The safety of people, facilities and property depends on the proper selection of cables. Various models and variants are available on the market, responding to the different needs of the installation and devices connected to it. The elements of the installation protection are equally important. What is automatic shutdown and how to choose the right cable for it?
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Why is a differential circuit breaker not permitted in fire protection installations?
Different types of protection are used in electrical installations. One of them is differential circuit breakers, which are designed to protect against electric shock. The mechanism of operation is based on detecting differences in current between wires (phase and neutral). Despite its common use, it cannot be used in fire protection installations. There are several reasons for such restrictions:
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a fire protection system requires reliable operation, the use of a differential circuit breaker generates the risk of unnecessary start-up when the system experiences a momentary drop in voltage and other current parameters; the risk of shutdown is quite high, therefore this circuit breaker is not suitable for use in an installation that requires precision operation; - the differential switch is characterized by too low resistance to electromagnetic interference, during a fire, when the wires are damaged, electromagnetic interference may occur, in such conditions the differential switch may not operate properly, and in the event of a threat, the most important thing is the reliability of the fire system;
- In the event of a fire, it is much more important to ensure the collective safety of people staying in the building, even at the cost of the risk of electric shock.
The idea behind using fire protection systems is to ensure continuity of operation, even in the event of a real threat. This is very important, which is why key systems use reliable safeguards that additionally do not carry the risk of automatic power outage.
What cross-section should the cable have for automatic shutdown to work?
One of the most important issues that must be considered when designing fire protection systems and their protection is the type and cross-section of the cables. Knowledge of the system's operating mechanism is important to perform calculations. When a short circuit occurs, a high short circuit loop impedance measurement is observed, which is why the right choice of cable is so important. First of all, the most important parameters should be considered, including:
- type of protection, whether it be a fuse, circuit breaker, etc.;
- protection operation time;
- short circuit current value;
- length of the circuit, if the distances are large, the further, the lower the short-circuit current value;
- resistivity of the material from which the wire is made (mainly copper or aluminum).
Analysis of the indicated factors allows for the optimal selection of the wire for automatic disconnection. The wires should comply with the PN-HD 60364-4-41 standard. Automatic disconnection of the power supply must occur within a strictly defined time, which is determined depending on the network layout and the rated voltage.
In installations, the most common network configuration is the TN variant. If the voltage in such a configuration is 230/400 V, the short-circuit current causes disconnection within 5 s for distribution circuits, while in the case of final circuits, such as lighting or sockets, disconnection occurs after only 0.4 s.
Another important issue is the permissible voltage drop. When selecting the appropriate wire cross-section, it should be ensured that the voltage drop to the established acceptable level is limited to no more than 3-5%. The exact value depends on the individual characteristics of the installation.
Another important issue is the calculation of the value of the so-called short-circuit current. This parameter depends on the cross-section of the wires and the distance from the power source. In wires with a larger cross-section, a lower value of resistance is observed in the short-circuit loop, which means that the value of the short-circuit current will be higher. The loop impedance also depends on the material from which the installation wires are made.
How to check the selected cable for automatic shutdown conditions?
Checking the automatic power off is crucial to assess fire safety. To do this, it is necessary to perform short-circuit loop impedance measurements. The obtained value must be sufficiently low, only then will the protection function correctly.
Cables and wires in fire protection systems must function reliably, the safety of people and property depends on it. That is why testing the system is so important. The first step is to verify the type of network system. The most common is TN, but there are also TT and IT variants.
If it is a TN variant, then the switch-off must occur in just 0.4 s for final circuits and sockets and in a maximum of 5 s for distribution circuits. It is very important to calculate the actual return loop impedance. For this purpose, the parameters of the wire resistance and the total length of the circuit must also be known. Control cables and the long-term current-carrying capacity of the wires are crucial for the system to function efficiently, ensuring optimal safety.