RCD (residual current device) is designed to prevent dangerous effects on humans and animals of electric current when touching live and other parts of appliances and electrical installations under voltage. Another important function of the device is the prevention of fires when leakage currents to the ground occur. The protective action is manifested in the disconnection of the mains in the following situations:
- short circuit of the body of the electrical appliance, under voltage, through the body to the ground;
- contact of current-carrying elements with grounded non-current-carrying parts of electrical installations as a result of insulation damage;
- change of grounding (PE) and neutral (N) conductors in the electrical circuit.
RCD also protects networks from power surges. To do this, a non-linear resistance is connected to the neutral at the input of the device and the phase at the output. Differential current flows through it when the voltage increases above 270 V, after which the RCD is turned off.
Protective devices differ in types and operating principles. One of the most practical is an RCD selective, providing targeted disconnection of load groups. Its feature is an underestimated speed response (type S or G). It is installed closer to the source, has a differential current rating of 100 or 300 mA and ensures that the next ordinary RCD located in front of the consumer is the first to disconnect.
Thus, modern protection of electric networks is based on the identification of malfunctions and disconnection of individual sections from systems operating in normal conditions.
How is an RCD arranged?
RCDs are also called residual current circuit breakers. The purpose remains the same: disconnect the circuit when a current leak occurs. The main element of the device is a toroidal transformer with several turns of the neutral and phase wires connected in the opposite direction. The resulting magnetic field during normal operation of the device remains equal to zero. A leak into the ground upsets the balance, voltage arises in the secondary winding, when a certain value is reached, the electric circuit is switched off using the starting and actuating mechanisms.
For RCDs, a grounding bus RE is needed. Otherwise, when the potential appears on the body of the appliance due to damaged insulation, there is no current leakage, and when you touch it and grounded metal parts (heating radiator, water pipes), you can get a noticeable electric shock. In this case, the protective device will work, but it will be better if it happens from a leak into the ground.
For reliable operation of the protective device, ground it. When working on such a circuit, the RCD will break the circuit even before touching the metal casing of the equipment or household appliances.
Types of RCD
RCDs are classified by their functions:
- AC - response to a suddenly appearing or increasing gradually alternating leakage current.
- A - additionally responds to a constant pulsating differential current, which may appear unexpectedly or increase gradually.
- B - response to direct and alternating pulsating leakage currents.
- S - selective RCD with an additional temporary shutdown delay.
- G - similar to S, but with less delay.
Which RCD choose?
The pulsating current in domestic conditions appears from washing machines, lighting dimmers, televisions, computers, power tools and other devices with switching power supplies. The absence of isolation transformers in devices with thyristor control significantly increased the likelihood of leakage of direct or alternating ripple current. Therefore, if earlier it was enough to establish the type of speaker, now type A or B. is needed.
Where to install an RCD?
- Public places in buildings where there is no increased risk of electric shock.
- In electrical circuits with a possible danger of electric shock (rooms with humidity above the norm, groups of outlets, household appliances, etc.).
- At the main entrance for protection against fire hazard. Usually an RCD selective is installed here.
- In floor switchboards, in apartment shields, in individual houses.
- In radial power supply systems: general selective RCD and separate along the discharge lines, with a choice of parameters guaranteeing selective operation.
- At the near protection levels, for example, 10 and 30 mA, 30 and 40 mA, etc., the current selectivity of the RCD is unlikely due to the high response speed. For the indicated values, it is ensured if a selective RCD of 100 mA is selected so that there is still a time delay.
- Due to aging of the insulation, a gradual increase in leakage currents does not always occur.
- With an instantaneous increase in the leakage current due to breakdown of insulation, any conventional RCD, sequentially located in the circuit, can work. This is due to the rapid and significant excess of the settings at once at several levels of protection.
The need to use selective RCD
RCD selective performs its fire protection function if modifications with a time delay of S or G are applied. They are subject to increased requirements for short circuit resistance, switching ability, dynamic and thermal resistance, etc.
Typically, a selective fire protection RCD is installed at the main input for a large leakage current.
RCDs cannot be used in circuits that cannot be suddenly turned off, as this can lead to emergency situations (fire or burglar alarms, danger to personnel, etc.).
In addition to RCDs, circuit breakers must have current selectivity . The first to operate are located closer to the area of overload or short circuit. In this case , the circuit breakers operate earlier than the short-circuit current reaches the limit value. This is necessary to prevent overloading of series-connected sections, since current flows through the contacts of their protective devices.
