How to choose the RCD by power: existing types of RCD + fineness of choice
The increase in the number of household appliances increases the risk of electric shock during its operation. Therefore, it is recommended to install protective systems in the premises to prevent current leakage.
It is important to understand how to choose a UZO so that household appliances are safe, and its operation is stable.
The principle of operation of the RCD
To prevent accidental electric shock when in contact with household and industrial electrical appliances, a protective shutdown device was invented.
It is based on a transformer with a toroidal core, which monitors the strength of the current on the "phase" and "zero". If its levels diverge, then the relay trips and the power contacts go off.
Normally, any electrical device has a leakage current. But its level is so small that it is safe for the human body.
Therefore, the residual current circuit breaker is programmed to operate at a current value that can cause electric shock to people or cause damage to the equipment.
For example, when a child shoves a bare metal pin into the socket, an electrical leakage through the body will occur, and the RCD will turn off the light in the apartment.
The speed of the device is such that the body does not experience any negative feelings.
Depending on the power of the connected equipment, the presence of intermediate protective devices and the length of the wiring, RCDs are used with different limit values of the differential currents.
The most common in everyday life protective devices with a threshold level of 10 mA, 30 mA and 100 mA. These devices are sufficient to protect most residential and office premises.
It should be remembered that the classic RCD does not protect the wiring from a short circuit and does not disconnect the power contactswhen the network is overloaded. Therefore, it is desirable to use these devices in combination with other mechanisms of electrical protection.
Despite the simplicity of the internal structure, the choice of RCD models on the market is quite large. Each device has a specific set of technical parameters that cannot be configured during operation.
To facilitate the selection of an RCD, the classification options for these devices should be considered.
- By speed of operationThe mechanism of the RCD is divided into conventional and selective. The first turn off the power contacts almost instantly, and the second - with a delay. Selective RCDs are used in multi-level systems where the trigger sequence is important.
- By type of relayRCDs are divided into electromechanical, breaking the contact mechanically, and electronic, preventing the flow of current through a semiconductor circuit.
- By type of current. AC type AC is disconnected from AC leakage, type A AC and DC.
- For additional features: without protection against network overloads. RCDs with short-circuit or high current tripping mechanisms are commonly called difavtomats.
- By constructional execution. There are RCDs attached to a DIN rail, to a wall, as well as devices in the form of a socket, a portable device, an adapter.
- By operating voltage: for 220V, 380V, combined.
- By volatility. There are RCD models capable and unable to disconnect the power load in the absence of operating voltage.
- By the number of connected poles: bipolar and quadrupole.
For the correct selection of an RCD, it is not enough to know its technical characteristics. In order for the device to effectively perform its protective function, you need to take into account when purchasing a length of home wiring, the power of the connected devices and some other parameters.
Rules for the selection of protective devices
Before buying an RCD, you can visit the forums of electricians to find advice on the reliability of a manufacturer.
However, to select the maximum and threshold current, the number of poles, the mounting scheme and other technical parameters must be strictly individual, based on the characteristics of the room and electrical wiring.
The safety device does not control the power consumption of the connected devices, but has limitations on the maximum permissible current.
thereforeIt is important to know how to choose the RCD for maximum power in order to correctly consider the energy consumption of each group of rooms when installing the wiring diagram.. After all, if the nominal current exceeds the threshold value for the device, it may burn out.
In apartments and private houses, a single-level or two-level RCD system is usually used. Each of them has its own characteristics.
With a single-level circuit with a single RCD, the rated current is calculated based on the total power of devices simultaneously connected to the network.
For example, when powering a washing machine2.4 kWlighting1.1 kWand other devices2.8 kW, RCD must pass(2400 + 1100 + 2800) / 220 = 28 A.
In this case, at a rated current of the protective shutdown device of 30 A, it does not burn out even when all household appliances and lighting work simultaneously.
When installing a single RCD, there may be a problem finding the breakdown location. In whatever room the leakage occurs, the electricity will be dislodged throughout the apartment. Therefore, it is better not to save and mount an extensive protection system.
There is a variant of an extensive single-level installation circuit of the RCD. In such a situation, the wires from the meter with the help of a special bus branch into several groups, each of which is controlled by a separate protective apparatus.
The calculation of the rated current for each RCD with an extensive single-level system is carried out separately. This takes into account the maximum power of devices that are potentially connected to the device.
For example, when only a washing machine with power consumption is connected to the RCD2.4 kWits rated current will have to be at least2400/220 = 10.9 A.
Optimal, in terms of safety and maintainability, is a two-level RCD system.
Its first level is set at the entrance to the apartment and provides fire safety. The rated current of this protective device must be no lower than the maximum capacity of the electricity metering device.
The second level of energy protection is placed on separate groups of consumers. These can be rooms, floors, extensions, street lighting, single sockets.
Second level devices usually cost less and have a lower rated current. The sum of its values for all installed devices must be less than that of the basic RCD at the entrance to the room.
For example,in case of protective devices of the second level with a rated current of 10 A, 16 A and 16 A, it will be necessary to mount an instrument with a minimum bandwidth of 10 + 16 + 16 = 42 A at the common input.
