cannot be a replacement for grounding. Let's imagine that a person touches the neutral and phase wires at the same time. Current will flow through the body, however, since there will be no leakage from the circuit, the RCD will not work. But when using a grounded circuit on the housings of electrical appliances, a dangerous potential cannot appear, since the current will immediately flow through the grounding wire into the ground, which will fix the machine and interrupt the power supply.
Many people know that in order to ensure safe operation of electrical wiring, it is necessary to install not only automatic devices, but also RCDs. But not everyone understands why they install it, as well as how this additional device works.
A residual current device (RCD) is designed to conduct currents under normal operating conditions and disconnect them by opening the contacts when a current leak occurs, which can arise for various reasons:
- When a person or animal touches live parts and an electric current flows through them of such a magnitude that can cause injury.
- In case of insulation damage and current leakage to the grounded housing.
- When there is contact between the neutral working (N) conductor and the protective grounding (PE) conductor.
- When changing the zero working (N) and zero protective (PE) conductors.
- When the zero (N) breaks or burns, which is accompanied by a person or animal touching the phase (L) conductor and current flowing through the body in contact with the ground.
When a phase is shorted to the body of an electrical installation, if for some reason it has not been grounded, there is a danger of electric shock when touching the elements of the body. To protect human health in such situations, a Protective Shutdown Device was created. If you accidentally touch live wires, the current flowing through a person to the ground can reach a dangerous value, sufficient to cause a tragic outcome.
The RCD device will de-energize a dangerous electrical installation at the moment a leakage current occurs (through a person to the ground or through grounded parts of the electrical installation), thereby preventing electric shock or the possibility of a fire.
It is obvious that an RCD can protect humans and animals in most situations that lead to electric shock, therefore in developed countries the use of an RCD is mandatory, and some manufacturers even integrate an RCD into the electrical circuits of their devices. The use of RCDs in Russia is also recommended by the Electrical Installation Rules (PUE), 7th edition of 1999.
What is the operating principle of protective devices based on?
The operation of RCD devices of electromechanical and electronic types is based on the measurement of leakage currents. The sensitive element capable of measuring leakage currents in the RCD is a differential transformer with three windings.
Such a transformer is a conventional current transformer on a toroidal core made of a ferromagnet. The first two windings in this transformer are formed by phase and neutral wires, through which current flows to the load and back. During normal operation, these currents are equal and the magnetic fluxes induced in the transformer core will also be equal, but directed in the opposite direction, which will cause their mutual compensation.
The result of adding magnetic fluxes will be a value equal to zero. And, therefore, nothing will happen in the third winding (control winding). If an insulation breakdown occurs on the housing, or a person touches live parts, then a current will flow through the phase wire, which will be greater in magnitude than the current in the neutral wire.
An increase in current in the phase wire will cause an increase in the magnetic flux generated in the first winding from the phase wire, and the total flux of magnetic induction will change and become non-zero. The resulting magnetic flux will cause current to be induced in the control winding connected to the high-precision executive relay, and if there is sufficient current in the control winding circuit, the relay will operate, which will set the release of the power contacts of the protective device in motion.
A dangerous electrical installation will be left without electricity. The response time of the RCD is a fraction of a second, during which the electric current cannot harm a person.
What is the difference between electronic and electromechanical devices?
The difference in the operation of electronic and electromechanical ouzo is the need of the former for an additional power source. This is due to the fact that the signal removed from the control winding of the differential transformer has low power. To amplify it, a special electronic circuit is used, which increases the signal magnitude many times and sends a powerful electrical impulse to the electromagnetic coil of the current release of the main contacts of the tripping device.
Typically, an electronic circuit is powered by the circuit it protects. At the same time, in the event of a break in the neutral wire, the electronic circuit breaker will remain without supply voltage and will not be able to protect the object.
Differences between devices designed for three-phase circuits
In three-phase electrical circuits, special RCDs are also used, designed specifically for three-phase consumers of electrical energy. The operating principle of these devices is based on comparing the differences in the sums of currents across all phase conductors (L1, L2, L3) with zero, which under normal conditions should be zero. When a current leak occurs, similar to a single-phase RCD, a splitting device is triggered, preventing electric shock or the occurrence of large leakage currents.
The difference between the devices is only in the design. If a 2-pole device has one differential transformer, then a 4-pole device has 3 of them - one for each phase. Accordingly, there are 3 actuators, each of which is connected to the release mechanism of the power contacts of the device.
The tripping of the release occurs from any phase in which the ouzo detects the flow of leakage current, regardless of the state of the others.
The operation of protective shutdown devices of the electronic or electromechanical type is based on the conversion of a low-power electrical signal taken from the control winding of the sensitive element into a mechanical action on the release mechanism.
Video about why install an RCD
An easy to understand explanation of why RCDs are installed and what types they are.
Basics purpose of RCD is the protection of people from electric shock in the event of a malfunction of electrical equipment (being energized as a result of insulation damage) as a result of accidental or unconscious human contact with live parts.
Also preventing fires caused by ignition of electrical wiring due to the flow of leakage currents.
Operating principle of RCD
Operating principle of RCD? - Many people ask this question.
