Main Distribution Board (MDB): Definition, Meaning, Requirements

Main distribution board (MDB): a distribution board that fulfills all the functions of a main electrical distribution for the supplied area assigned to it and where the voltage is measured for operating the electric supply system for safety services [defined in the IEC 60364-7-710-2021].

The main distribution board is a low-voltage switchgear that is installed in a building or its detached part. The MDB is used to distribute electrical energy between the distribution and final circuits connected to it in the entire electrical installation of a building or in some part of it. By means of the switchgear and controlgear installed in the MDB as well as the protective equipment, the electrical circuits connected to it are protected and controlled.

Main distribution board of a residential complex
Figure 1. Main distribution board of a residential complex.

The main distribution board in an installation will also normally provide a main switch (known as an incomer) which switches the phase and neutral lines for the whole supply.

MDB Basic Requirements

  • A TN-C system shall not be used downstream of the main distribution board.
  • In Norway, the use of a PEN conductor downstream of the main distribution board is not allowed.
  • The electric source for safety services shall automatically take over, if the voltage of one or more incoming line conductors of the main distribution board of the building with the main power supply drops to less than 85 % of the nominal voltage and during more than 0,5 s
  • The MDB should be located in a building. Only then can its components be guaranteed reliable and long-term operation, as they will be protected from the adverse effects of the external (outside the building) environment.
  • The minimum degree of protection provided by an main distribution board enclosure against mechanical impact, if necessary, shall be defined by the relevant assembly standards. Mechanical impact tests, where required by the specific assembly standard, are to be carried out in accordance with the test requirements of the specific assembly standard.
  • The degree of protection provided by any MDB against contact with live parts, ingress of solid foreign bodies and water is indicated by the IP code according to IEC 60529:1989, IEC 60529:1989/AMD1:1999 and IEC 60529:1989/AMD2:2013.

After installation in accordance with the MDB manufacturer’s instructions, the degree of protection of an enclosed assembly shall be at least IP2X. The degree of protection provided from the front of a dead-front assembly shall be at least IPXXB.

For assemblies for outdoor use having no supplementary protection, the second characteristic numeral shall be at least 3.

  • The apparatus and circuits in the assembly shall be so arranged as to facilitate their operation and maintenance with the required protection against electric shock.
  • Hazardous-live-parts shall be completely covered with insulation that can only be removed by destruction or by the use of a tool.
  • The 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 and lacquers alone are generally not considered to satisfy the requirements for basic insulation. This does not preclude the use of specifically designed insulating coatings that fulfil the dielectric requirements.
  • Air-insulated live parts shall be inside enclosures or behind barriers. The enclosures or barriers shall provide a degree of protection of at least IPXXB.
  • Horizontal top surfaces of accessible enclosures having a height equal to or lower than 1,6 m above the standing area shall provide a degree of protection of at least IPXXD.
  • All exposed-conductive-parts of the main distribution board shall be interconnected together and to the protective conductor of the supply or via an earthing conductor to the earthing arrangement.
  • With metal parts of the main distribution board where abrasion-resistant finishes are used (e.g. gland plates with powder coatings), interconnections providing earth continuity requires the removal or the penetration of the coating.
  • A protective conductor within the MDB shall be designed so that it can withstand the highest thermal and dynamic stresses arising from earth fault currents in external circuits supplied through the assembly in its installed location.
  • In the case of a main distribution board containing structural parts, frameworks, enclosures, etc. made of conducting materials, a protective conductor, if provided, need not be insulated from these parts. Conductors to voltage-operated fault detection devices, including the conductors connecting them to a separate earth electrode, shall be insulated when specified by their manufacturer. This can also apply to the earth connection of the transformer neutral.
  • Switching devices and components shall be installed and wired in the MDB in accordance with instructions provided by their manufacturers and in such a manner that their proper functioning is not impaired by interaction, such as heat, switching emissions, vibrations, electromagnetic fields, which are present in normal operation. In the case of electronic equipment, this may necessitate the separation or screening of all electronic signal processing circuits.
  • When fuses are installed, the original manufacturer shall state the type and rating of the fuse-links to be used.
  • Functional units mounted on the same support (mounting plate, mounting frame) and their terminals for external conductors shall be so arranged as to be accessible for mounting, wiring, maintenance and replacement.
  • Barriers for manual switching devices shall be so designed that the switching emissions do not present a danger to the operator.
  • To minimize danger when replacing fuse-links, interphase barriers shall be applied, unless the design and location of the fuses makes this unnecessary.
  • Unless otherwise specified in the relevant product standard, the colours of indicator lights and push-buttons shall be in accordance with IEC 60073:2002.
  • The busbars shall be arranged in such a manner that an internal short-circuit is not to be expected. They shall be rated at least in accordance with the requirements concerning the short-circuit withstand strength and designed to withstand at least the short-circuit stresses limited by the protective device(s) on the supply side of the busbars.
  • The minimum cross-sectional area of the neutral conductor within a three-phase and neutral circuit shall be:

