Probably, there is no longer such a person who would not hear about RCD (or in decoding of the abbreviation - protective shutdown device). Actually, the term itself determines the purpose of this device. In other words, its role is to relieve voltage from the connected electrical network in the event of an emergency.
Due to this, it is possible to prevent the occurrence of a fire, which is usually caused by a fire in the wiring. But what types of RCDs exist and what is the principle of operation of this protective device?
Leakage current
The functionality of the RCD is somehow related to this definition, but what does the leakage current mean? To put it in simple words, then this is its flow from a conductor with a phase to the ground along a path that is not intended for this. For example, the metal casing of any electrical appliance, water pipes, metal bars of reinforcement, raw plastered walls.
The causes of current leakage can be several:
- Aging of wires, which is inevitable during long-term operation.
- Mechanical damage.
- Thermal impact on the wiring when the electrical equipment is in overload mode.
The risk of current leakage should not be underestimated. In the absence of a protective shutdown device VD1-63 (for example) and in case of violation of the insulation of the wires at the above objects (metal case of the device, etc.), a potential appears. Once a person touches them, he becomes a conductor, and the current will go into the earth through his body. At the same time, its value can be different, which causes certain consequences, up to a fatal outcome.
To guarantee personal safety, your home must be equipped with appropriate protective equipment. In particular, we are talking about RCDs or, as an option, differential automata are suitable.
How does an RCD work?
Such devices, in addition to RCDs, also have other names:
- differential automata;
- differential current machines.
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 else it is called a phase) and at the output (in other words, zero).
You can draw a certain parallel and compare the principle of operation of the residual current device (RCD) with a balance or a balancer. When equilibrium is maintained, everything functions normally. That is, the input current value is equal to the output. If the equilibrium changes, this affects the quality of the state of the entire system. In other words, with a discrepancy in the testimony, the RCD breaks the circuit.
Such a difference at the input and output, at which the RCD is triggered, is selected by 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 an electric circuit in case of breakdown on the case itself, and up to the moment of exposure to a person.
Single phase circuit
In a single-phase circuit, current values are compared in relation to phase and zero, as, in fact, as described above. Mentioned equilibrium is achieved only in the case of a whole insulating sheath of wiring. Imbalance can occur if it is damaged, which causes leakage current.
Three phase circuit
In a three-phase network, the operating principle 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 imbalance is determined. Moreover, 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 we will analyze the information obtained using a specific example. A two-pole RCD is installed in the home electrical distribution panel. An input two-wire cable (phase with zero) is connected to its upper contacts, and wiring (also phase and zero) goes to some load from the lower terminals. Let it 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 mode of operation of the equipment, the electrons, starting their way from the input cable, pass the RCD, move along the phase conductor to the boiler heater. From there, the particles move along the neutral wire towards the protective shutdown device and are sent to the ground. In this case, the current value at the input and output is the same, only the directions are different.
If the insulation of the wiring is damaged, then part of the current through the coolant (water) began to flow to the body of the device, and after that it goes to ground through the ground. The remainder will also rush to the RCD through a 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 determined by the protective 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 housing with potential or a bare wire under voltage. A leakage of current will occur through the human body, the device is able to instantly detect this and stop the supply of electricity.
More details about the RCD device and the principle of operation
The design of the protective device will 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 current leakage. Typically, the device itself consists of the following basic elements:
- differential current transformer;
- mechanism for breaking an electric circuit;
- electromagnetic relay;
- test node.
The transformer is connected to two counter windings (phase and zero). In normal operation of the electrical network, these conductors create countercurrent magnetic fluxes in the transformer core. Due to this, their total value is zero, since they mutually cancel each other out - equilibrium is maintained.
The secondary transformer winding is connected to an electromagnetic relay and is still at rest. The occurrence of current leakage immediately changes things. Different current values begin to flow along the “phase” and “zero”. Proceeding from this, on the transformer core, the magnitude of the magnetic fluxes will already be nonzero, that is, the equilibrium is upset - the fluxes become different not only in direction but also in value.
This leads to the appearance of current in the secondary winding, and when its readings reach the set value, an electromagnetic relay is activated. It, in turn, is associated with a trip mechanism that immediately breaks the chain.
Test node
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 fact, this is a normal resistance (load connected bypassing the transformer). A similar mechanism simulates a leakage of current, with the help of which the operation of the RCD is checked.
And how does such a check work? There is a special “TEST” button on the protective electric 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 current value at the input and output will be different, the created imbalance will trigger the trip mechanism.
