In some systems, it is possible to provide special protection for the installation against short circuit or overload. When should individual protection of wires be used, and in what situations is the possibility of common protection permitted?
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Individual protection of low voltage cables
When performing electrical installations, it is necessary to take appropriate care to protect low-voltage cables. This is important in the context of safety, both for users and devices operating in the network.
The following individual protections may be used in low-voltage installations:
- overload protection - it protects the installation against overheating and damage to the insulation; so-called overcurrent circuit breakers (automatics) or fuses are used;
- short circuit protection - protection against damage caused by a sudden increase in current in the circuit, overcurrent circuit breakers or fuses with high breaking capacity are used;
- differential protection - their task is to protect against electric shock, they detect leakage current that appears when, for example, wire insulation is damaged;
- surge protection - a surge arrester is used to protect against sudden, short-term voltage increases, which occur, for example, as a consequence of lightning discharges; without surge arresters, devices connected to the network could be damaged at the moment of a sudden increase;
- thermal protection - are necessary in systems where installation cables are exposed to high temperatures.
Choosing the right accessories is also a very important issue. Power cables and wires , or more precisely their cross-section, must be adapted to the needs of the installation and the devices operating in it.
How to solve the problem of selectively disconnecting a short-circuited wire?
Selective disconnection of a short-circuited wire is a situation that creates a significant problem in electrical systems. It consists in the fact that undesirable operation of protections occurs. Protections, when they operate incorrectly, cause disconnection not only of the system or circuit affected by the failure, but also of others. This problem can be solved quite easily, but it is necessary to select appropriate protections and their proper configuration.
The first solution used is to use the principle of selectivity of protections. The idea of such a solution is that properly placed protections help to switch off only the area that is as close as possible to the fault. A short circuit or overload does not cause problems with the operation of other circuits.
Another way to solve the problem of selective disconnection is to use overcurrent circuit breakers. When designing installations, it is important to remember that they should be adapted to the requirements, thanks to which they can protect selected parts of the system even better. In practice, there are many types of circuit breakers (depending on the type of characteristic, these are B, C or D circuit breakers). It is very important to apply the gradation principle when installing subsequent protections. This means that the elements located closer to the circuit breaker had a higher rated current than those located further from the power source.
A good solution may also be to use switches with a delayed action time. Properly selected and installed, they are responsible for ensuring that the first to be switched off are the parts of the installation that are located as close as possible to the point of the short circuit or overload.
Depending on the type of installation, fuses and differential protection can also be used. It is very important that the elements used are properly coordinated with each other, only then will they provide proper safety.
In what installations can common overcurrent protection be used?
In some installations, it is possible to use common overcurrent protections if such a procedure is justified by safety and functionality reasons, of course within the framework of applicable legal regulations. Such a solution can be used in particular:
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in receiving installations in residential buildings - in single-family houses and apartments, e.g. for plug-in sockets located within one room, provided that the load does not exceed the capacity of the cable, or for lighting in one circuit, also within one room; - in low-current installations, such as monitoring or alarm installations;
- in industrial installations and switchboards - this applies to certain groups of devices or machines with a similar operating nature, most often devices with a low rated current and not causing a high load are protected in this way, or in the case of auxiliary circuits;
- in emergency power supply installations, such as UPS devices, common protections can be used when the receivers are connected in a group and connected to the same power source;
- In photovoltaic installations and other renewable energy systems, one protection usually provides protection for several modules.
Control cables with common protections must be selected taking into account the principle of selecting wires. Important is, among other things, the total long-term current-carrying capacity of the wires, regardless of whether it is a series connection or parallel connection. The number of devices, their type and characteristics, including technical parameters, are also of great importance.
The key issue in the selection of cables is the use of cables, connectors and conductors in compliance with applicable long-term current-carrying capacity standards.