– for circuits with a line conductor cross-sectional area up to and including 16 mm2, 100 % of that of the corresponding lines;

– for circuits with a line conductor cross-sectional area above 16 mm2, 50 % of that of the corresponding lines with a minimum of 16 mm2.

NOTE. There are specific applications when a smaller neutral conductor is acceptable to the user.

  • For certain applications, which lead to high values of zero sequence harmonics (e.g. third order harmonics), higher cross-sections of the neutral conductor may be required as these harmonics of the lines are added in the neutral conductor and lead to high current load at higher frequencies. This is subject to special agreement between the main distribution board manufacturer and the user.
  • The design of the auxiliary circuits shall take into account the auxiliary circuit(s) earthing and ensure that an earth fault shall not cause unintentional dangerous operation.
  • In general, auxiliary circuits shall be protected against the effects of short-circuits. However, a short-circuit protective device shall not be provided if its operation is liable to cause a danger. In such a case, the conductors of auxiliary circuits shall be arranged in such a manner that a short-circuit is not to be expected.
  • Except for the cases mentioned in 8.6.6 [2], the method and the extent of identification of conductors, for example by arrangement, colours or symbols, on the terminals to which they are connected or on the end(s) of the conductors themselves, is the responsibility of the main distribution board manufacturer and shall be in agreement with the indications on the wiring diagrams and drawings. Where appropriate, identification according to IEC 60445 shall be applied.
  • The protective conductor (PE, PEL, PEM or PEN) shall be readily distinguishable by location and/or marking or colour. If identification by colour or marking is used it shall be in accordance with IEC 60445. When the protective conductor is an insulated single-core cable, this colour identification shall be used, preferably throughout the whole length.
  • Any neutral or mid conductor of the main circuit shall be readily distinguishable by location and/or marking or colour (see IEC 60445 where blue is required).
  • If special precautions are required at the place of installation to ensure proper cooling, the main distribution board manufacturer shall furnish the necessary information (for instance indication of the need for spacing with respect to parts that are liable to impede the dissipation of heat or produce heat themselves).
  • The coordination of protective devices within the assembly with those to be used externally to the assembly shall be the subject of an agreement between the assembly manufacturer and the user. Information given in the assembly manufacturer’s catalogue may take the place of such an agreement.
  • If the operating conditions require maximum continuity of supply, the settings or selection of the short-circuit protective devices within the main distribution board should, where possible, be so coordinated that a short-circuit occurring in any outgoing circuit is cleared by the SCPD installed in the circuit without affecting the other outgoing circuits, thus ensuring selectivity of the protective system.

What is the Difference Between a Consumer Unit and a Distribution Board?

In the United Kingdom, a distribution board designed for domestic installations is known as a consumer unit.

Consumer unit (may also be known as a consumer control unit or electricity control unit). A particular type of distribution board comprising a type-tested co-ordinated assembly for the control and distribution of electrical energy, principally in domestic premises, incorporating manual means of double-pole isolation on the incoming circuit(s) and an assembly of one or more fuses, circuit-breakers, residual current operated devices or signalling and other devices proven during the type-test of the assembly as suitable for such use.

References

  1. IEC 60364-7-710-2021
  2. IEC 61439-1-2020