What Is a Consumer Unit?

The term ‘consumer unit (may also be known as a consumer control unit or electricity control unit)’ is officially defined within the 2018 edition of BS 7671 as:

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.

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In the UK, a type A distribution board intended to be operated by ordinary persons (DBO) used principally for domestic (household) installations and having a maximum incoming unit rating of 100 A and a maximum outgoing circuit rating of 63 A, is known as a “consumer unit” or “customer distribution board”.

A consumer unit may be installed at the origin of the electrical installation of a domestic premise, which is a low-voltage switchgear designed for the input of electricity into the electrical installation of the domestic premise and its distribution in the electrical installation of the domestic premise among the electrical equipment.

To perform these functions, the consumer unit is equipped with low-voltage switchgear and controlgear, through which control and protection of the final circuits departing from it is performed. The consumer unit can also meter the electrical energy consumed. In this case, the consumer unit is equipped with an electricity meter.

General view of the consumer unit
Figure 1. General view of the consumer unit with closed (A) and open (B) door.

Main Types of Consumer Unit

In the UK, there are four main types of consumer unit:

  1. Main switch consumer unit.
  2. Dual RCD consumer unit.
  3. High integrity consumer unit.
  4. RCD incomer consumer unit.

Main Components of the Consumer Unit

  • Enclosure.
  • Neutral bar/s.
  • Earth bar.
  • Din rail.
  • Busbar.
  • Main switch.
  • Mini Circuit Breaker (MCB).
  • Residual Current Device (RCD).
  • Residual current operated circuit-breaker with integral overcurrent protection (RCBO).
  • Surge protection device (SPD).
  • Blanks.

Note. At the origin of consumer units, circuit breakers for household use are usually installed corresponding to IEC 60898-1. The preferred value of rated current for circuit breakers in this standard is set to 32 A.

The preferred rated currents for household residual current devices corresponding to IEC 61008-1 and IEC 61009-1 are also set to 32 A.

Note. Residual current devices, which have a rated residual operating current (IΔn) of 30 mA, can only be of the general type. According to the requirements of IEC 61008-1 and IEC 61009-1 such RCDs operate without time delay. There are usually several final electrical circuits connected to the consumer unit, protected by RCDs having a rated residual operating current of 10 and 30 mA. Therefore, the use of a general type RCD on the origin of the consumer unit eliminates its selective operation with RCDs that protect the final electrical circuits.

Consumer Unit Requirements

  • Consumer units must meet the requirements of the IEC 61439 “Low-voltage switchgear and controlgear assemblies” complex standards.
  • Consumer units can be wall-mounted or recessed in a recess.
  • Consumer units can be single-phase and three-phase and correspond to class I and class II electrical equipment. At the origin of the consumer unit may be installed a switching device.
  • Structural elements of class I consumer unit (frames, shells and other parts) are usually made of steel, which has a protective coating. Class II consumer unit enclosures are usually made of insulating materials that are resistant to combustion when exposed to a glow-wire heated to a temperature of (850±10)°C. If consumer units are embedded in hardly flammable walls, their casings must be resistant to the effects of a glow-wire heated to a temperature of (650±10)°C. Class II consumer unit enclosures may also be made of steel. Insulating elements of the consumer unit of classes I and II, which fasten live parts, must be resistant to ignition when exposed to a glow-wire heated to a temperature of (960 ± 10) ° C.
  • The degree of protection against contact with live parts in places accessible to the touch and against ingress of foreign solid bodies when the door is closed shall not be less than IP3X in consumer units of class I and IP4X in consumer units of class II. The degree of protection provided by the operational panel when the door is open and at the entry and exit points of conductors in the wall-mounted version of the consumer unit must be at least IP2XC. In consumer units with screw-in type fuses, the degree of protection on the operating panel must be at least IP2X.
  • Wall-mounted and recessed in a recess consumer units must have suitable structural elements for fixing them. The recessed in a recess consumer units must have a frame that covers the edges of the recesses.
  • Consumer units can be made with or without doors. Behind the door of the consumer unit there shall be the operational panel with the apparatus control elements, which in combination with other structural elements of the unit shall exclude access to its live parts.
  • In consumer units with electricity meters, there must be structural elements that can be sealed to prevent access to the metering circuits (from the origin of the consumer unit to the origin of the meters). The operational panel of the consumer unit may be such an element.
  • In the consumer units, the protective apparatus of the final electrical circuits must be marked with serial numbers. In the consumer panel, the apparatus must be provided with places for recording its purpose. These records can be given in tables placed on the inner sides of the consumer units doors.
  • In the consumer units must be provided with terminals designed to connect the conductors of external electrical circuits. The input terminals must ensure the connection of conductors with a cross-section of 2.5 to 16 mm2. Terminals for bonding conductors shall be capable of connecting conductors with a cross-section of 10 mm2. The terminals for conductors of final electrical circuits must be designed for the connection of conductors with a cross-section of 1.5 to 6 mm2. One conductor must be connected to each protective or neutral conductor terminal.
  • In class I consumer units, the neutral conductor terminals must be insulated from the conductive enclosures in the same way as the phase conductor terminals, and the protective conductor terminals must have electrical connections to the enclosures. In class II consumer units, the terminals for protective and neutral conductors must be insulated from the conductive enclosures in the same way as the terminals for the phase conductors.
  • The exposed-conductive-parts of class I consumer units must be effectively connected to each other and to the PE protective busbar. The resistance between the protective busbar and each exposed-conductive-part of the consumer unit must not exceed 0.1 Ω.
  • Internal electrical circuits of consumer units should be made with insulated copper wires. Wires must have insulation rated for 660 VAC.
  • Insulated wires of internal electrical circuits must be laid in such a way that they do not touch non-insulated live parts, sharp edges of the consumer unit enclosure. The bending radii of wires must be at least six times their outside diameter. Wires must not have intermediate twists, soldered and other connections. The insulation resistance of the internal circuits of the consumer unit in the cold state shall not be less than 10 MΩ.
  • 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 60445:2021. 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-point conductor of the main circuit shall be readily distinguishable by location and/or marking or colour (see IEC 60445:2021 where blue is required).
  • Metal parts of consumer units must have protective paint, powder polymer and other coatings.
  • Within domestic (household) premises, consumer units and similar switchgear assemblies shall have their enclosure manufactured from non-combustible material, or be enclosed in a cabinet or enclosure constructed of non-combustible material. NOTE: Ferrous metal, e.g. steel, is deemed to be an example of a non-combustible material.
  • In low voltage assemblies to the BS EN 61439 series, e.g. consumer units incorporated devices and components shall only be those declared suitable according to the assembly manufacturer’s instructions or literature.
  • Incorporated components inside the assembly can be from different manufacturers. It is essential that all incorporated components should have had their compatibility for the final enclosed arrangements verified by the original manufacturer of the assembly and be assembled in accordance with their instructions e.g. the consumer unit, distribution board manufacturer. The original manufacturer is the organization that carried out the original design and the associated verification of the low voltage switchgear and control gear assembly to the relevant part of the BS EN 61439 series. If an assembly deviates from its original manufacturer’s instructions, or includes components not included in the original verification, the person introducing the deviation becomes the original manufacturer with the corresponding obligations.

References

  1. BS 7671
  2. BS EN 61439-3:2019
  3. IEC 61439-1-2020

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