Guidelines to the Construction of an LV Assembly Complying with The Standards IEC
The following prescriptions are aimed at ensuring that the protective measures required are taken when the assembly is installed in the electrical plant, in compliance with the relative standards.
1| Protection Against Direct Contact
Protection against direct contact can be obtained both by means of the assembly construction itself as well as by means of complementary measures to be used during installation.
The protective measures against direct contact are:
Protection by insulation of live parts: Live parts shall be completely covered with insulation which can only be removed by destruction. This insulation shall be made of suitable materials capable of durably withstanding the mechanical, electrical and thermal stresses to which the insulation may be subjected in service. Paints, varnishes, lacquers and similar products used alone are generally not considered suitable for providing adequate insulation for protection against direct contact.
Protection by barriers or enclosures: All external surfaces shall provide a degree of protection against direct contact of at least IPXXB. Horizontal top surfaces of accessible enclosures having a height equal to or lower than 1.6 m shall provide a degree of protection of at least IPXXD. The distance between the mechanical means provided for the protection and the live parts they protect shall not be less than the values specified for the clearances and creepage distances. All barriers and enclosures shall be firmly secured in place. Taking into account their nature, size and arrangement, they shall have sufficient stability and durability to resist the strains and stresses likely to occur in normal service without reducing clearances.
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2| Protection Against Indirect Contact
The user shall indicate the protective measure which is applied to the installation for which the ASSEMBLY is intended.
The protective measures against indirect contact are:
Protection by using protective circuits: A protective circuit (coordinated with a device for automatic supply disconnection) can be realized either separately from the metal enclosure or the enclosure itself can be used as part of the protective circuit.
The exposed conductive parts of an assembly which do not constitute a danger either because they cannot be touched on large surfaces or grasped with the hands because they are of small size (e.g. screws, nameplates, etc.) need not be connected to the protective circuits.
Manual operating means, such as levers, handles and other metal devices, shall be either electrically connected in a secure manner with the parts connected to the protective circuits or provided with additional insulation adequate for the maximum insulation voltage
of the assembly. Metal parts covered with a layer of varnish or enamel cannot generally be considered to be adequately insulated to comply with these requirements.
For lids, doors, cover plates and the like, the usual metal screwed connections and metal hinges are considered sufficient to ensure continuity provided that no electrical equipment requiring earthing is attached to them. In this case, the live parts shall be connected by a protective conductor with a cross-section at least equal to the maximum cross-sectional area of the phase conductor which supplies the assembly.
The cross-sectional area of protective conductors (PE, PEN) in an assembly intended to be connected to external conductors shall be determined through one of the following methods:
a) the cross-sectional area of the protective conductor shall not be less than the appropriate value shown in the following table.
If a non-standard value results from the application of Table the larger standardized cross-section nearest to the calculated value shall be used.
The values of this Table are valid only if the protective conductor (PE, PEN) is made of the same material as the phase conductor. If not, the cross-sectional area of the protective conductor (PE, PEN) is to be determined in a manner that produces a conductance equivalent to that which results by applying Table-1.
For PEN conductors, the following additional requirements shall apply:
The minimum cross-sectional area shall be 10 mm2 for a copper conductor and 16 mm2 for an aluminium one;
The cross-sectional area of the PEN conductor shall not be lower than that of the neutral conductor*;
The PEN conductors need not be insulated within an assembly;
The structural parts shall not be used as a PEN conductor. However, mounting rails made of copper or aluminium may be used as PEN conductors;
for certain applications in which the current in the PEN conductor may reach high values, for example, large fluorescent lighting installations, a PEN conductor having the same or higher current carrying capacity as the phase conductors may be necessary; this shall be subject to a special agreement between manufacturer and user.
* The minimum cross-sectional area of the neutral in a three-phase circuit plus neutral shall be:
for circuits with a phase conductor of cross-sectional area S ≤ 16 mm2100% of that of the corresponding phases;
- for circuits with a phase conductor of cross-sectional area S > 16 mm2, 50% of that of the corresponding phases with 16 mm2 minimum. It is assumed that the neutral currents do not exceed 50% of the phase currents.
b) the cross-sectional area of the protective conductor (PE, PEN) may be calculated with the aid of the following formula:
This formula is used to calculate the cross-section of the protective conductors necessary to withstand the thermal stresses caused by currents of duration in a range between 0.2s and 5s, where:
Sp is the area of the section expressed in mm2; I is the r.m.s. value of the fault current (in AC) flowing through the protective device, expressed in A, for a fault of negligible impedance;
t is the trip time of the breaking device in seconds;
k is a factor whose value depends on the material of the protective conductor, on the insulation and on other elements, as well as on the initial and final temperature.
of factor k for insulated protective conductors not incorporated in bare cables or bare protection conductors in touch with cable coatings.
Note: it is presumed that the initial temperature of the conductors is 30°C.
The exposed conductive parts of a device that cannot be connected to the protective circuit through its own fixing means shall be connected to the protective circuit of the assembly by means of a conductor, whose cross-section shall be chosen according to the following Table:
S: cross-sectional area of the phase conductor
Protection realized with measures other than the use of protective circuits: Electrical assemblies can provide protection against indirect contact by means of the following measures which do not require a protective circuit:
a) electrical separation of the circuits;
b) full insulation.
3| Management in the safety of the assembly
The use of the assembly shall guarantee the usual safety protections, both in case of operation as well as in case of replacement of small components, such as lamps and fuses, on behalf of ordinary personnel, if such procedure is followed.
More complex and dangerous operations may be performed by authorized personnel only and are related to the carrying out of particular procedures and the use of particular safety components, as regards the accessibility of the assembly, for:
inspections and controls;
maintenance;
extension works also in the presence of live parts