Selective circuit breaker: how to protect an electrical circuit, respecting selectivity

The selective circuit breaker is part of the electrical circuit. It performs a protective function and prevents the burning of electrical wiring, damage to the connected to it equipment in the event of a short circuit or exceeding the rated voltage in specific areas chains.

Selectivity principle

Selectivity in electrics and electronics is usually called the property of a protective mechanism, which is expressed in the ability to isolate a malfunctioning element and turn it off.

Selective defense happens:

• Absolute. It works only in the area where a breakdown or short circuit occurs.
• Relative. The fuses located above are activated when, for various reasons, other protective devices do not work.

A striking example of selectivity is the principle of operation of a switchboard in a house. Wiring diagram for each room (bedroom, kitchen, bathroom, etc.) ) has its own fuse. When a short circuit or voltage drop occurs in a specific place, absolute protection is triggered, the whole house is not de-energized.

If a separate fuse fails, the absolute protection takes over. The device installed at the entrance completely cuts off the power supply.

Connection of protective devices

The selectivity of circuit breakers is usually classified based on the way they are connected.

According to this principle, network protection can be:

• Complete. In a chain of serially connected devices, the one that is located near the emergency area is triggered earlier.
• Partial. Its action is similar to the full one with the only difference that it works up to a certain value of the overcurrent.
• Temporary. Devices connected in series have equal current characteristics, but different delay times for operation. This period gradually increases from the problem area to the power source. Timing selectivity allows one fuse to insure the other in the event of a disconnection.
• Current. Its main parameter is the maximum current cutoff. Devices are located as it decreases from power sources to load objects.
• Time-current. Automata that respond to current and time are divided into four groups. The first group (A), used in electronic circuits, shows the maximum efficiency. The fourth group (D) is used in electric drive systems, which are characterized by a large starting current.
• Zone. It functions under the control of measuring instruments that transmit data to the control center. Based on the information received, it is determined which of the devices will be disabled. This type of protection requires additional power supplies; it is difficult and costly to implement. It is used in the industrial field.

The fastest is the energy type of protection. Fuses connected according to this scheme act so quickly that the short circuit does not have time to reach its maximum.

Calculation of protective indicators

The role of selective devices in electrical circuits is most often performed by standard switches. For their action to be effective, you need to carry out the correct calculations and settings. When placing the switch near the power source, the rule expressed by the formula Ic must be observed. O. afterbirth. ≥ Kn. o. ∙ I grade before

It uses the following parameters:

• Iso.final. The current at which the protection is triggered.
• 1st grade before Short-circuit current at the end of the protection zone farthest from the power supply.
• Kn.about. Reliability factor. Depends on the range of indicators.

The setting of protective equipment, in which the time interval is an important factor of regulation, takes place according to the tc formula. O. afterbirth. ≥ tк. prev + ∆t. It contains tс. O. afterbirth. and tk. prev. denote the periods after which cutoffs of fuses installed near and far from the power source are triggered, respectively. The ∆t index characterizes the time stage of selectivity, which is determined from the catalog.

To ensure maximum protection of the power grid, not only numerical calculations are used, but also a selectivity map is drawn up. It shows in the form of graphs the time-current indicators of the switches placed in the circuit in turn and having one voltage source. When compiling such a map, scale plays an important role. It should be such that all design points are visible. It is also important that the map contains information on the highs and lows of short circuits at points in the system.

Despite all the advantages of selective protection, its installation is not always advisable. It shows its effectiveness only in case of significant power failures. The grounds for the installation of selective protective devices are the high rates of rated circuit breakers, obtained as a result of preliminary calculations.

Role of circuit breakers

The key to the safe operation of the electrical circuit is a clear protection system, which is based on the principle of dividing the common circuit into separate sections. This separation allows you to quickly identify the problem and eliminate it.

By installing selective switches, it is possible to achieve the correct compatibility of all used protections and ensure:

• safety of life of people and work of electrical appliances;
• automatic detection of a faulty area and its repair;
• supplying electricity to sections of the circuit located in the immediate vicinity of the damaged areas;
• maintaining the quality of the supplied electricity.

The fact that single pole circuit breakers mounted on a phase conductor can cope with a mains protection function does not mean that two-pole circuit-breakers must be completely refuse. These are the devices protect the system from currentsremaining in neutral after disconnecting the phase conductor.

Two-pole circuit breakers are installed on a DIN-rail of a switchboard instead of two single-pole devices. They show high efficiency of work in apartments and private houses. Disconnection of such a device serves as a guarantee for the safe conduct of any work with electricity.

A circuit breaker with two poles, can act as a protective and control mechanism in systems with technological (kinematic, mechanical, etc.) communication. If you provide power to two such devices from independent lines, then if one of them fails, the other will turn off.

In accordance with the rules for electrical installations, the selection of selective switches should be carried out taking into account the rated currents of electrical receivers. This will help to avoid triggering of protective equipment in case of short-term overloads.