Protection of electrical networks with voltage up to 1 kV. Choice of protection

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Section 3. Protection and automation

Protection of electrical networks with voltage up to 1 kV. Choice of protection

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Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE)

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3.1.8. Electrical networks must be protected against short-circuit currents, providing the shortest possible disconnection time and selectivity requirements.

The protection should provide disconnection of the damaged section in case of a short circuit at the end of the protected line: one-, two- and three-phase - in networks with dead-earthed neutral; two- and three-phase - in networks with isolated neutral.

Reliable disconnection of the damaged section of the network is ensured if the ratio of the lowest rated short-circuit current to the rated current of the fuse link or circuit breaker release is not less than the values given in 1.7.79 and 7.3.139.

3.1.9. In networks protected only from short-circuit currents (not requiring overload protection in accordance with 3.1.10), with the exception of extended networks, for example, rural, utility networks, it is allowed not to carry out the design verification of the multiplicity of short-circuit current given in 1.7.79 and 7.3.139, if provided condition that in relation to the long-term permissible current loads of conductors given in the tables of Ch. 1.3, protective devices had a multiplicity of not more than:

300% for the rated current of the fuse link;
450% for the setting current of a circuit breaker having only an instantaneous overcurrent release (cut-off);
100% for the rated current of the release of a circuit breaker with a non-adjustable inverse current characteristic (regardless of the presence or absence of a cutoff);
125% for the pick-up current of a circuit breaker release with an adjustable inverse current-dependent characteristic; if there is still a cut-off on this circuit breaker, then its multiplicity of the operating current is not limited.

The presence of protection devices with overestimated current settings is not a justification for increasing the cross section of conductors in excess of those indicated in Ch. 1.3.

3.1.10. Indoor networks made of openly laid conductors with a combustible outer sheath or insulation must be protected from overload.

In addition, the following must be protected from network overload indoors:

lighting networks in residential and public buildings, in commercial premises, utility rooms of industrial enterprises, including networks for household and portable electrical receivers (irons, kettles, stoves, room refrigerators, vacuum cleaners, washing and sewing machines, etc.), and also in fire hazardous areas;
power networks at industrial enterprises, in residential and public buildings, commercial premises - only in cases where, due to the conditions of the technological process or the mode of operation of the network, a prolonged overload of conductors may occur;
networks of all types in hazardous areas - in accordance with the requirements of 7.3.94.

3.1.11. In networks protected against overloads (see 3.1.10), conductors should be selected according to the rated current, while the condition must be ensured that, in relation to the long-term permissible current loads given in the tables of Ch. 1.3, protective devices had a multiplicity of not more than:

80% for the rated current of the fusible link or the setting current of the circuit breaker, which has only the maximum instantaneous release (cut-off), - for conductors with PVC, rubber and similar insulation in terms of thermal characteristics; for conductors laid in non-explosive industrial premises of industrial enterprises, 100% is allowed;
100% for the rated current of the fusible link or the setting current of the circuit breaker, which has only the maximum instantaneous release (cut-off), - for cables with paper insulation;
100% for the rated current of the circuit breaker release with a non-adjustable inverse current-dependent characteristic (regardless of the presence or absence of a cutoff) - for conductors of all brands;
100% for the starting current of the release of the circuit breaker with an adjustable characteristic inversely dependent on the current - for conductors with PVC, rubber and similar insulation in terms of thermal characteristics;
125% for the pick-up current of a circuit breaker release with an adjustable current-inverse characteristic - for cables with paper insulation and vulcanized polyethylene insulation.

3.1.12. The long-term permissible current load of branch conductors to short-circuited electric motors must be at least:

100% of motor rated current in non-hazardous areas;
125% of motor rated current in hazardous areas.

The ratios between the long-term permissible load of conductors to short-circuited electric motors and the settings of protective devices in any case should not exceed those specified in 3.1.9 (see also 7.3.97).

3.1.13. In cases where the required permissible continuous current load of the conductor, determined according to 3.1.9 and 3.1.11, does not coincide with the data in the tables of permissible loads given in Ch. 1.3, it is allowed to use a conductor of the nearest smaller section, but not less than that required by the rated current.

See other articles Section Rules for the installation of electrical installations (PUE).

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