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Section 3. Protection and automation
Protection of electrical networks with voltage up to 1 kV. Places of installation of protective devices
Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE)
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3.1.14. Protective devices should be located, if possible, in places accessible for maintenance in such a way that the possibility of their mechanical damage is excluded. Their installation must be carried out in such a way that when operating with them or during their action, danger to the operating personnel and the possibility of damage to surrounding objects are excluded.
Protective devices with exposed live parts must only be serviced by qualified personnel.
3.1.15. Protective devices should be installed, as a rule, in network places where the conductor cross-section decreases (towards the place of electricity consumption) or where it is necessary to ensure the sensitivity and selectivity of protection (see also 3.1.16 and 3.1.19).
3.1.16. Protection devices must be installed directly at the points of connection of the protected conductors to the supply line. It is allowed, if necessary, to take the length of the section between the supply line and the branch protection device up to 6 m. The conductors in this section may have a cross section smaller than the cross section of the supply line conductors, but not less than the cross section of the conductors after the protection device.
For branches performed in hard-to-reach places (for example, at high altitudes), protective devices can be installed at a distance of up to 30 m from the branch point in a place convenient for maintenance (for example, at the input to the distribution point, in the starting device of the electrical receiver, etc.). In this case, the cross section of the branch conductors must be at least the cross section determined by the rated current, but must provide at least 10% of the throughput of the protected section of the supply line. Laying of branch conductors in these cases (with branch lengths up to 6 and up to 30 m) should be carried out with combustible outer sheath or insulation of conductors - in pipes, metal hoses, or boxes, in other cases, except for cable structures, fire and explosive zones, - open on structures provided that they are protected from possible mechanical damage.
3.1.17. When protecting networks with fuses, fuses must be installed on all normally unearthed poles or phases. Installation of fuses in zero working conductors is prohibited.
3.1.18. When protecting networks with a dead-earthed neutral by automatic switches, their releases must be installed in all normally ungrounded conductors (see also 7.3.99).
When protecting networks with isolated neutral in three-wire networks of three-phase current and two-wire networks of single-phase or direct current, it is allowed to install releases of circuit breakers in two phases for three-wire networks and in one phase (pole) for two-wire networks. In this case, within the same electrical installation, protection should be carried out in the same phases (poles).
Releases in neutral conductors are allowed to be installed only on the condition that when they are triggered, all energized conductors are disconnected from the network at the same time.
3.1.19. It is allowed not to install protection devices, if it is expedient under the operating conditions, in places:
1) branches of conductors from the busbars of the shield to devices installed on the same shield; in this case, the conductors must be selected according to the rated current of the branch;
2) reducing the section of the supply line along its length and on branches from it, if the protection of the previous section of the line protects the section with a reduced section of conductors or if the unprotected sections of the line or branches from it are made with conductors selected with a section of at least half the section of the conductors of the protected section of the line;
3) branches from the supply line to low-power electrical receivers, if the line supplying them is protected by a device with a setting of not more than 25 A for power receivers and household appliances, and for lamps - in accordance with 6.2.2;
4) branches from the supply line of the conductors of the measurement, control and signaling circuits, if these conductors do not go beyond the corresponding machines or switchboard, or if these conductors go beyond their limits, but the electrical wiring is made in pipes or has a non-combustible sheath.
It is not allowed to install protective devices at the points of connection to the supply line of such control, signaling and measurement circuits, the disconnection of which can lead to dangerous consequences (shutdown of fire pumps, fans that prevent the formation of explosive mixtures, some auxiliary mechanisms of power plants, etc.) . In all cases, such circuits must be carried out by conductors in pipes or have a non-combustible sheath. The cross section of these circuits must not be less than those given in 3.4.4.
 See other articles Section Rules for the installation of electrical installations (PUE).
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