RCD selective: principle of operation and types

The RCD (protective shutdown device) is designed to prevent dangerous electric shock to human and animals when touched by live parts and other parts of appliances and electrical installations under voltage. The next important function of the device is to prevent fires when leakage currents appear on the ground. The protective action is manifested in the disconnection of the mains supply in the following situations:

• Closing of the body of the electrical appliance, which is under tension, through the body to the ground;
• Contact of current-carrying elements with earthed non-conductive parts of electrical installations as a result of insulation damage;
• The change of the ground (PE) and zero (N) conductors in the electrical circuit.

The RCD also protects the 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 rises above 270 V, after which the RCD trips.

Protective devices differ in the types and principles of operation. One of the most practical is the RCD selective, which provides the target disconnection of load groups. Its feature is a low speed response characteristic (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 normal RCD located in front of the user will be disconnected first.

Thus, the modern protection of electric networks is based on the identification of faults and the disconnection of individual sections from operating in normal modes of systems.

How is the RCD arranged?

The RCD is also called a differential current switch. The function remains the same: disconnect the circuit when a leakage occurs. The main element of the device is a toroidal transformer with several turns of zero and phase wires connected in the opposite direction. The resulting magnetic field remains zero for normal operation. A leak into the ground breaks the balance, in the secondary winding there is a voltage, when reaching a certain value, the electrical circuit is disconnected with the starting and actuating mechanisms.

A PE earthing bar is required for the RCD. Otherwise, when there is potential on the body of the appliance due to damaged insulation, there is no leakage of current, and if you touch it and grounded metal parts (radiator heating, water pipes), you can get a tangible electric shock. In this case, the protective device will work, but it will be better if it happens from a leak into the ground.

To ensure the safe operation of the protective device, ground it. When operating under this scheme, the RCD breaks the circuit even before touching the metal casing of equipment or household appliances.

Types of RCDs

RCDs are classified according to the functions to be performed:

• AC - a response to a sudden or gradually increasing alternating leakage current.
• A - additionally triggers a constant pulsating differential current, which can appear unexpectedly or increase gradually.
• B - response to a constant and variable pulsating leakage currents.
• S - selective RCD with additional time delay for tripping.
• G is similar to S, but with a smaller delay.

Which RCD to choose?

Pulsating current in domestic conditions appears from washing machines, lighting dimmers, televisions, computers, power tools and other devices with impulse power units. The absence of isolation transformers in devices with thyristor control significantly increased the likelihood of leakage of a constant or alternating pulsating current. Therefore, if earlier it was sufficient to set the type of the AC, now type A or B is needed.

Where to install the RCD?

1. Public places in buildings where there is no increased risk of electric shock.
2. In electrical circuits with a possible danger of electric shock (rooms with humidity above the norm, a group of outlets, household appliances, etc.).
3. At the main input to protect against fire hazard. Usually, the RCD is selective.
4. In floor distribution boards, in apartment shields, in individual houses.
5. In radial power supply systems: a general selective RCD and separate by outgoing lines, with a choice of parameters guaranteeing selective triggering.
6. In the near protection stages, for example, 10 and 30 mA, 30 and 40 mA, etc., the current selectivity of the RCD action is unlikely because of the high operating speed. For the specified values, it is ensured if a selective RCD 100 mA is selected so that there is still time delay.
7. Due to the aging of the insulation, a gradual increase in the leakage current does not always occur.
8. With an instantaneous increase in the leakage current, due to a breakdown of the insulation, any conventional RCD, which are in series in the circuit, can trigger. This is due to the rapid and significant excess of settings at once on several stages of protection.

The need to use selective RCDs

RCD selective performs its function of fire protection if modifications with a time delay of S or G are applied. They are subject to increased requirements for resistance to short circuits, switching capacity, dynamic and thermal resistance, and so on.

Usually the main input sets a selective fire protection RCD for a large leakage current.

RCDs can not be used in circuits that can not be switched off suddenly, as this can lead to emergencies (fire or burglar alarm, danger to personnel, etc.).

In addition to RCD, the current selectivity should have automatic switches. The first to be triggered is located near the site of overload or short circuit. In this case, the circuit breakers are activated earlier than the short-circuit current reaches the limit value. This is necessary to prevent overloading of series connected sections, since the current passes through the contacts of their protective devices.