The advantage of a two-tier system is the ability to turn off individual groups of electrical appliances in the presence of current leakage. This allows you to repair appliances or find problems with insulation in the wall without de-energizing the entire apartment.
Calculation of the required differential current
Each model of RCD operates at a certain level of differential current arising between two conductors of the cable. Therefore, it is important to know how to choose for the home an UZO with safe characteristics.
When calculating the threshold differential current of an RCD, several parameters are taken into account at once:
- wire length to the device consuming electricity;
- natural leakage current in the technique;
- power devices.
The general formula for determining a difftock is as follows:
IΔ = (0,4Irask (A) + 0,01 L wire (m)) / 1000
For example, let’s take the above described scheme of electrical appliances and their power. Let the cable length to each group of home appliances be 12 m.
The calculation of the parameters of the RCD for the above scheme will be as follows:
- IΔmash= (0.4 * 2800/220) + 0.01 * 12 = 5.21 mA;
- IΔosv= (0.4 * 1100/220) + 0.01 * 12 = 2.12 mA;
- IΔroz= (0.4 * 2400/220) + 0.01 * 12 = 4.48 mA.
According to the recommendations, the threshold current of the device should be three times the calculated differential. What is associated with increased electrical load in the first second of the inclusion of household appliances.
If this rule is not followed, frequent false positives of the RCD are possible, which will cause problems for consumers.
Therefore, for each group of electrical devices under consideration, the minimum value of the threshold differential current will be as follows:
- 5.21 mA * 3 = 15.63;
- 2.12 mA * 3 = 6.36;
- 4.48 mA * 3 = 13.45.
That is, a RCD with 30 mA difftok will be needed for a washing machine and a group of sockets, and a 10 mA device will be enough for a lighting group.
Such characteristics of the devices will ensure the normal functioning of the equipment and will protect people from electric shocks. It is not recommended to install RCDs with a parameter higher than 30 mA for these purposes.
When a two-level scheme, the leakage current of the main protective device located at the entrance to the room is selected in the range of 100-300 mA.
These RCDs are triggered by the breakdown of old or damaged insulation inside the walls. Thus, the premises are protected from fire in case of hidden defects in electrical wiring.
RCD trip time
In a two-tier system, the appearance of a significant leakage current can lead to the operation of a protective apparatus at both levels.
To eliminate such a situation, a selective protective device can be installed as a basic one. Its response time is 150-500 ms, which is several times longer than that of a standard RCD (20-40 ms).
With this selection of devices, only the power supply at the second level will be disconnected, which will not lead to the disappearance of electricity in the entire apartment.
As for conventional RCDs, the shorter their reaction time, the safer they are. This fact must be considered when purchasing them.
Choosing a reliable manufacturer
Directly protective function of the RCD does not depend on its manufacturer.A device of any company, with the exception of obviously defective models, will shut off the power supply if the differential current exceeds the threshold value.
The disadvantages of security devices may be as follows:
- false positives;
- increased buzz;
- heating during operation;
- shakiness of the case, which can lead to damage during installation;
- small warranty period.
The more reliable and authoritative the manufacturer of the RCD, the smaller the disadvantages listed will be its equipment.
However, with increasing quality, the price will also increase. The most reliable manufacturers of protective devices are:
- Schneider Electric;
- General Electric.
When buying an RCD, it should be remembered that this device is not installed for the sake of compliance with building codes, but to preserve the health and life of loved ones.
thereforedo not buy products from dubious manufacturers. They can not only fail, but also lead to a fire hazard situation..
There are other criteria for choosing a protective device, but they are much less important for the safety of consumers.
General rules for selection and installation
In addition to the criteria for the selection of RCDs, there are general useful recommendations for the purchase and installation of this equipment.
They will help you not to make a mistake and immediately get a model suitable for a particular apartment or house.
Tips for choosing the following:
- It is recommended to take RCDs, which, when triggered, disable not only the phase, but also “zero”.
- In the framework of the circuit monitored by the apparatus there should be no grounded electrical appliances.
- The device should operate at short-term voltage drops of 50% of the nominal, which can occur in the first moments of a short circuit.
- The terminals of the RCD must be made of weakly oxidized material and equipped with a reliable system of fixing the wires.
- The advantage when buying should be given to devices with the function of protection against short circuit and overload.
- RCDs of the second level may not be installed on safe equipment groups, for example, on ceiling light sources.
- It is recommended to install devices with a threshold difftokom 10 mA on the shower stalls and jacuzzi.
- Attention should be paid to the possibility of connecting to the apparatus of aluminum wires.Some devices do not work with them correctly.
It is possible to install the correctly selected RCD on your own.. This process is not very different from installing an outlet or switch.
It is important to carefully consider the wiring diagram and make it as indicated on it.
Conclusions and useful video on the topic
The choice of RCDs with consideration of options, as well as explanations of the features of various schemes for their connection:
Rules for the selection of RCDs, Part 1:
Rules for the selection of RCDs, part 2:
It is better to entrust the choice of a suitable RCD, especially when installing two-level systems, to professionals.It is easier to invite an experienced electrician to the house once and consult with him than to change the unsuitable product in the store. After all, the health and lives of loved ones who will use household electrical appliances are at stake..