Section 1 General Rules

Selection of conductors for heating, economic current density and corona conditions. The choice of sections of conductors for heating

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

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1.3.2. Conductors for any purpose must meet the requirements for maximum permissible heating, taking into account not only normal, but also post-emergency conditions, as well as conditions during repairs and possible uneven distribution of currents between lines, bus sections, etc. When checking for heating, a half-hour maximum is accepted current, the largest of the average half-hour currents of a given network element.

1.3.3. For intermittent and short-term operating modes of electrical receivers (with a total cycle duration of up to 10 minutes and a working period of no more than 4 minutes), the current reduced to the long-term mode should be taken as the calculated current for checking the cross-section of heating conductors. Wherein:

1) for copper conductors with a cross-section of up to 6 mm2, and for aluminum conductors up to 10 mm2, the current is taken as for installations with long-term operation;

2) for copper conductors with a cross-section of more than 6 mm2, and for aluminum conductors with a cross-section of more than 10 mm2, the current is determined by multiplying the permissible long-term current by the coefficient where Tp.v is the duration of the operating period expressed in relative units (on duration in relation to the cycle duration).

1.3.4. For a short-term operating mode with a switching duration of no more than 4 minutes and breaks between switching on sufficient to cool the conductors to ambient temperature, the maximum permissible currents should be determined according to the standards for intermittent short-term duty (see 1.3.3.). When the duration of switching on is more than 4 minutes, as well as during breaks of insufficient duration between switching on, the maximum permissible currents should be determined as for installations with a long operating mode.

1.3.5. For cables with voltages up to 10 kV with impregnated paper insulation that carry less than rated loads, a short-term overload indicated in table may be allowed. 1.3.1.

1.3.6. For the period of liquidation of the post-emergency regime, an overload of up to 10% is allowed for cables with polyethylene insulation, and for cables with polyvinyl chloride insulation up to 15% of the nominal load during peak loads lasting no more than 6 hours per day for 5 days, if the load during the remaining periods of time of these days is not exceeds nominal.

For the period of liquidation of the post-emergency regime for cables with voltages up to 10 kV with paper insulation, overloads are allowed for 5 days within the limits specified in table. 1.3.2.

For cable lines that have been in operation for more than 15 years, overloads should be reduced by 10%.

Overloading of cable lines with a voltage of 20 - 35 kV is not allowed.

Table 1.3.1. Permissible short-term overload for cables with voltages up to 10 kV with impregnated paper insulation

Table 1.3.2. Permissible overload for the period of post-emergency liquidation for cables with voltage up to 10 kV with paper insulation

1.3.7. The requirements for normal loads and post-accident overloads apply to cables and the connecting and termination couplings and terminations installed on them.

1.3.8. Zero working conductors in a four-wire three-phase current system must have a conductivity of at least 50% of the conductivity of the phase conductors; if necessary, it should be increased to 100% of the conductivity of the phase conductors.

1.3.9. When determining permissible long-term currents for cables, bare and insulated wires and busbars, as well as for rigid and flexible conductors laid in an environment whose temperature differs significantly from that given in 1.3.12 - 1.3.15. and 1.3.22., the coefficients given in table should be applied. 1.3.3.

Table 1.3.3. Correction factors for currents for cables, bare and insulated wires and busbars depending on ground and air temperatures

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