Types of selective RCDs

For a selective RCD, it is important to sustain a pause, in order to trigger the device of a general type located below the circuit. In this case, the device with a temporary switch-off delay passes the leakage current through itself and does not work. The delay interval for models may vary. For products marked S, it is 0.15-0.5 s, for example, the RCD 63a 100mA is selective, with the possibility of adjusting the delay. The choice will be best if they are installed at the entrance of the supply cable of the apartment. Some foreign models have time delays even higher. They are designed to disconnect the circuit when a fire hazard occurs. The longer the protection is deactivated, the greater the probability of ignition of the insulation.

When G is labeled, the device operates 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 groups of loads in the network, a separate protective device is connected before each, and a selective fireproof 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 this connection scheme, it is easier to detect a malfunction. If the normal RCD is faulty or does not respond to a fault in the circuit, then a selective RCD (300 mA or 100 mA) will trip and disconnect the entire network.

To ensure selectivity, the following instrument settings are needed:

• Set the response time of the selective RCD, if it is possible;
• Set the required trip parameters depending on the leakage current.

The characteristics of the disconnection of the RCD of selective action must exceed the others by not less than 3 times. Only in this case the device will be guaranteed to work.

RCD parameters

Two time parameters of the RCD are determined by Russian standards:

• The tripping time is the interval from the appearance of the cut-off leakage current Δi until the arc is extinguished;
• The maximum failure time for a device of type S is the time interval between the onset of occurrence of Δi and the opening of contacts.

The last parameter determines the selectivity of the RCD. Its limiting value is 0.5 s. In this case it should be taken into account that for the protection of people, the opening should occur within 10-30 ms, to prevent the ignition of insulation - up to 500 ms. RCD type S selective is widely used where it is necessary to exclude false positives from the effects of interference or voltage surges.

For the disconnection speed of the network, the RCDs are divided as follows:

• General application - without delay;
• Type G - 10-40 ms;
• Type S - 40-500 ms.

Leakage currents always occur in electrical circuits. In sum, they should not be higher than 1/3 of the nominal Δi of the device. It is considered that for 1 A load there is 0.4 mA of leakage current of the consumer, and for 1 m of the length of the phase wire - 10 μA. The protective device is adjusted by the value of the total current of natural leakage. If this is not done, there may be frequent false alarms. It should be noted that the device with Δi = 100 mA no longer protects a person from electric shock.

When designing electrical networks, it is possible not to specify the type of RCD until the experts require it. But you need to prove your choice in advance. It is important that the rated current of the device is higher than the current of the intended load. In addition, the RCD is installed only in a common pair with a circuit breaker. You can install one differential automaton instead of two devices. It will cost less, but you should correctly choose the parameters.

RCD protects in two-wire networks, where there is no protective conductor. But it works only after touching a dangerous place.

How to choose a fireproof RCD?

Selective RCD 63A, 300mA is usually installed at the entrance as fire-fighting.

Many use conventional models of a general type, installing in the house protection devices at 30 mA. Here, the "partial" selectivity function is performed due to a large difference in the pickup currents. At the same time, money is saved on the price difference. In addition, conventional RCDs provide safety better due to faster response when capturing leakage currents. The difference in the behavior of the instruments lies in the fact that the selective device does not turn off first with a differential current equal to or greater than 300 mA. Such a situation is already extraordinary and the question is not whether it is worth going to the control board, which can be on a street post. With such a large current, the usual RCD will probably work, if an accident occurred on the line. Here and so it will be clear where to look for a malfunction.

Thus, a fireproof RCD can be installed as selective or conventional.

Manufacturers of RCDs

Legrand Group is a well-known in the world manufacturer of electrical building systems. Leading positions provide the highest production culture and large investments in the creation of new electrical products. For Russia, the group delivers the entire list of electrical equipment, starting from outlets and switches to sophisticated 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. The time delay (0-1.3 s) and sensitivity are adjustable. In combination with automatic machines, high-sensitivity or basic protective devices are used.

Prices for RCDs remain high, as in other brands.

