The situation when a short circuit or overload in the socket in the kitchen is turned off not the corresponding automatic switch, and the introductory switch in the whole apartment or even on the floor panel, is familiar to many. This phenomenon is called selective disruption, and it turns a local problem into a global one (power outage). Instead of isolating the faulty site, the system shuts down the entire building, forcing the owner to spend time searching for the cause and turning the power back on.
The fundamental task of any power supply system is to ensure reliability and safety. Protection selectivity This is a key principle to achieve this goal. If this principle is violated, the electrical grid becomes vulnerable, and the risks of damage to expensive equipment increase many times over. Understanding the mechanisms of protection devices is necessary not only for design engineers, but also for everyone who wants to protect their home.
In this article, we will discuss in detail why there is a mismatch between the work of machines, what types of selectivity exist and how to avoid errors when assembling a shield. Violation of the logic of the work of protection It can be very expensive, especially if it comes to production equipment or server racks, where every second of downtime leads to financial losses.
What is selectivity and why is it critical
Selectivity (or selectivity) is the ability of protective equipment to turn off only the emergency section of the network, leaving the rest of the system in working condition. Imagine the plumbing: if a pipe burst in the bathroom, the tap is blocked at this site, and not the valve at the entrance to the house, leaving neighbors and the kitchen without water. In electricians, the role of such valves plays switch-off.
Perfect selectivity ensures that when a supercurrent occurs in the consumer circuit, the machine closest to the accident site will work. The upper apparatus (e.g., the input machine) must remain on. This allows you to quickly localize (fault) and minimize damage. When selectivity is impaired, a false or excessive shutdown occurs, which disrupts the operation of the entire power system of the facility.
⚠️ Warning: A complete lack of selectivity can lead to cascading blackouts, where one minor malfunction shuts down an entire enterprise or residential complex, causing panic and disruption of vital systems.
There are several types of selectivity, each based on different physical principles. Temporary selectivity This means that the time of the machine is different, and current - the difference in the current rates. Energy and logical selectivity are also distinguished, which are more often used in complex industrial systems with electronic cleavers.
Main types of violation of protection
Violation of selectivity most often occurs due to the incorrect selection of characteristics of automatic switches. When the down and down curves of the machine intersect or are too close together, it becomes impossible to predict the behavior of the system in an accident. In the area of overlapping characteristics, both devices can work simultaneously or the one that should be silent will work.
One common problem is ignoring temperature factors and real load currents. Automata from different manufacturers can have a variety of characteristics, even if on paper they are identical. Heat disengagement One brand can work at a current of 1.13 In in an hour, and another - in 50 minutes, which if the close arrangement of the settings will lead to chaos.
- 🔌 Current disturbance: Automatic setups are picked up too close, and when overloaded, both devices work simultaneously.
- ⏱️ Temporary Disturbance: Time-current characteristics intersect in the short circuit zone, which leads to unpredictable choice of device to shut down.
- 📉 Energy disturbance: the energy transmitted through the lower automaton exceeds the permissible for the cable, but the higher automaton is triggered faster, preventing the lower one from working out its characteristic.
Particular attention should be paid to selectivity for short circuits. In this zone, the currents are huge, and the response time is calculated in milliseconds. If the group B automatic and the group C automatic are standing consecutively without a proper gap, with a current KZ 500, the Amperes can knock out both, or worse, only the upper one. Logical selectivityThe system, implemented with the help of controllers, completely eliminates such situations, but in household shields it is practically not used due to the high cost.
Why do machines from one manufacturer work better?
The use of equipment of one brand and one series significantly increases the likelihood of selectivity, since the factory calibrations of the uncouples are made according to uniform standards, and their time-flow curves predictably correlate with each other.
Temporary and current selectivity: what is the difference
The differences between the two types are fundamental to understanding the principles of protection. Current selectivity It is based on the principle of "stepping" of nominal currents. The higher machine must always have a higher denomination than the lower one. For example, if the line is a 16A machine, the introductory should be at least 25A or 32A. However, a simple difference in denominations is not enough, it is important to consider the classes of disconnection (B, C, D).
Temporary selectivity This is due to delaying the response time. The higher machine "waits" longer, allowing the lower to turn off the accident. In everyday conditions, this is provided by the design of the machines themselves: an instant electromagnetic sever works without delay, but if the currents of the KZ are small, a thermal sever that has inertia comes into play. In industrial systems, special time relays are used.
| Comparison parameter | Current selectivity | Temporary selectivity |
|---|---|---|
| Principle of action | Difference in the magnitude of the cutoff current | Difference in delay time |
| Where applicable | Household and simple industrial networks | Complex industrial networks |
| Dependence | Depends on the frequency of the current KZ | Depends on the setting of the time exposure |
| Example | B16 and B40 automatic | Automatic devices with electronic disengagement |
Often these methods are combined. For example, in the zone of small overloads, current selectivity works (the difference in denominations), and in the zone of large short-circuit currents, where thermal cleavers do not have time to work out, the reaction speed of electromagnetic coils is important. If the outage curves cross, selectivity is broken. The critical point is the zone of transition from thermal protection to electromagnetic protection, where failures occur most often.
Typical errors in the assembly of the electric shield
The most common mistake is installing machines of the same denomination sequentially. For example, at the entrance to the house there is a C40 machine, and for each group there is also a C40. In this case, with KZ on any line with a probability of 50% will turn off the input machine, leaving without light the whole house. This is a gross violation of design rules.