Types of selective RCD
For selective RCD, it is important to pause, so that the general type device, located lower in the circuit, can work. In this case, the device with a time delay of shutdown passes a leakage current through itself and does not work. The delay interval for models may vary. For products with marking S it is 0.15-0.5 s, for example, RCD 63a 100mA is selective, with the possibility of setting the delay. The choice will be best if they are installed at the inlet of the apartment’s power cable. Some foreign models have time delays even higher. They are designed to disconnect the circuit in the event of a fire hazard. The longer the protection shuts off, the greater the likelihood of ignition of the insulation.
When marking G, the device responds within 0.06-0.08 s. The device is fast enough to respond to network problems. It should be installed below the RCD type S selective. With two-stage protection, it can be installed on the main input, since the speed of the RCD connected below is still higher.
If there are several load groups in the network, a separate protective device is connected in front of each, and a selective fire-protective RCD is connected to the input. Then, if one of the lines fails, only it will be de-energized, and the rest will remain connected. With such a wiring diagram, it is easier to detect a malfunction. If a conventional RCD is found to be faulty or does not respond to a malfunction in the circuit, then a selective RCD will trigger (300 mA or 100 mA) and disconnect the entire network.
To ensure selectivity, the following instrument settings are needed:
- set the response time of the selective RCD, if it provides such an opportunity;
- set the required trip parameters depending on the magnitude of the leakage current.
The disconnection characteristics of the RCD of selective action should exceed the others by at least 3 times in magnitude. Only in this case the device will be guaranteed to work.
RCD parameters
Two time parameters of the RCD are determined by Russian standards:
- tripping time - the period from the occurrence of the breaking leakage current ∆i to the moment of extinction of the arc;
- the maximum failure time for a device of type S is the time interval between the onset of ∆i and the opening of contacts.
The last parameter determines the selectivity of the RCD. Its limit value is 0.5 s. It should be borne in mind that, in order to protect people, opening must take place within 10-30 ms, to prevent ignition of the insulation - up to 500 ms. Type S selective RCD is widely used where it is necessary to exclude false alarms from the influence of interference or power surges.
According to the network shutdown speed, the RCDs are divided as follows:
- general use - without delay;
- type G - 10-40 ms;
- type S - 40-500 ms.
Leakage currents always occur in electrical circuits. In total, they should not be higher than 1/3 of the nominal ∆i device. It is believed that 0.4 mA of consumer leakage current falls on 1 A of the load, and 10 μA on 1 m of the phase wire. The protective device is adjusted according to the value of the total natural leakage current. If this is not done, frequent false positives can occur. It should be borne in mind that a device with ∆i = 100 mA will no longer protect a person from electric shock.
When designing electrical networks, you can not specify the type of RCD, while experts do not require. But justify your choice in advance. It is important that the rated current of the device is higher than the current of the expected load. In addition, the RCD is installed only in common with a circuit breaker. You can install one differential machine instead of two devices. It will cost less, but you should choose the right parameters.
RCD protects in two-wire networks where there is no protective conductor. But it works only after touching a dangerous place.
What to choose a fire protection RCD?
Selective RCD 63A, 300mA is usually installed at the input as a fire.
Many people use ordinary models of a general type, installing 30 mA protection devices in the house. Here the function of "partial" selectivity is fulfilled due to the large difference in the operating currents. At the same time, money is saved on the price difference. In addition, conventional RCD provides better safety due to faster response when catching leakage currents. The difference in the behavior of the devices is that the selective device will not turn off first when the differential current is equal to or greater than 300 mA. A similar situation is already extraordinary and the question is not whether to go to the control panel, which can be on a street post. With such a large current, the usual RCD will probably work if an accident occurs on the line. Here and so it will be clear where to look for a malfunction.
Thus, a fire protection RCD can be installed both selective and conventional.
RCD manufacturers
Legrand Group is a world famous manufacturer of electrical building systems. The leading position is ensured by the highest production culture and major investments in the creation of new electrical products. For Russia, the group supplies the entire list of electrical equipment, starting from sockets and switches to complex control systems.
Selective RCD Legrand is electronic and electromechanical type (indicated on the front panel). Depending on the version, it is installed on the side or bottom of the circuit breakers. Adjustable time delay (0-1.3 s) and sensitivity. In combination with automatic machines, they are used as highly sensitive or basic protective devices.