By ABB, the most complete RCDs are represented by the F 200 series - from 16 A to 125 A. For the home network, there is enough RCD 63A, 100 mA selective. For leakage currents for household appliances, a 30 mA device is usually used. As a fire protection at the input of a private house, a selective RCD ABB (63A, 300mA) is used for four-pole for a three-phase network, as one of the most reliable. He is not inferior in quality to products of the brand Legrand. For an apartment with a single-phase input there will be a bipolar device. The photo below shows the selective RCDs ABB 63A, 300mA.

The maximum current that the apparatus can withstand is 3 to 10 kA (indicated on the front panel). It is a short-term, not a working current. The RCD is capable of withstanding a pause until the circuit breaker is disconnected.

The firm is one of the leading, but the prices are very high. Consumers often prefer abb models, since security is the most expensive. The differential unit ABB DDA200 AP-R type A and AC is produced. It provides a delay time of 10 ms, although this is not a selective ABB RCD. The curve of the disconnect characteristic is located between the selective and the conventional RCD. The device has increased resistance to false triggering compared to general purpose devices.

The percentage of rejects for selective RCD ABB, as well as for the rest of the products, is only 2%, thanks to which there are practically no problems in operation. Electromechanical devices are significantly more reliable than electronic ones and in all have pros, except for the price. RCDs with an electronic actuator already starting to appear are not inferior in terms of mechanical reliability.

In the market you can find products twice cheaper, and in terms of quality are not inferior to ABB. The company also produces the FH 200 series, which has a slightly lower price, but it loses considerably as the F 200 products. In particular, it does not have such reliable wire fastening contacts that quickly start to hang out, which affects the quality of the work.

If you buy a selective RCD ABB, then only in specialized stores, and not in questionable places. Counterfeiting is dangerous because it is not capable of protecting a person properly. On the modular equipment, in the list of which the UZO has also fallen, the self-wheelers pay much attention due to the high cost.

The national group of companies IEK produces about 7 thousand items of products that meet world standards and ensure reliable operation of power grids.

High requirements are imposed on RCDs. On the one hand, they must work reliably, protecting people from electric shock, and wiring - from the danger of ignition. But in this case, devices installed at different stages of electrical circuits must act selectively, disabling individual areas. These conditions, as well as GOST 51326.1, correspond to the selective IED of IEK type VD1 63S.

The product group is represented by nominal currents of 25-80 A, and the differential currents are 100 mA and 300 mA. Products are cheaper than famous brands and are widely used as input fire extinguishers. In this case, the selectivity of the protection is ensured by the high values of the cutoff currents and the time delays for switching off the circuits.

Selection of protective devices

If the electricity consumption occurs according to a simple scheme, a sinusoidal current flows through the circuit. The leakage will be of a similar shape and here you can use devices such as AC.

In modern household appliances, control schemes with phase cutoff are increasingly being used. The device of the AS type will not react to them, and it is better to use an RCD type A, which also reacts to a sinusoidal current. The devices can be used together, for example, for lighting with incandescent lamps, the type of AC is suitable, and for outlets that can be connected to devices with impulse regulation - type A. But if you have to change the lighting to energy saving lamps with brightness control by phase cutoff, Replace the device type AS on A. Otherwise it will not work.

For the purpose of separating the operation by the levels of electrical circuits, selective devices must be used. At the main input, the type S is set, at the second level - G, and then the instantaneous triggering devices.

The RCD is selected one step higher in the rated current than the circuit-breaker connected in pairs, which can operate for a long time when the load is exceeded. If the input is an automatic machine at 50 A, it is suitable for the RCD selective 63A.

According to the requirements of the standards, the faceplate panels indicate the nominal voltage values, as well as the continuous and tripping current Δi. If there is a designation of a sinusoid, it is a type of speaker. The presence of two positive half-periods under it means Type A. Selective RCDs are denoted by the letters S and G. The rated short-circuit current is indicated in the box. The device must withstand its rise to maximum, until the machine goes off. Usually the current does not have time to reach the limit value. The RCD switches off the circuit with the defect in advance, until the conductor has warmed up and the insulation has ignited.

Conclusion

In electrical household networks, current and time selectivity is used. For this, the protective devices are installed sequentially in a tree-like scheme, where one switch is common. The basis of the principle of action is the reduction of the time of current flow through the body with direct or indirect contact to the elements of electrical installations under voltage. RCD selective is installed at the entrance and performs fire protection function.