Basics purpose of RCD is to protect people from harm 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 variables in these windings magnetic fluxes, 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 let’s imagine that a person touched an electrical appliance that, as a result of damage to the insulation, found itself under 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 human body in this case, it can be thought of 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.
The 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 possibility of fires that could be caused by a fire in the electrical wiring.
In certain situations, the RCD, the operating principle of which is quite simple, stops supplying the protected voltage line. This happens if a person touches live parts of electrical installations, and non-current-carrying elements that become energized as a result of an insulation breakdown. Another reason for contact opening is the occurrence of current leakage to the electrical installation body or ground.
Consideration of the operating principle of RCDs in general and with a specific example
When inexpensive apartments are rented from a developer, all electrical equipment, including RCDs and automatic circuit breakers, as well as wiring and circuit breakers, are already 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.
An RCD (the operating principle is based on determining incoming and outgoing currents at the entrance to the system) can respond to minimal leakages and perform its protective function. To measure leakage, a sensitive element such as a differential transformer with three windings is installed in the device.
The principle of operation of an RCD can be easily understood using a specific example. If a person touches the live parts of the installation, or an insulation breakdown occurs on its body, the amount of current flowing through the phase wire will exceed the amount of current in the neutral wire.
The total (resulting) flux of magnetic induction, in this case, will certainly change, will differ from zero and will cause the induction of current in the control winding. The relay to which the winding is connected will operate, and the contact release of the power protective device will be set in motion.
As a result, a dangerous electrical installation is de-energized in a split second, ensuring the safety of human health.
Connecting an RCD to a single-phase network: basic rules
The RCD diagram is indicated on the device body and allows you to understand the principle of its operation, correctly connect the device to the electrical circuit protection circuit, avoiding incorrect operation of the device or its failure.
The RCD circuit by which it is connected to the power supply system depends on various parameters and factors. In residential premises, as a rule, single-phase 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 single-phase network, but also familiarize yourself with the safety rules.
The principle of operation of the RCD and the connection diagram imply the use of two wiring wires connected to the input terminals, and two wires to the output of the device, connected to the corresponding output terminals. The device should only be installed when the voltage is turned off. Before installation, you need to make sure that there is enough space in the panel for the selected device.
And its connection diagram is quite simple. There are several options for installing this device, but the principle, in general, remains the same.
The most common and affordable option is to place the device at the entrance to the house/apartment. The disadvantage of this option is that when the device is triggered, the entire living room 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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RCD with rated current 40 A
RCBO with overcurrent protection OptiDin VD63 with rated current up to 63A
Residual current device(abbr. RCD; more precise name: residual current device controlled by differential (residual) current, abbr. RCD-D) or residual current switch (VDT) or protective switching device (ZOU) - a mechanical switching device or a set of elements that, when the differential current reaches (exceeds) a given value under certain operating conditions, should cause the contacts to open. May consist of various individual elements designed to detect, measure (compare with a given value) differential current and close and open an electrical circuit (disconnector).
The main task of the RCD is to protect people from electric shock and from fire caused by current leakage through worn wire insulation and poor-quality connections.
Combined devices that combine an RCD and an overcurrent protection device are also widely used; such devices are called RCD-D with built-in overcurrent protection, or simply diffautomatic. Often differential automatic devices are equipped with a special indication that allows you to determine for what reason the operation occurred (from overcurrent or differential current).
Purpose
RCDs are intended for
- Protecting people from electric shock when indirect touch(person touching open conductive non-current-carrying parts of an electrical installation that become energized in the event of insulation damage), as well as when direct touch(person touching live parts of an electrical installation that are energized). This function is provided by an RCD of appropriate sensitivity (cut-off current no more than 30 mA).
- Preventing fires when leakage currents occur on the housing or on the ground.
Goals and operating principle
The operating principle of an RCD is based on measuring the balance of currents between the current-carrying conductors entering it using a differential current transformer. If the balance of currents is disturbed, then the RCD immediately opens all included in it contact groups, thus disconnecting the faulty load.
The RCD measures the algebraic sum of currents flowing through the controlled conductors (two for a single-phase RCD, four for a three-phase, etc.): in the normal state, the current “flowing” through one conductors should be equal to current, “flowing” through others, that is, the sum of the currents passing through the RCD is zero (more precisely, the sum should not exceed the permissible value). If the amount exceeds the permissible value, then this means that part of the current passes in addition to the RCD, that is, the controlled electrical circuit is faulty - there is a leak in it.