RCD prices remain high, as with other brands.
The ABV firm most fully represents RCDs of the F 200 series - from 16 A to 125 A. For a home network, RCD 63A is sufficient, 100 mA selective. For leakage currents, a 30 mA device is usually used for household appliances. As a fire protection at the entrance of a private house, selective ABB RCD (63A, 300mA) four-pole for a three-phase network is used, as one of the most reliable. Legrand brand products are not inferior to it in quality. For an apartment with a single-phase input, there will be a two-pole device. The photo below shows a selective RCD ABB 63A, 300mA.
The maximum current that the device is capable of withstanding is 3 to 10 kA (indicated on the front panel). It is a short-term, not a working current. The RCD is able to withstand a pause until the machine disconnects the circuit.
The company is one of the leading, but the prices are very high. Consumers often favor abb models as security is the most expensive. ABB DDA200 AP-R differential block type A and AC is available. It provides a delay of 10 ms, although this is not a selective RCD ABB. The curve of the shutdown characteristic is located between the selective and conventional RCD. The device has increased resistance to false alarms compared to general-purpose devices.
The percentage of rejects on selective ABB RCDs, as well as on other products, is only 2%, due to which there are practically no problems in operation. Electromechanical devices are much more reliable than electronic ones and have advantages in everything, except for the price. RCDs are already starting to appear with an electronic actuator, not inferior in reliability to mechanical.
On the market you can find products half the price, but not inferior in quality to ABB. The company also produces the FH 200 series, which has a slightly lower price, but loses significantly in quality to the F 200 products. In particular, it does not have such reliable conductor fastening contacts that quickly begin to dangle, which affects the quality of work.
If you purchase selective RCD ABB, then only in specialized stores, and not in dubious places. A fake is dangerous because it is not able to protect a person properly. On modular equipment, which also included RCDs, self-fingerers pay great attention due to their high cost.
The domestic group of companies IEK produces about 7 thousand items of products that meet international standards and ensure reliable operation of electric networks.
RCDs have high requirements. On the one hand, they must operate reliably, protecting people from electric shock, and wiring from the danger of ignition. But at the same time, devices installed at different stages of electrical circuits must act selectively, disconnecting individual sections. These conditions, as well as GOST 51326.1, correspond to a selective RCD IEK type VD1 63S.
The group of products is represented by rated currents of 25-80 A, and differential currents are 100 mA and 300 mA. Products are cheaper than those of famous brands and are widely used as introductory fire fighting devices. At the same time, the selectivity of protection is ensured by large cutoff currents and time delays for disconnecting circuits.
Selection of protective devices
If electricity is consumed in a simple manner, a sinusoidal current flows through the circuit. The leak will be of a similar shape and here you can use devices such as AC.
In modern household appliances, phase-locked control circuits are increasingly being used. A device of type AC will not respond to them and here it is better to use an RCD of type A, which also responds to a sinusoidal current. Devices can be used together, for example, type is suitable for incandescent lighting, and type is suitable for outlets to which devices with pulse regulation can be connected. But if you have to change the lighting to energy-saving lamps with brightness control by phase cut-off, you will also have to Replace the AC device with A. Otherwise, it will not work.
In order to separate the operation according to the levels of electrical circuits, it is necessary to use selective devices. On the main input, type S is set, on the second level - G, and then instantaneous triggers.
The RCD is selected one step higher in rated current than a circuit breaker connected to it, which can operate for a long time when the load is exceeded. If at the input there is a 50 A machine, an RCD selective 63A will be suitable for it.
According to the requirements of the standards, the front panels of the devices indicate the nominal values of voltage, as well as continuous and breaking current ∆i. If there is a designation of a sinusoid - this is a type of speaker. The presence under it of two positive half-periods means type A. Selective RCDs are indicated by the letters S and G. The rated short-circuit current is indicated in the box. The device must withstand its growth to a maximum until the machine shuts down. Typically, the current does not have time to reach the limit value. The RCD disconnects the defective circuit in advance, until the conductor has heated up and the insulation has not ignited.
Conclusion
In electrical household networks, current and time selectivity is used. To do this, the protective devices are installed in series according to the tree diagram, where one switch is common. The basis of the principle of action is to reduce the time the current flows through the body when directly or indirectly touching live electrical components. RCD selective is installed at the inlet and performs a fire function.