Selective RCD: device, purpose, scope of application + diagram and connection nuances

Among the variety of protective devices, there is a selective RCD, which experts note as a practical and optimal option for organizing protection. How does this type of device stand out from others in a related group?

What properties does a selective device have and how high is its sensitivity? Let's try to figure it out with a short review in this direction.

Purpose and principle of operation

Relay devices for electrical networks, designed to provide protection from direct contact in hazardous areas, as well as equipment protection, are represented by a variety of design.

Features of selective devices

A distinctive feature of a selective device is the presence in the circuit of a time delay function for turning off the circuit that powers the load.

Typically, this parameter exceeds 40 ms - this means that selective devices are not designed to protect against damage from direct contact.

The range of devices used for protection during the operation of electrical networks provides a wide choice. Almost all types of RCDs can be used in single-phase or three-phase networks

Also among the features of selective devices, it should be noted their good stability in response to current and voltage surges. Thanks to this property, the risk of false alarms and, accordingly, circuit shutdowns is almost completely eliminated.What selectivity of circuit breakers is is described in detail in this material.

As a rule, in practice, devices are used whose rated current is in the range of 25-100 A. In this case, the value of the differential leakage current is in the range of 0.1-0.3 A.

Two-pole and four-pole versions of the device are manufactured. Each type is actively used as part of branched cascade circuits.

Operating principle and design of RCD type S

The distinctive features of selective devices are limited only to those indicated above.

In all remaining design functionality, there is virtually no significant difference between selective devices and general-purpose devices.

The operating principle of the modules has the sole purpose of preventing possible current leaks, which poses a potential threat to users of various electrical equipment. Selective type RCDs also act to prevent damage to equipment.

The principle of operation, accordingly, remains standard - applicable to all protective devices from the RCD group:

1. There is a differential transformer in the design.
2. Thanks to the transformer, a comparison of the control currents is carried out.
3. The difference is transmitted to the sensing element.
4. If the difference exceeds the set control parameter, a cutoff occurs.

That's the whole operating principle in general terms. However, it is also worth noting such a feature as the dependence of the devices on power supply.

In practice, two design options for selective-type RCDs (and general ones too) are used. One of the options involves external power supply, while the other excludes it completely.

Four-pole module integrated into a single-phase voltage network.The switching is carried out on the basis of a traditional solution, when an input machine, an electrical energy meter and then a selective RCD are used

It is clear that protective device designs that do not use an external power circuit appear to be more reliable than those that require an energy source for operational efficiency.

Since the differential transformer is, in fact, the dominant design element, special requirements are imposed on this part of the RCD circuit.

The magnetic core of the DT must have a strict linear magnetization characteristic.

This is approximately how the processes occur inside the magnetic core of a selective type RCD and the flow of currents through the existing working windings. By changing the current indicators of each line, the leakage factor is controlled

The temperature properties of the magnetic core should ensure high-quality operation over a wide temperature range. Therefore, to manufacture this element, a special material is used - amorphous iron or the like.

Other parts of the design of a selective RCD device are sensitive magnetoelectric relays - direct-acting elements, often called threshold elements.

In some designs, relays are replaced by electronics, but the principle remains the same.

Normal and emergency modes

When operating an S-type RCD, until the presence of a leakage current (differential current) is detected, the conductors forming an electrical circuit in the magnetic field of the core pass equivalent rated load currents.

A diagram clearly showing the internal processes of a selective-type module when operating the device in operating mode.The required level of sensitivity is ensured by setting it according to the cutoff current

These currents, equal in magnitude, induce magnetic fields of multidirectional action inside the core.

Their total flux turns out to be zero, which explains the absence of current on the secondary winding of the diesel generator. Its zero current has no effect on the cutoff sensing element. The RCD remains on.

Otherwise, when the described circuit is disturbed, the balance of currents is also disturbed. As a result, a current of a certain magnitude is generated on the secondary winding of the DT.

As soon as this value exceeds the threshold value of the trigger element of the selective protective device, it will operate. Which will activate the executive blocking system - cutting off the load power circuits. The RCD will turn off and cut off the load circuit.

Traditional areas of application of the device

As noted above, this modification of protective devices is not used for protection against direct contact.

Most often, the devices are used as interlocks in case of a possible fire in electrical wiring or system mechanisms.

Direct contact with equipment, especially household equipment, is a common occurrence. However, in the event of a current leak, such contact is fraught with serious consequences. An S-type RCD will not protect against direct contact in leakage conditions, but there are other types of devices for this

These same RCDs are used as protective equipment against short circuits in the power supply circuits of valuable expensive installations/devices/equipment or in the supply circuits of important technological systems.

