How to choose the voltage of the cables in LV

It is quite common to know how to calculate the section of a conductor, but another parameter must also be taken into account when calculating a laying, the level of insulation or nominal voltage, which is almost always set by the ITC-BT 20, which usually contains mandatory minimum values ​​for each installation system.

If the installation of a home is going to work at 230V voltage, installing a 300V cable with nominal compound voltage would not be correct, since the REBT requires minimum nominal voltage values ​​according to the installation system. Likewise, these minimum nominal voltage values ​​are usually higher than the commonly used voltages. Let’s review the minimum voltages required in the different sections of the ITC-BT 20.

The insulation levels of power cables, previously called nominal voltage and currently rated voltage, are defined by the values ​​of simple voltage and compound voltage (Uo/U):

-Uo: rated effective voltage at an industrial frequency between each conductor and the screen or sheath, for which the cable and its accessories have been designed.

-U: Rated effective voltage at an industrial frequency between any two conductors, for which the cable and its accessories have been designed.

Minimum required voltage of 450/750 V

-protective tubes (section 2.2.1 of ITC-BT 20)

-construction holes (section 2.2.6)

-protective channels of IP4X degree or higher (section 2.2.7)

-moldings (section 2.2.7)

The minimum required voltage of 0.6/1 kV

-Insulated conductors fixed directly on the walls (section 2.2.2)

They have exposed cables without extra protection, so the cable must include its own protection (sheath) as required by this section of ITC-BT 20 when it prescribes that the cables are provided with insulation and sheath.

Buried insulated conductors (section 2.2.3)

We find a section with somewhat ambiguous wording since it admits that the cables may not have a cover and assigned voltage of 0.6/1 kV. Even so, when referring to ITC-BT 07, it indicates that they are cables with sheath and rated voltage of 0.6/1 kV. In this same line, buried cables are prescribed in other ITC-BTs such as number 15.

Thus, it is required that these cables have a cover since burying them directly could damage their insulation. When they are buried under a tube, they are already protected by it, but the laying phase can damage the insulation if simple insulated conductors are used. In addition, these are conduits that are likely to be affected by water seepage, so mechanical protection of the cable is necessary from its manufacture, providing it with a cover and the maximum level of insulation within the LV (0.6/1 kV ).

Aerial conductors (section 2.2.5)

This section refers to the ITC-BT 06 where it is explained that the cables must have a rated voltage of not less than 0.6/1 kV and comply with UNE 21030. These are braided cables for aerial networks stretched on supports or perched on facades typically outdoors.

This point 2.2.5 applies to cables that are not within point 2.2.2. Cables stapled to interior walls would be included, with Polirret Feriex CPRO and Al Polirret CPRO-type cables suitable for interiors or exteriors.

No minimum voltage required

-Insulated conductors directly embedded in structures (section 2.2.4)

A minimum rated voltage level is not required, but cables must have insulation and sheathing. See next section.

-Insulated conductors in tray or tray support (section 2.2.9)

Cables installed in trays must be insulated and covered. There is no specific minimum voltage level required. The rated compound voltage of the cable shall be equal to or greater than the circuit voltage. Commonly used 0.6/1 kV cables type RV-K (Retenax CPRO Flex) or RZ1-K (AS) (Afumex Class 1000 V (AS)) are typically used.

It seems logical to think about the need for a cover for cables in a tray or tray support since it is a channel without a cover and therefore the cable is exposed to the danger that some mechanical aggression could affect it. For this, the cable covers protect conductors and insulation.

Other cases

-Insulated conductors under protective channels of a degree lower than IP4X (section 2.2.7)

Cables with insulation and sheath with a minimum rated voltage of 300/500 V are required.

-Special voltage installations (ITC-BT 37)

Special voltages are understood to be installations that exceed 500 V effective value in alternating current or 750 V arithmetic mean value in direct current. In this case, the nominal voltage required for the cable will not be less than 0.6/1 kV.

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