Probably, there is no longer such a person who would not have heard about the RCD (or in the decoding of the abbreviation - a residual current device). Actually, the term itself defines the purpose of this device. In other words, its role is to remove voltage from the electrical network connected to it in the event of an emergency.
This can prevent fire, which is usually caused by wiring fires. But what types of RCDs exist and what is the principle of operation of this protective device?
Leakage current
The functionality of an RCD is somehow related to this definition, but what does leakage current mean? In simple words, this is its flow from a conductor with a phase into the ground along a path that is not intended for this. For example, the metal case of any electrical appliance, water pipes, metal barsfittings, damp plastered walls.
There can be several reasons for current leakage:
- Aging of wires, which is inevitable during long-term operation.
- Mechanical damage.
- Thermal effect on the wiring when electrical equipment is operating in overload mode.
The danger of current leakage should not be underestimated. In the absence of a residual current device VD1-63 (for example) and if the insulation of the wires is broken on the above objects (the metal case of the device, etc.), a potential appears. As soon as a person touches them, he becomes a conductor, and the current will go into the ground through his body. At the same time, its value can be different, which causes certain consequences, up to death.
To guarantee personal safety, your home must be equipped with appropriate protective equipment. In particular, we are talking about RCDs or, alternatively, differential automata are suitable.
How does an RCD work?
Such devices, in addition to RCDs, also have other names:
- differential automata;
- residual current circuit breakers.
These definitions more accurately characterize these electrical appliances in terms of functionality and principle of operation. The operation of the RCD is as follows: the device is able to perceive the difference in current at the input (or otherwise it is called a phase) and at the output (in other words, zero).
You can draw some parallel and compare the principle of operation of a residual current device (RCD) withscales or balance. As long as the balance is maintained, everything functions normally. That is, the input value of the current is equal to the output. If the equilibrium changes, this affects the quality of the state of the entire system. In other words, if there is a discrepancy in the readings, the RCD breaks the circuit.
Such a difference at the input and output, at which the RCD is triggered, is selected an order of magnitude less than the value that can cause serious physical damage to a person. As a rule, it is 15-40 mA. RCDs can open the electrical circuit in the event of a breakdown on the case itself, and before the impact on a person.
Single phase circuit
In a single-phase circuit, the comparison of current values is carried out in relation to phase and zero, as, in fact, was described above. The mentioned balance is achieved only in the case of a complete insulating sheath of the wiring. An imbalance can occur if it is damaged, which causes leakage current.
Three-phase circuit
In a three-phase network, the principle of operation of the residual current device takes into account the values of the neutral conductor and the sum of the three phases. Actually, on this basis, the presence of an imbalance is determined. However, in any case, if there is any difference between the input and output current, this may indicate an insulating breakdown. That is, the fact of the presence of current leakage is present, which means the device will immediately work.
From theory to practice
Now let's take a look at some specific example of the information received. In the home electrical distribution boxa bipolar RCD installed. An introductory two-core cable (phase with zero) is connected to its upper contacts, and from the lower terminals the wiring (also phase and zero) goes to some kind of load. Let this be the outlet to which the water heater is connected. Protective grounding of the equipment case is carried out directly bypassing the RCD.
In the normal operation of the equipment, the electrons, starting their way from the input cable, pass through the RCD, move along the phase conductor to the heating element of the boiler. From there, the particles move already along the neutral wire towards the residual current device and are sent to the ground. In this case, the value of the current at the input and output is the same, only the directions are different.
If the wiring insulation is damaged, then part of the current through the coolant (water) began to flow to the device case, and after that it goes into the ground through the ground. The remainder will also rush to the RCD along the neutral wire, but in this case its value will be less than the incoming indicator, and by an amount equal to the value of the leakage current. This difference will be detected by the protection device and if the value is higher than the trip setting, it will open the circuit.
The RCD will work in a similar way when a person touches a case with potential or a bare live wire. Current leakage will occur through the human body, the device is able to instantly detect this and cut off the power supply.
More details about the RCD device and the principle of operation
Design of the protective devicewill allow you to more accurately imagine the principle of operation of the RCD and how exactly it is able to respond in a timely manner to a current leakage. Usually the device itself consists of the following main elements:
- residual current transformer;
- electric circuit breaking mechanism;
- electromagnetic relay;
- check node.
The transformer is connected to two opposite windings (phase and zero). During normal operation of the electrical network, these conductors create magnetic fluxes in the opposite direction in the transformer core. Due to this, their total value is equal to zero, since they mutually compensate each other - the balance is maintained.
The secondary transformer winding is connected to an electromagnetic relay and is still at rest. The occurrence of a current leakage immediately changes the situation. Different current values begin to flow along the “phase” and “zero”. Based on this, the value of magnetic fluxes on the transformer core will already be different from zero, that is, the balance is disturbed - the fluxes become different not only in direction, but also in value.
This causes a current in the secondary winding, and when its readings reach the set value, the electromagnetic relay is activated. This, in turn, is connected to a trip mechanism that immediately breaks the circuit.
Check knot
We have already familiarized ourselves with the principle of operation and the purpose of the device (RCD), but what is the role of the test node? In essence, this is the usualresistance (load connected bypassing the transformer). A similar mechanism simulates a current leakage, with the help of which the operability of the RCD is checked.
