The team at ELECSA look at what a distribution circuit is and the recording of circuit details.
A distribution circuit is a circuit that supplies a distribution board or other switchgear and does not extend directly to current-using equipment or a means of connection for such equipment.
An example of a distribution circuit as shown in Fig 1, that may be encountered in a domestic installation is a supply to another building, such as, for example, a detached garage, shed or greenhouse.
Load centres and supplies to other buildings
The circumstances relating to distribution circuits in electrical installations in dwellings are diverse, and it is not possible to identify examples for all such circumstances.
However, the following two examples may serve to provide an indication where such circuits may be used to advantage.
- A distribution circuit feeding a distant internal distribution board or consumer unit from a switch-fuse or circuit-breaker at the origin of the installation. The use of such a circuit may be advantageous where the electrical loads are some considerable distance from the origin.
- A distribution circuit feeding a detached building located at some distance from the intake position in the dwelling.
It is a requirement of Regulation 411.3.1.2 that in each installation, main protective bonding conductors complying with Chapter 54 connect extraneous conductive-parts to the main earthing terminal. This regulation also requires that where an installation serves more than one building, main protective bonding conductors must connect to the extraneous-conductive-parts in each building. A detached garage may be one example of such a building.
Where any metallic pipes enter a building have an insulating section at their point of entry, as shown by Fig 2, there is no requirement to provide protective equipotential bonding (Regulation 411.3.1.2 refers).
The significance of Regulation 411.3.1.2 in the context of a building that is supplied via a distribution circuit is that the protective conductor associated with the distribution circuit may also serve as a main protective bonding conductor and should have a cross-sectional area complying with Regulation 544.1.1.
For an installation where PME conditions do not apply (TN-S or TT system), the copper equivalent cross-sectional area of the main protective bonding conductors must be not less than half that required for the earthing conductor of the installation, subject to a minimum of 6mm2. The cross-sectional area need not exceed 25mm2 where the bonding conductor is of copper.
For an installation where PME conditions do apply and where there may be metallic services extending to other buildings, or the building is supplied by a distribution circuit contains extraneous-conductive-parts (such as metallic structure in contact with Earth), the associated protective conductor of the distribution circuit must have the cross-sectional area not less than that required by Table 54.8 of BS 7671 in relation to the neutral conductor of the distributor’s supply cable.
For supply neutral conductors with a copper equivalent cross-sectional area of not more than 35mm2, the minimum cross-sectional area of copper main protective bonding conductors need not exceed 10mm2.
Alternatively, the earthing system for the remote building could be segregated from the PME earthing terminal and connected to an installation earth electrode forming part of a TT earthing system, all other requirements of BS 7671 for a TT system being met.
Assessment of current demand
When designing a new installation, the following two items will need to be considered by the installation designer:
1) Total connected load – the total electrical rating of all devices such as lamps, electrical heaters and the like that are to be connected to a distribution system.
2) Maximum demand – the largest current normally carried by connected items of equipment, switches and protective devices when operational in normal use; it does not include the levels of current flowing under overload or fault conditions.
A consumer unit or a distribution board must be of sufficient rating to carry the total connected load. The cable supplying a final circuit must be capable of carrying the full-load current indefinitely without deterioration. However, a distribution cable may be sized to carry the maximum demand expected in normal operation on the basis that not all of the total connected load will be switched on simultaneously.
Alternatively, where a diversity factor is known from experience of a particular type of installation, the current demand of a distribution circuit may be calculated by applying a diversity factor to the sum of the connected loads.
Other methods of calculating the current demand of a distribution circuit may be used. Such methods are often based on known links between current demand and other criteria, such as floor area and population.
For a distribution circuit, the disconnection times given in Table 1 are permitted for AC circuits supplied at a nominal voltage, (U0), of 230 V.
Recording of circuit details
As with all circuits, distribution circuits are required to be inspected and tested and the results recorded on the Electrical Installation Certificate.
Regulation 514.9.1 and Section 644 call for circuit details and test results to be provided for all circuits in an electrical installation. However, there is sometimes a tendency to overlook the provision of this information for distribution circuits (or sub-mains as they are sometimes called), where such circuits form part of the installation. The requirement to provide such information applies to distribution circuits as much as for final circuits.
One situation where this type of omission of data occurs is where a circuit-breaker, located at the origin, feeds a single distribution circuit to a remote distribution board or consumer unit (see Figure 1).
Without this recorded data the certification for the installation is incomplete. This does not apply to meter tails making the short connection between the consumer unit or main switch to the electricity meter or distributor’s isolator.
For more detail on testing and recording of circuit details, refer to the updated 18th Edition publication of: Inspection, testing and certification produced by Certsure, which can be obtained from: www.shop.niceic.com/publications