It is common practice to introduce selective devices when constructing complex cascade electrical circuits, where each branch involves a different type of load with different currents.

Cascade circuit solution for electrical network wiring using selective type protection modules. One of the common options used for electrifying residential buildings

With this configuration of the electricity branching system using selective devices, reliable protection is provided in individual areas.

And also, each individual RCD in the event of an accident provides the ability to quickly determine the defect.

Connection diagrams for selective cut-off RCDs

Actually, the circuit solutions in this case theoretically do not have any features that would distinguish them from the construction of circuits with other types of devices in the group.

Another question is, in what sequence should we include, say, selective cutoff and direct touch cutoff?

A typical design of the device, which ensures blocking of the power supply in case of current leakage and direct contact. The cut-off current setting is usually at least 30 mA

If we consider the selective shutdown device in a single version, then in this case it is an element of the simplest circuit and is mounted as standard:

1. The circuit breaker is installed first.
2. Next comes the RCD type S.
3. Then the load chain.

Meanwhile, protection is used in a variety of ways to use electrical networks.

For example, it is necessary to ensure high reliability of the operation of a three-phase electric motor. How to organize protection through a selective type RCD in this case?

Another design option is a four-pole device, which is more convenient in terms of electrical wiring configuration options. With this module it is easier to manipulate the ground bus wiring

A four-pole device is suitable here, with the help of which you can organize a protection circuit against short circuits (short circuits) of the windings.

The connection is also made through an intermediate RCD insert. That is, the machine turns on first, the second number is selective protection, the third is the electric motor.

Circuit design option with an electric motor. A simple and quite effective solution to protect the motor from possible phase-to-phase short circuits or short circuits to the frame

A single-phase circuit for standard needs, such as lighting and power supply, can be constructed quite simply by using a two-pole device and several circuit breakers.

Single-phase channels are routed to each individual room through circuit breakers, which are powered by the phase coming from the protection device.

This, one might say, is a classic circuit solution, which is used in most cases by owners of municipal housing, owners of houses and cottages.

A classic circuit version with one current protective cut-off device for wiring several electrical circuits. A common solution in old houses where the use of an earthen bar was not envisaged

Modern housing projects involve organizing circuits with the mandatory presence of a grounding bus. Therefore, such solutions are characterized by minor changes/additions to the wiring diagram.

In particular, the grounding conductor (PE), which is the same integral part as the zero bus, becomes an additional element of the wiring.

An alternative circuit solution with the same sectoral variation of wiring and using the same devices.Only in this option a grounding bus is already used. This example seems more effective in terms of protection

In apartments, houses, and cottages, selective protection devices are a mandatory addition to the apartment electrical panel when residents use household appliances:

• washing machine;
• dishwasher;
• powerful electric stove (stove).

Moreover, this type of device (selective) acts, as a rule, as the second protective stage, while at the first stage there are RCD cutoffs for direct direct contact.

That is, there is a group connection of devices and this is a really effective option in terms of the safety of operation of household electrical networks.

Nuances of connecting type S modules

Actually, the nuances are the same as those that accompany the process of connecting standard protective devices.

The terminals of each device have a specific purpose (phase, neutral) and are designated accordingly.

Terminal blocks on the protection device and symbols for connecting electrical lines. Also shown is the designation of the button for performing a test operation for correct operation of the RCD.

During installation, it is unacceptable to change the positions of the terminals regarding their purpose in relation to the power circuits.

If you connect a zero bus instead of a phase, there is at least a risk of failure of the device itself. It is extremely difficult to confuse two points, but in practice this happens.

Another nuance is the adjustment of the module to the existing electrical circuit in terms of current cutoff.

If the design does not include current adjustment options, you should select the correct device according to its technical and operational characteristics.

Finally, a mandatory connection nuance is device testing in the mode of supplying power to the load circuit.

This function is simple and requires only one action - activation of a special button, which is designated on the case/in the documentation as “Test”.

Conclusions and useful video on the topic

An accessible and understandable video explanation of the selectivity of the protective device:

The video demonstrates the practice of switching on a group of devices and the actual operation of devices in emergency situations:

You can often find arguments regarding the ease of connecting an RCD to the electrical network of a private home. Along with reasoning, it is often recommended to do this work with your own hands. The motive is well known – saving. However, skimping on your own safety is not the best choice. Therefore, such actions for installing a selective protection device should always be performed by a professional electrician.

Do you have experience using or connecting selective RCDs and can you supplement our material with interesting information on the topic of the article? Please write your comments and ask questions in the block below.