Selectivity of electrical network protection - what is it?

Selectivity in electrical and power engineering is the most important concept, since its main function was, will be protection of electrical devices from failure remains, due to any malfunctions electrical installations. It is thanks to this protective function that the lifespan of devices and devices is preserved, which allows electrical equipment to work longer and more reliably. In this article we will try to figure out what is the selectivity of the protection of the electrical network and what is its principle of operation.

What it is?

First of all, the concept of "selectivity" includes a protective mechanism and the well-functioning operation of certain devices, consisting of separate elements connected in series with each other. Often, such devices are various types of circuit breakers, fuses, RCDs, etc. The result of their work is the prevention of "combustion" of electrical mechanisms in the event of a threat. A diagram of the selective operation of circuit breakers and RCDs in the panel is provided below:

The advantage of this system is its ability to turn off only the necessary sections, while the rest of the system remains in working order. In this case, the only condition is the consistency of the protective devices with each other.

Main functions

So, the main functions of selective protection are:

• ensuring the safety of electrical appliances and employees;
• instant identification and shutdown of the power zone in which the breakdown occurred, without other shutdowns that will stop the supply of electricity in places of stable operation of equipment;
• reducing the influence of negative consequences on the rest of the electrical mechanisms;
• reducing the load on composite installations and preventing breakdowns in the faulty area;
• ensuring the highest possible continuous power supply of high quality;
• ensuring the continuity of the work process;
• providing qualified support in the event that the protection itself, which is responsible for opening, fails;
• support for the optimal functioning of the installation;
• ensuring ease of use and economic efficiency.

Selective protection types

The selectivity of protective equipment is divided into the following types:

1. Complete. Two devices with a serial connection are involved; when exposed to overcurrents, only one protection is triggered, which is closer to the fault zone.
2. Partial. It is similar to complete, but protection is valid only up to a certain overcurrent indicator.
3. Temporary. Several machines with the same current characteristics, but with different time delay, are included in the circuit. As a result, from the closest to the malfunction, to the most distant circuit breaker, the devices insure each other (for example, the closest one will work in 0.02 s, the next one after 0.5 s, and the last one after 1 s, if the other 2 do not work).
4. Current. Roughly speaking, the principle of operation of the current selectivity of the protections is similar to the time one, but the delay occurs not in time, but in the magnitude of the current. For example, circuit breakers are installed at input 25A, then 16A, and then 10A. At the same time, they may have the same shutdown time.
5. Time-current. In addition to the reaction of protection mechanisms to current, the time of this reaction is also determined.
6. Zone. When a violation of the current threshold is detected, the operation of the installation allows you to accurately determine the faulty zone and turn off the electricity supply only in it.
7. Energy. All breakdown prevention processes take place in the molded case of the circuit breaker. The disconnection takes place in such a short time that the mark for the maximum current value does not reach its result.

Also, the selectivity of protection can be absolute and relative. In the first case, only the damaged section of the circuit is turned off. Fuses installed in electrical appliances work according to this principle. Relative selectivity protects not only "its own area", but also the neighboring one, if the absolute selective protection has not worked in it.

Selectivity map

It is imperative to mention the selectivity map, which you will need "like air" for overcurrent protection. The map itself is a certain diagram, built in the axes, where all sets of time-current characteristics of the installed devices are displayed. An example is provided below:

We have already said that all protective devices must be connected in turn one after the other. And the characteristics of these devices are displayed on the map. The main rules for drawing cards are: protection settings must come from one voltage; the scale must be selected with the expectation that all boundary points will be visible; it is necessary to indicate not only the protective properties, but also the maximum and minimum indicators of short circuits at the design points of the circuit.

It should be noted that in today's practice the absence of selectivity maps in projects is firmly entrenched, especially at low voltages. And this is a violation of all design standards, which, in the end, is the result of a power outage at consumers.

Finally, we recommend watching a useful video on the topic:

Now you know what the selectivity of the protection of the electrical network is and what it is for. If you have questions, you can ask them on our forum for electricians.

It will be interesting to read:

• How to choose a circuit breaker for your home
• Why does the RCD in the dashboard work?
• Reasons for power outages