Consumer’s installations

Part 4 of the lET Regulations deals with the application of protective measures for safety and Chapter 53 with the regulations for switching devices or switchgear required for protection, isolation and switching of a consumer s installation. 

The supply to a domestic, commercial or small industrial consumer s installation is usually protected at the incoming service cable position with a 100 A high breaking capacity (HBC) fuse. The maximum, that is, worst case value of external earth fault loop impedance outside of the consumer s domestic installation is ... 

At the main s intake position, the supplier will provide a sealed HBC fuse and a sealed energy meter to measure the consumer s electricity consumption. It is after this point that we reach the consumer s installation. 

Where a combined neutral and earth conductor is used to supply a consumer s premises, the supplier must meet the protective multiple earthing requirements of Regulation 7, as well as ensure that the consumer s installation meets the criteria set out in the same Regulation for equipotential bonding and earthing. The neutral conduetor of PME supplies must not be connected through to metalwork in caravans or boats (refer to Part 6 of BS 7671 and Chapter 10 of this book for clarification). 

Part VII addresses the supplies to installations and covers matters sueh as precautions against supply failure the suitability and maintenance of distributor or meter operator equipment on consumers premises the cireum-stances under which supplies can be connected and discoimeeted and the information that must be provided to consumers. A new requirement in Regulation 24 would mean distributors having to offer earth facilities with new connections at low voltage. Moreover, under Regulation 25, the distributor should not provide a connection if there are reasonable grounds to believe that the consumer s installation, or other distributor s network, does not comply with BS 7671 or other relevant standards. 

An Introduction to Liquefied Petroleum Gas Code of Practice No. 1, Installation and Maintenance of Fixed Bulk LPG Storage at Consumer s Premises Part 1 Design and Installation Part 2 Small Bulk Installations for Domestic Purposes... 

A 20 A radial socket outlet circuit is wired in 2.5 mm PVC cable incorporating a 1.5mm CPC. The cable length is 30m installed in an ambient temperature of 20°C and the consumer s protection is by 20 A MCB Type B to BS EN 60898. The earth fault loop impedance of the supply is 0.5 fi. Calculate the total earth fault loop impedance Zs, and establish that the value is less than the maximum value permissible for this type of circuit. 

Other items of equipment at this position are the energy meter and the consumer s distribution unit, providing the protection for the final circuits and the earthing arrangements for the installation. 

The power factor at which consumers take their electricity from the local electricity supply authority is outside the control of the supply authority. The power factor of the consumer is governed entirely by the electrical plant and equipment that is installed and operated within the consumer s buildings. Domestic consumers do not have a bad power factor because they use very little inductive equipment. Most of the domestic load is neutral and at unity power factor. 

Electricity supply authorities discourage the use of equipment and installations with a low power factor because they absorb part of the capacity of the generating plant and the distribution network to no useful effect. They, therefore, penalize industrial consumers with a bad power factor through a maximum demand tariff, metered at the consumer s intake position. If the power factor falls below a datum level of between 0.85 and 0.9 then extra charges are incurred. In this way industrial consumers are encouraged to improve their power factor. 

The power factor of the consumer is governed entirely by the electrical plant and equipment that is installed and operated within the consumer s buildings. 

Consumers with large energy requirements often find it more economical and convenient to receive electricity at 11 kV and to transform it down to the lower values as required. In all cases, however, it is essential that the consumer s electrical installation and equipment, both fixed and portable, meet good standards of design, conshuction and protection, are adequately maintained and correctly used. 

The standard is also commonly used in civil litigation concerning the safety of consumer s electrical installations. 

The electricity supply companies generally use single-core PVC-insulated and sheathed conductors without further protection for the connections between their cut-outs, metering equipment and the consumer s main switchfuse or circuit breaker, and very often this wiring is not properly secured. It is not, therefore, as safe as the site installation wiring, so access to it should be restricted to authorised and competent persons. Any building or... 

The first test is between the current-carrying conductors and earth and is done by connecting the phase(s) and neutral conductors together at the supply point and measuring the insulation between them and the consumer s earthing terminal. Both on positions in two-way switched circuits should be tested (see Fig. 17.7). This test is not, of course, appKcable to TN-C installations. 

Older installations usually have an armoured paper insulated and lead sheathed service cable providing an SNE supply. The consumer s earthing terminal should be connected to the lead sheath of this service cable. Look out for earthing to the incoming water service pipe, which is no longer permitted. Check that the size of the earthing conductor complies with BS 7671, section 543, or, if the consumer has his or her own buried earth electrode, is in accordance with both sections 542 and 543. 

Check the position of the meter. The Electricity Supply Regulations require it to be installed in the consumer s premises unless it is more reasonable for it to be located elsewhere, in which case it should be accessible to the consumer. This is more likely to occur in blocks of flats and housing in multiple occupation. In such premises the supply authority may bring in only one service cable on to a busbar and then feed each tenant via cables in conduit or sometimes in MIMS cable. The latter is not a good choice because the insulation is hygroscopic and if the end seals fail, the cable can break down. It may also fail if subject to HV transients, so such cables should be fitted with suppressors. If the conduit or sheath of the MIMS cable is used as the earthing conductor, check that the connections are satisfactory and of low resistance and any joints in the conduit provide adequate continuity. 

Post-World War II installations will usually have a consumer unit, which is a small distribution board with a double pole isolating switch and a phase busbar to feed the fuses or MCBs controlling the final circuits. There will be terminal blocks for the neutrals and protective conductors. The earth terminal block may be utilised as the consumer s earthing terminal. Some of the older models may have wood frames and/or may be backless. As BS 7671 requires connections to be made in non-flammable enclosures, the wooden framed type do not comply and the backless ones are acceptable only if mounted on non-flammable material. If rewirable fuses to BS 3036 are used, check that the correct size of fuse wire has been used. For cartridge fuses, check that blown fuses have not been repaired with a bit of fuse wire spanning the contacts or the cartridge replaced by a nail, hairpin or the like. There should be only one final circuit connected to each fuse carrier or MCB. 

---