And how does such a check work? There is a special “TEST” button on the protective electrical device, it is designed to supply current from the phase to the test resistance, and then to the neutral, bypassing the transformer. Due to this, the value of the current at the input and output will be different, the created imbalance will put the trip mechanism into action.
If during the test the RCD did not turn off, you should refrain from installing it. This procedure must be carried out regularly - at least once a month. This is a basic fire safety requirement that should not be neglected!
Types of residual current devices
RCD classification involves several types of protective devices. In this case, various indicators act as a criterion:
- mounting method;
- number of poles;
- type of current in the network;
- delay time;
- activation method;
- rated current values.
Let's consider each of them separately.
Mounting method
According to this classification, protective devices can be fixed type, which are usually intended for installation in electrical distribution boards. In addition, there are portable devices, as well as adapters for installation in sockets.
Number of poles
Depending on the number of poles, protective devices can be two-pole orfour-pole. The first option is used in single-phase electrical circuits to protect a person from electric shock or to avoid a fire. Such devices have only two poles - for phase (L) and zero (N).
Four-pole RCDs are no longer two, but 4 terminals - three phases (L) and one zero (N). In other words, they are designed for use in a three-phase circuit.
Type of mains current
According to this criterion, RCDs, in turn, are divided into several subspecies.
Type A is, in fact, a variation of type AC, only takes into account the values of pulsating current. Based on this, the RCD-A type has a more complex design, and due to this, better protection is provided. Accordingly, for such protective shutdown devices, the price is noticeably higher than the RCD-AS type.
Type B - capable of handling DC and AC differential currents. As a rule, such protective devices are relevant for industrial facilities.
The type of AC corresponds to an alternating sinusoidal current, which increases gradually or suddenly. If necessary, the device reacts instantly.
Delay time
As for the delay time, the RCD type S has the value of this indicator is 0.1-0.5 seconds. It is recommended to install it if there are several protective devices. Type G instruments have a selection function and the delay time varies from 0.05 to 0.09 s. But there is also an RCD without trip delay.
Residual current device type Soften installed at the entrance of electricity to a residential building or private property for fire protection purposes.
Activation method
Here there is a division into subspecies - electromechanical and electrical protective devices. The first type does not depend on the value of the mains voltage. Their main indication of operation is the indication of differential current in the damaged zone.
As for electrical safety devices, it is important that there is voltage in the network. They require an external source to function. Compared to electromechanical RCDs, such devices are more reliable in use.
Rated current values
Here the division is as follows. Depending on the values of the rated load current, these are 16 A, 20 A, 25 A, 32 A, 40 A, 63 A, 80 A, 100 A (Amps). Based on the rated residual breaking current, these are 10 mA, 30 mA, 100 mA, 300 mA, 500 mA (milliamps).
Connecting an RCD
Residual current devices 25 A and others like that are specially designed for power supply circuits according to the TN-S or TN-C-S system with the connection of a protective neutral PE bus, which is connected to the cases of all electrical devices by means of wires.
It is worth noting that the RCD is not able to protect electrical wiring from short circuits and overloads. In this regard, it is necessary to have an automatic switch, and it must be located in front of the electric meter. This is the only way to ensuremaximum protection in your home.
It should be understood that RCD and circuit breaker are not the same thing. More on this later. As for the installation of RCDs, rooms with a high risk zone are:
- bathroom;
- kitchen;
- cellar;
- garage.
To protect the electrical wiring in these rooms, it is advisable to use protective devices.
Connection in the apartment
Modern residential buildings mainly use three-phase, and sometimes even five-phase circuits. However, in houses built back in the era of the USSR, the wiring is often single-phase, and in addition, the neutral and protective conductors are combined into one. In other words, there is no grounding element in such a system.
The connection diagram with RCD in the apartment will look like this:
- Introductory machine.
- Electricity meter.
- RCD 30 mA.
- Electrical wiring.
If the apartment has power consumers of electricity, which, for example, can be an electric oven or a washing machine, then you need to install an additional RCD.
Connection in a private home
The connection sequence for private real estate may look slightly different:
- Introductory machine.
- Electricity meter.
- RCD in the range of 100-300 mA, based on the amount of electricity consumed by all equipment that is available.
- Protective devices for individual current consumption. Usually in this case the range is alreadyless than 10-30mA.
You can make the connection, if necessary, by yourself or use the services of professional electricians.
The difference between an RCD and a circuit breaker
Now it should be clear what is the difference between a residual current device and a circuit breaker. The main feature is the different principle of operation of both devices. The role of automata is reduced mainly to saving connected electrical appliances from excessive current values. At the same time, they are able to withstand loads that are “too tough” for RCDs. What can be said about the safety of human life?!
For a better understanding, it is worth giving an example. There is an electrical device in which the body is grounded. At one fine moment, a short circuit occurs, to which the machine quickly reacts and de-energizes the entire circuit.
But otherwise, the insulation layer of the wire may be damaged. This can occur due to mechanical damage, wear over a long service life, moisture ingress. Or the case of the device is simply not grounded. Then a current leakage will inevitably occur, albeit a small one. In this case, the machine will not work, as it is not designed for such work.
Visually, the leak is also impossible to detect, but one has only to touch the body of the device, as a person can receive a serious discharge of current. This can be avoided if an RCD is present in the circuit. The residual current circuit breaker is able to detect even small leaks and immediately stopspower supply.