The second common mistake is mixing devices of different classes and manufacturers without checking their compatibility. Installation of a class "C" (standard) machine in front of a class "B" (sensitive) with close denominations can lead to the fact that when starting a powerful engine (for example, a pump) the upper automatic machine will work, since its instantaneous shutdown range is wider and can block the initiation current of the lower one.
☑️ Checking the selectivity of the shield
Limiting power outage (PCS) is also often ignored. If the entrance is a machine with a PCS 6 kA, and on the line where possible KZ with a current of 10 kA, there is a machine with a PCS 4 kA, then in an accident the lower machine can simply stick or burn without breaking the chain. Then the top one will work, but the cost of the mistake can be fatal.
⚠️ The use of cheap, uncertified automata of unknown brands makes the concept of selectivity meaningless. Their real-world characteristics may differ from those claimed by 30-40%, leading to unpredictable network behavior.
How to calculate and choose the right machines
To ensure proper protection, selectivity tables provided by equipment manufacturers (ABB, Schneider Electric, Legrand, IEK) are used. These tables show which combinations of automatics guarantee selective shutdown. The simple rule of 2-level difference (e.g. 16A and 25A, or 16A and 32A) does not always work, especially for classes C and D.
The calculation should take into account the real load currents and the stock factor. Nominal current The automatic is selected based on the cross-section of the cable and the power of consumers, but to ensure selectivity, sometimes you have to increase the cross-section of the cable to be able to put the machine of a larger denomination on the upper level. This creates the necessary current gap.
- 📊 Use network simulation software if the project is complex.
- 🏭 Give preference to modular systems of one manufacturer for the entire shield.
- 🔍 Consider the ambient temperature, as it affects the operation of the thermal disengagement.
It is important to remember about the selectiveness of the leakage current, if ultrasound (differential machines) are used. Here the rule applies: the higher RCD should have a leakage current at least 3 times greater than the lower one (for example, 30 mA and 100 mA), and necessarily a longer exposure time (selective RCD is marked with the letter "S").
When assembling the shield, always leave a power reserve on the introductory machine. If the group of machines gives a total of 63A, it is better to put the introductory not 63A, but 80A or 100A to avoid false shutdowns when all devices work simultaneously.
Consequences of Ignoring Protection Rules
Ignoring selectivity leads not only to inconvenience, but also to real fire-hazardous situations. If the machine does not work where it is needed, or it works too slowly, the wires at the site of the damage can overheat, melt the insulation and ignite nearby materials. Electrodynamic effects Short-circuit currents can mechanically destroy the contacts if they do not open instantly.
In the commercial sector, the consequences are even more serious. Stopping the production line due to the disconnection of the general input instead of one machine entails a simple, spoilage of products and fines. Recovery of complex automation after complete shutdown can take hours, whereas replacing the fuse at a particular node would take minutes.
In addition, frequent false shutdowns of introductory machines lead to their premature wear. The mechanical life of any switching apparatus is limited by the number of on/off cycles. Constant work "at the limit" or in abnormal modes reduces the life of expensive equipment.
Properly selected selectivity is not just a theoretical requirement of the PUE, but saving money on repairing wiring, equipment and preserving the nerves of the property owner.
Practical recommendations for home masters
If you change the wiring yourself or assemble a shield, stick to proven schemes. Do not try to save money, putting all the machines of the same denomination "just not knock out". On the contrary, a competent scheme can knock out more often on a particular device, but will save the rest of the house. Use class B and C machines correctly: class B for lighting and sockets, class C for engines and appliances with large initiation currents.
Always check the tightness of contacts. Poor contact on the input machine can cause it to heat and heat out even without overloading current, which is often mistaken for a violation of selectivity. Diagnostics It should begin with a visual inspection and inspection of the quality of installation.
⚠️ Warning: Never replace a burned machine with a device with a large denomination "so that it does not knock out." This is a direct road to fire, as the wiring will not withstand the increased current.
For complex cases where standard methods do not help (for example, old networks with poor insulation state), it is worth considering installing arc-breaking protection devices (UPDS) or a voltage control relay with a protection function. However, the basic rule remains: the lower automaton should be “faster” or “more sensitive” than the higher one.
What to do if the introductory machine knocks out, and group wholes?
This is a classic sign of a violation of selectivity or malfunction of the input machine itself. First, check if the network is overloaded. If the load is normal, it is possible that an automaton with a defect or a class of disconnection is at the entrance, not corresponding to the group (for example, at the input "B", but on the groups "C"). Requires replacement of the input machine on the device with a large denomination or other class, or revision of the group scheme.
Can you provide selectivity machines from different manufacturers?
Theoretically, it is possible if their time-current characteristics do not overlap. However, in practice, it is difficult to guarantee this due to the spread of parameters. Manufacturers guarantee selectivity only within their series. Mixing brands (for example, ABB on input and IEK on groups) is permissible only with a large current supply (at least 2-3 stages of the difference in denominations).
Does the ageing of the automaton affect selectivity?
Yes, it does. Over time, the bimetallic plate of the heat releaser can "tire" and start to work at lower currents or, conversely, stick. The electromagnetic coil can also change properties. Therefore, old machines (over 10-15 years) are recommended to be changed, even if they are visually serviceable, since their real characteristics no longer correspond to the factory.