If during the test the RCD has not disconnected, 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 circuit breakers
The classification of RCDs involves several varieties of protective devices. At the same time, various indicators act as a criterion:
- installation method;
- number of poles;
- type of current in the network;
- delay time;
- activation method;
- rated current.
Let's consider each of them separately.
Mounting method
According to this classification, the protective devices can be of a stationary type, which are usually designed for installation in distribution electrical switchboards. In addition, there are portable devices, as well as adapters for installation in outlets.
Number of poles
Depending on the number of poles, protective devices can be bipolar or quadrupole. The first option is used in single-phase electrical circuits to protect a person from electric shock or to prevent 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 each (L) and one zero (N). In other words, they are designed for use in a three-phase circuit.
Type of current in the network
According to this criterion, RCDs, in turn, are divided into several subspecies.
Type A is, in fact, a type of AC type, only takes into account the values of the ripple current. Based on this, the UZO-A type has a more complex structure, and due to this, better protection is provided. Accordingly, for such protective shutdown devices, the price is significantly higher than the UZO-AS type.
Type B - is able to work with differential currents of constant and variable nature. As a rule, such protective devices are relevant for industrial facilities.
Speaker type corresponds to an alternating sinusoidal current, which increases gradually or suddenly. If necessary, the device responds instantly.
Delay time
As for the delay time, the RCD type S has a value of 0.1-0.5 seconds. It is recommended to install it if there are several protective devices. Type G devices have a select function, and the delay time varies from 0.05 to 0.09 s. But there is also an RCD without a delay in operation.
Type S residual current circuit breakers are often installed at the input of electricity to a residential building or private property for fire protection purposes.
Activation method
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 sign of tripping is the indication of the differential current in the damaged area.
As for electrical safety devices, it is important that there is voltage in the network. For their performance any external source is required. Compared with electromechanical RCDs, such devices are more reliable in use.
Rated current
Here the division is as follows. Depending on the values of the rated load current, it is 16 A, 20 A, 25 A, 32 A, 40 A, 63 A, 80 A, 100 A (Ampere). Based on the indices of the rated breaking differential current, these are 10 mA, 30 mA, 100 mA, 300 mA, 500 mA (milliamps).
RCD connection
25 A residual current circuit breakers and other similar ones are specially designed for power supply circuits according to the TN-S or TN-CS system with the PE protective zero bus connected, which is connected to the housings of all electrical devices via wires.
It is worth noting that the RCD is not able to protect the electrical wiring from short circuit and overload. In this regard, it is necessary to have a circuit breaker, and it should be located in front of the electric meter. This is the only way to provide maximum protection in your home.
It should be understood that the RCD and the circuit breaker are not the same thing. More on this later. As for the installation of RCDs, then premises with an increased risk zone are:
- bathroom;
- kitchen;
- basement;
- garage.
To protect the wiring in these rooms, it is advisable to use protective devices.
Connection in the apartment
In modern residential buildings, mainly three-phase, and sometimes even five-phase, circuits are used. However, in houses built back in the Soviet era, wiring is often single-phase, and in addition, the neutral and protective conductors are combined into one. In other words, in such a system there is no grounding element.
The connection diagram from the RCD to the apartment will look like this:
- Introductory machine.
- Electricity metering device.
- RCD 30 mA.
- Electrical wiring.
If the apartment has power consumers of electricity, which, for example, can be an electric furnace or a washing machine, then it is necessary to install an additional RCD.
Connection in a private house
The connection sequence for private real estate may have a slightly different look:
- Introductory machine.
- Electricity metering device.
- RCD in the range of 100-300 mA, based on the amount of electricity consumed by all the equipment that is available.
- Protective devices for individual current consumption. Usually in this case the range is already less than 10-30 mA.
If necessary, you can make the connection yourself or use the services of professional electricians.
The difference between an RCD and a circuit breaker
Now it should be clarified what is the difference between the residual current circuit breaker and the circuit breaker. The main symptom is the different principle of operation of both devices. The role of automatic machines is reduced mainly to saving connected electrical devices from excessive current values. Moreover, 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 electric device with a grounded housing. At one point, a short circuit occurs, which the machine quickly responds to and de-energizes the entire circuit.
But in another case, the insulating layer of the wire may be damaged. This can occur due to mechanical damage, wear over a long service life, or moisture. Or the case of the device is simply not grounded. Then a current leak will inevitably occur, albeit a small one. In this case, the machine will not work, as it is not designed for such work.
Visually, leakage is also impossible to detect, but if you just touch the body of the device, a person can get a serious discharge of current. This can be avoided if an RCD is present in the circuit. The residual current device can detect even minor leaks and immediately cuts off the power supply.