What Is a Final Electric Circuit (Branch Circuit)? Definition, Meaning, Requirements

Definitions

Final electric circuit (IEC, UK) or (branch circuit, US): an electric circuit incorporating current-using equipment and/or socket-outlets [defined in the IEC 60050-826-2022].

Branch circuit: the circuit conductors between the final overcurrent device protecting the circuit and the outlet(s) [defined in the NEC].

Some regulatory documents use the terms “group electric circuit”, “group circuit” and “group network” instead of the correct term “final electric circuit”.

Final circuit: a circuit connected directly to current-using equipment, or to a socket-outlet or socket-outlets or other outlet points for the connection of such equipment [defined in the BS 7671].

Meaning

The final electric circuits of the electrical installation of a building are intended to distribute electricity to current-using equipment. They are connected to the main distribution boards, floor distribution boards and other low-voltage switchgear of the electrical installation of a building. These circuits usually include protective devices, wires and cables and the final electrical equipment connected to them, such as electric lights, socket-outlets, electric heaters, washing machines, refrigerators, electric tools, etc.

Figure 1 below shows an example of final electrical circuits:

Final electrical circuits
Figure 1. Example of a diagram with final circuits

The name of the electric circuit in question – final circuit, given to it in the International Electrotechnical Dictionary (IEV), underlines the main function it fulfils in the electrical installation of a building, namely to provide electrical energy to current-using equipment converting it to other forms of energy and to socket-outlets.

Final Circuit Requirements

1. The maximum disconnection time stated in Table 41.1 [2] shall be applied to final circuits with a rated current not exceeding:

  • 63 A with one or more socket-outlets, and
  • 32 A supplying only fixed connected current-using equipment.

2. Additional requirements for circuits with luminaires in TN- and TT-systems: in premises designed to accommodate a single household, additional protection by a residual current protective device (RCD) with a rated residual operating current not exceeding 30 mA shall be provided for a.c. final circuits supplying luminaires.

3. In Norway, where more installations are likely to have galvanic connection to the same distribution network, all final circuits in IT installations with galvanic connection to a public IT distribution network need to be disconnected within the time specified for a TN system (see Table 41.1) in the event of a fault of negligible impedance between the line conductor and an exposed-conductive-part or a protective conductor in the circuit or
equipment.

4. Final circuits and current-using equipment shall be protected against insulation faults as follows:

  • a) In TN and TT earthing systems, RCDs with a rated residual operating current IΔn ≤300 mA shall be used. Where resistive faults may cause a fire, e.g. for overhead heating with heating film elements, the rated residual operating current shall be IΔn ≤30 mA.
  • b) In IT earthing systems, insulation monitoring devices monitoring the whole installation or RCMs (residual current monitoring devices) in the final circuits, both with audible and visual signals, shall be provided. Alternatively, RCDs with a rated residual operating current as specified in a) may be used. In the event of a second fault see Part 41 [3] for disconnection times.

Mineral insulated cables and busbar trunking systems are not considered likely to cause a fire from insulation faults and therefore need not be protected.

NOTE. Cables with metallic coverings are recommended. The metallic covering should be connected to the protective conductor.

5. In Norway, final circuits in IT installations connected to a public low-voltage distribution network shall, for protection against fire, be protected in accordance with the requirements for protection against electric shock as specified in 411.6.1 of IEC 60634-4-41:2017.

6. It is recommended that special measures be taken to protect against the effects of arc faults in final circuits:

  • in premises with sleeping accommodation;
  • in locations with risks of fire due to the nature of processed or stored materials, i.e. BE2 locations, (e.g. barns, wood-working shops, stores of combustible materials);
  • in locations with combustible constructional materials, i.e. CA2 locations (e.g. wooden buildings);
  • in fire propagating structures, i.e. CB2 locations;
  • in locations with endangering of irreplaceable goods.

NOTE 1. A material is considered to be non-combustible if, in compliance with ISO 1182 [31] and ISO 1716 [32], it does not support combustion.

In a.c. circuits, the use of arc fault detection devices (AFDD) in compliance with IEC 62606 will satisfy the above-mentioned recommendation.

If used, an AFDD shall be placed at the origin of the circuit to be protected.

NOTE 2. An AFDD is a device intended to mitigate the effects of arcing faults by initiating disconnection of the circuit when an arc fault is detected, see Annex B [3].

The use of AFDDs does not obviate the need to apply one or more measures provided in other clauses in this standard.

NOTE 3. National committees may decide if the use of AFDDs is made a requirement or a recommendation in their national standard.

7. In the USA, special measures to protect against the effects of arc faults in final circuits are required in accordance with NFPA 70 article 210.12 [4].

8. In the UK socket outlets can be supplied through ring final circuits protected by a 32 A protective device complying with IEC 60269, IEC 60898, IEC 60947-2, or IEC 61009-1, wired with copper conductors having phase and neutral conductors with a minimum crosssectional area of 2,5mm2 except for 2 core mineral insulated cables complying with the relevant IEC standard for which the minimum cross-sectional area is 1,5 mm2. Such ring final
circuits are deemed to meet the requirements of 433.1 [5] if the current-carrying capacity (Iz) of the cable is not less than 20 A, and if, under the
intended conditions of use, the load current in any part of the ring is unlikely to exceed for long periods the current carrying capacity (Iz) of the cable.