As you know from an electrical engineering course, electric current flows from the network along the phase wire through the load and returns back to the network through the neutral wire. This pattern formed the basis for the operation of the RCD.
If these currents are equal, I in = I out, the RCD does not respond. If I in > I out, the RCD senses a leak and trips.
That is, the currents flowing through the phase and neutral wires must be equal (this applies to a single-phase two-wire network; for a three-phase four-wire network, the current in the neutral is equal to the sum of the currents that flow in the phases). If the currents are not equal, then there is a leak, to which the RCD reacts.
Let us consider the principle of operation of the RCD in more detail.
The main element of the design residual current devices is a differential current transformer. This is a toroidal core on which windings are wound.
During normal operation of the network, the electric current flowing in the phase and neutral wires creates alternating magnetic fluxes in these windings, which are equal in magnitude but opposite in direction. The resulting magnetic flux in the toroidal core will be equal to:
Ф ∑ = Ф L - Ф N = 0
As can be seen from the formula, the magnetic flux in the toroidal core of the RCD will be equal to zero, therefore, the EMF will not be induced in the control winding, and the current in it, accordingly, will not be induced either. In this case, the residual current device does not work and is in sleep mode.
Now imagine that a person touched an electrical appliance that, as a result of insulation damage, was exposed to phase voltage. Now, in addition to the load current, an additional current will flow through the RCD - leakage current.
In this case, the currents in the phase and neutral wires will not be equal. The resulting magnetic flux will also not be zero:
Under the influence of the resulting magnetic flux, an EMF is excited in the control winding, and under the influence of the EMF, a current arises in it. The current generated in the control winding activates a magnetoelectric relay that disconnects the power contacts.
The maximum current in the control winding will appear when there is no current in one of the power windings. That is, this is a situation when a person touches a phase wire, for example in a socket, in this case no current will flow in the neutral wire.
Despite the fact that the leakage current is very small, RCDs are equipped with magnetoelectric relays with high sensitivity, the threshold element of which is capable of responding to a leakage current of 10 mA.
Leakage current This is one of the main parameters by which an RCD is selected. There is a scale of rated differential tripping currents of 10 mA, 30 mA, 100 mA, 300 mA, 500 mA.
It should be understood that the residual current device reacts only to leakage currents and does not work during overloads and short circuits. The RCD will not work even if a person simultaneously touches the phase and neutral wires. This happens because the human body in this case can be represented as a load through which an electric current passes.
Because of this, instead of RCDs, differential circuit breakers are installed, which by their design combine both an RCD and a circuit breaker.
Checking the functionality of the RCD
In order to monitor the serviceability (operability) of the RCD, a “Test” button is provided on its body, when pressed, a leakage current (differential current) is artificially created. If the residual current device is working properly, then when you press the “Test” button it will turn off.
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Today we will discuss with you an interesting article on the principle of operation of an RCD.
What is an RCD? Why is it needed?
The residual current device (RCD) is intended for:
- protecting people from the occurrence of a fault in an electrical installation
- disconnecting the voltage in case of accidental or mistaken contact with live parts of the electrical installation during a current leak
- protection against ignition of electrical wiring during a ground fault (housing)
Alternatives to RCDs have appeared on the electrical goods market - these are differential circuit breakers. Their peculiarity is that they combine both an RCD and a circuit breaker.
Differential machines take up less space, but the cost is several times higher. But we will talk about all the features of differential machines in the following articles. In order not to miss anything interesting, subscribe to receive news.
The operating principle of the RCD is based on the response of the current sensor to the changing input value of the differential current in the conductors.
The current sensor is an ordinary one, which is designed in the form of a toroidal core. The operating current setting is set on a magnetoelectric relay, which has very high sensitivity.
RCDs made with a relay control element are very reliable and trouble-free.
But the development of electrical engineering does not stand still, so not so long ago electronic RCDs appeared, in which the controlling body is not a relay, but a special electronic circuit.
The relay acts on the actuator, which in turn opens the electrical circuit.
The actuator consists of:
- contact group (selected for maximum current - look at the RCD passport)
- springs (to open the electrical circuit in case of abnormal operation)
To independently check the serviceability of the RCD, you must press the “Test” button. This creates an artificial current leakage, which is sufficient to trigger the RCD. Thus, you can independently check the RCD without involving specialists. The RCD check using the “Test” button must be carried out monthly. For a more thorough check of the RCD, we carry out.
Now we will look at the principle of operation of the RCD in more detail.
RCD operation under normal network conditions
In the normal state of electrical wiring (without leaks), the operating current (I1=I2) flows counter-parallel and induces magnetic fluxes (Ф1=Ф2) of the same magnitude in the secondary winding of the current transformer, which compensate each other. At this moment, the relay does not operate, because the current in the secondary winding of the current transformer is close to zero.
RCD operation during leakage
In case of accidental or mistaken contact with live parts of an electrical installation, a leakage current appears. At this moment, the magnitude of the currents passing through the current transformer is disrupted (I1 is not equal to I2), so a current (not balance) will appear in the secondary circuit of the current transformer, which will be sufficient to trigger the relay. The relay activates the spring mechanism and the RCD is switched off.
See the figure below for what the RCD looks like from the inside.