Detection of leakage currents using an RCD is an additional protective measure, and not a replacement for overcurrent protection using fuses, since the RCD does not react in any way to faults if they are not accompanied by a current leak (for example, a short circuit between the phase and neutral conductors).
RCDs with a residual current of about 300 mA or more are sometimes used to protect large areas of electrical networks (for example, in computer centers), where a low threshold would lead to false alarms. Such low-sensitivity RCDs perform a fire-fighting function and are not effective protection from electric shock.
Example
Internal structure of the RCD connected to the power cord break
The photo shows internal structure one of the types of RCD. This RCD is intended for installation into a power cord break; its rated current is 13 A, with a differential tripping current of 30 mA. This device is:
- RCD with auxiliary power supply;
- performing automatic shutdown when the auxiliary source fails.
This means that the RCD can only be turned on if there is supply voltage; if the voltage fails, it will automatically turn off (this behavior increases the safety of the device).
The phase and neutral conductors from the power source are connected to contacts (1), the RCD load is connected to contacts (2). The protective grounding conductor (PE conductor) is not connected to the RCD in any way.
When you press the button (3), the contacts (4) (as well as another contact hidden behind the node (5)) close, and the RCD passes current. The solenoid (5) keeps the contacts closed after the button is released.
The coil (6) on the toroidal core is the secondary winding of the differential current transformer, which surrounds the phase and neutral conductors. The conductors pass through the torus but have no electrical contact with the coil. In the normal state, the current flowing through the phase conductor is exactly equal to the current flowing through the neutral conductor, but these currents are opposite in direction. Thus, the currents cancel each other out and there is no EMF in the coil of the differential current transformer.
Any leakage of current from the protected circuit to grounded conductors (for example, a person standing on a wet floor touching a phase conductor) leads to an imbalance in the current transformer: “more current flows through the phase conductor” than returns through the zero conductor (part of the current flows through human body, that is, in addition to the transformer). An unbalanced current in the primary winding of a current transformer leads to the appearance of an emf in the secondary winding. This EMF is immediately recorded by the tracking device (7), which turns off the power to the solenoid (5). The disconnected solenoid no longer holds the contacts (4) in the closed state, and they open under the action of the spring force, de-energizing the faulty load.
The device is designed in such a way that shutdown occurs in a split second, which significantly reduces the severity of the consequences of electric shock.
The test button (8) allows you to check the functionality of the device by passing a small current through the orange test lead (9). The test wire passes through the core of the current transformer, so the current in the test wire is equivalent to an imbalance of the current-carrying conductors, that is, the RCD should turn off when the test button is pressed. If the RCD does not turn off, it means it is faulty and must be replaced.
Application
In Russia, the use of RCDs became mandatory with the adoption of the 7th edition of the Electrical Installation Rules (PUE). Typically, in the case of household electrical wiring, one or more RCDs are mounted on a DIN rail in the electrical panel.
Many manufacturers of household devices that can be used in damp areas (for example, hair dryers) provide a built-in RCD for such devices. In a number of countries, such built-in RCDs are mandatory.
Conditions for triggering the RCD:
- Direct human contact with live parts and contact with the ground.
- Damage to the main insulation and contact of live parts with a grounded body.
- Replacement of neutral and grounding conductors.
- Replacement of phase and neutral conductors and human contact with live parts and its simultaneous contact with the “ground”.
- Breakage of the neutral conductor before (and after the RCD) and a person touching live or live parts and simultaneously making contact with the ground.
Examination
It is recommended to check the performance of the RCD monthly. The easiest way to check is to press the " test”, which is usually located on the body of the RCD (as a rule, the “test” button has an image of a large letter “T”). The button test can be performed by the user, meaning that qualified personnel are not required. If the RCD is working properly and connected to electrical network, then when you press the “test” button it should work immediately (that is, turn off the load). If after pressing the button the load remains energized, then the RCD is faulty and must be replaced.
The push button test is not a complete test of the RCD. It may be triggered by a button, but will not pass a full laboratory test, which includes measuring the residual current and operating time.
In addition, pressing the button checks the RCD itself, but not whether it is connected correctly. Therefore, a more reliable test is to simulate a leak directly in the circuit, which is the load of the RCD. It is advisable to perform such a test at least once for each RCD after its installation. Unlike pressing a button, a test leak must be carried out only qualified personnel.