In the UK socket outlets can be supplied through ring final circuits protected by a 32 A protective device with or without unfused spurs.

9. Final circuits for ME equipment and ME systems shall be for the exclusive use of such equipment.

10. In group 2 medical locations, a medical IT system, including the requirements of 710.411.6.3.101 [6] and 710.512.1.101 [6], shall be used for final circuits and where the same final circuit is connected to ME equipment or an ME system, located within the patient environment. Exceptions can be made for final circuits for:

  • equipment with a rated power greater than 5 kVA,
  • X-ray equipment,
  • the supply of the motors of fixed operating tables.

In medical locations of group 2, the supply to final circuits for socket-outlets for ME equipment and ME systems used for life-support of the patient, shall not be automatically disconnected in the event of a first fault.

11. In medical IT systems, a signal lamp or display shall be fitted to indicate the availability of supply for each final circuit supplying socket-outlets intended for ME equipment.

NOTE. Where socket-outlets are fitted with supply indicator luminaires, the emission of green light is preferred.

Switched socket-outlets shall not be used for medical IT systems.

The configuration of socket-outlets shall be as follows:

  • a single socket-outlet supplied by an individually protected circuit; or
  • several socket-outlets separately supplied by a minimum of two final circuits, with a maximum of six socket-outlets per final circuit.

12. For a final circuit with a number of socket-outlets or connection units intended to supply two or more items of equipment, where it is known or reasonably to be expected that the total protective conductor current in normal service will exceed 10 mA, the circuit shall be provided with a high integrity protective conductor connection complying with the requi rements of Regulation 543.7.1 [7, in the UK] . The following arrangements of the final circuit are acceptable:

  • A ring final circuit with a ring protective conductor. Spurs, if provided, require high integrity protective conductor connections complying with the requirements of Regulation 543.7.1 [7]
  • A radial final circuit with a single protective conductor:

– the protective conductor being connected as a ring, or

– a separate protective conductor being provided at the final socket-outlet by connection to the metal conduit or ducting, or

– where two or more similar radial ci rcuits supply socket-outlets in adjacent areas and are fed from the same distribution board, have identical means of short-circuit and overcurrent protection and circuit protective conductors of the same cross-sectional area, then a second protective conductor may be provided at the fi nal socket-outlet on one circuit by connection to the protective conductor of the adjacent circuit.

  • Other circuits complying with the requirements of Regulation 543.7. 1 [7].

13. In circuits, whatever the type of earthing system, the following disconnection devices shall be provided [7, UK]:

  • In final circuits supplying socket-outlets with rated current not exceeding 32 A, an RCD having them characteristics specified in Regulation 415.1 .1 [7]
  • In final circuits supplying socket-outlets with rated current more than 32 A, an RCD with a rated residual operating current not exceeding 100 mA

14. A final circuit intended for the fixed connection of a supply to a mobile home or a residential park home shall be individually protected by an RCD having a rated residual operating current not exceeding 30 mA accessible to the consumer. Devices selected shall disconnect all live conductors [7].

NOTE: This final circuit will be from the connection/metering point to the consumer and not the distribution circuit to the connection/metering point.

15. Final circuits intended for fixed connection for the supply to houseboats shall be protected individually by an RCD having the characteristics specified in Regulation 415.1. 1 [7]. The device selected shall disconnect all poles, including the neutral.

16. Each final circuit shall be protected by an overcurrent protective device which disconnects all live conductors of that circuit [7, in the UK].

17. For a TN system, the final circuit supplying a charging point for electric vehicles shall not include a PEN conductor [7, in the UK].

18. Where an installation comprises more than one final circuit, each final circuit shall be connected to a separate way in a distribution board. The wiring of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated [7, in the UK].

19. The line and neutral conductors of each final circuit shall be electrically separate from those of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated [7, in the UK].

20. The wiring of every final circuit and distribution circuit intended to supply one or more items of equipment, such that the total protective conductor current is likely to exceed 10 mA, shall have a high integrity protective connection complying with one or more of the following [7, in the UK]:

  • A single protective conductor having a cross-sectional area of not less than 10 mm2, complying with the requirements of Regulations 543.2 and 543.3 [7]
  • single copper protective conductor having a cross-sectional area of not less than 4 mm2, complying with the requirements of Regulations 543.2 and 543.3 [7], the protective conductor being enclosed to provide additional protection against mechanical damage, for example, within a flexible conduit
  • Two individual protective conductors, each complying with the requirements of Section 543 [7]. The two protective conductors may be of different types, e.g. a metal conduit together with an additional conductor of a cable enclosed in the same conduit.

References

  1. IEC 60050-826-2022
  2. IEC 60364-4-41-2017
  3. IEC 60364-4-42-2014
  4. NFPA 70
  5. IEC 60364-4-43
  6. IEC 60364-7-710:2021
  7. BS 7671