Restrictions
An RCD can significantly improve the safety of electrical installations, but it cannot completely eliminate the risk of electric shock or fire. The RCD does not respond to emergency situations, if they are not accompanied by leakage from the protected circuit. In particular, the RCD does not respond to short circuits between phases and neutral.
The RCD will also not work if a person is under voltage, but no leakage has occurred, for example, when a finger touches both the phase and neutral conductors at the same time. Provide electrical protection from such touches is impossible, since it is impossible to distinguish the flow of current through the human body from the normal flow of current in the load. In such cases, only mechanical protective measures are effective (insulation, non-conductive casings, etc.), as well as disconnecting the electrical installation before servicing it.
Some types of RCD ( RCD-D with auxiliary power supply, see ) need power, which they receive from the protected circuit. Therefore, a potentially dangerous situation is when in the protected circuit above the RCD the neutral conductor is disconnected, but the phase conductor remains energized. In this case, the RCD will be unable to disconnect the circuit, since the potential difference in the protected circuit is insufficient for the operation of the RCD. The so-called electromechanical RCDs do not require power and are therefore free from this disadvantage.
Story
In the early 1970s, most RCDs were produced in circuit breaker-type enclosures. Since the early 1980s, in the United States, most household RCDs have been built into electrical outlets. In Russia, RCDs began to be used much later - approximately from 1994-1995. And until now, RCDs are mainly used for installation in an electrical panel on a DIN rail, and built-in RCDs have not yet become widespread.
RCD classification
By mode of action
- RCD without auxiliary power supply
- RCD-D with auxiliary power source:
- performing automatic shutdown in case of failure of the auxiliary source with and without time delay:
- automatically restarting when the auxiliary source is restored
- not automatically restarting when the auxiliary source is restored
- not producing automatic shutdown in case of failure of the auxiliary source:
- capable of shutting down if a dangerous situation arises after failure of an auxiliary source
- unable to shut down in the event of a hazardous situation following failure of an auxiliary source
- performing automatic shutdown in case of failure of the auxiliary source with and without time delay:
By installation method
- stationary with installation of fixed electrical wiring
- portable with installation by flexible wires with extension cords
By number of poles
- single-pole two-wire
- bipolar
- two-pole three-wire
- three-pole
- three-pole four-wire
- four-pole
By type of protection against overcurrent and overcurrent
- without built-in overcurrent protection
- with built-in overcurrent protection
- with built-in overload protection
- with built-in protection against short circuits
For loss of sensitivity in case of double grounding of the neutral working conductor
Under consideration
If possible, regulate the disconnecting differential current
- unregulated
- adjustable:
- with discrete regulation
- with smooth regulation
In terms of resistance to impulse voltage
- allowing the possibility of switching off during impulse voltage
- impulse voltage resistant
According to operating conditions
- RCD-D type AC - a protective shutdown device that responds to an alternating sinusoidal differential current that occurs suddenly or slowly increases;
- RCD-D type A - a protective shutdown device that responds to alternating sinusoidal differential current and pulsating direct differential current, occurring suddenly or slowly increasing;
- RCD-D type B. The RCD responds to alternating, direct and rectified differential currents.
- RCD-D type S - selective (with a shutdown time delay), this may be necessary where an automatic transfer switch is used.
- RCD-D type G - the same as S, but with a shorter time delay.
Application RCD type It is advisable in certain cases, for example, in circuits containing consumers with thyristor control without an isolating transformer. Type B RCDs are used in industrial electrical installations with mixed power supply - alternating, rectified and direct currents.
RCD characteristics
Characteristics common to all RCDs
³=== Only for RCD-D without built-in short circuit protection ===
- Type of short circuit protection
- Rated conditional short-circuit current Inc - the effective value of the expected current specified by the manufacturer, which the RCD-D, protected by the short-circuit protection device, can withstand under given operating conditions without irreversible changes that impair its functionality
- Rated conditional differential current at short circuit I Δc - the value of the expected differential current specified by the manufacturer, which the RCD, protected by the short-circuit protection device, can withstand under given operating conditions without irreversible changes that impair its functionality
See also
Notes
Links
- GOST R 50807-95 (2003) Protective devices controlled by differential (residual) current. General requirements and test methods (IEC 755-83).
- SP 31-110-2003 “Design and installation of electrical installations of residential and public buildings”
- HTML version of the educational and reference manual "UZO". Publishing house "Energoservice", 2003.
Dear guests, I am glad to welcome you to the pages of the Electrician’s Notes website.
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 about all the features differential automata we'll talk about it in future 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.