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Are the Wiring Regulations a legally binding document?
No. BS 7671 is not legally binding in itself, but it may be used in court to demonstrate compliance with a statutory regulation or when quoted in a contract. Most wiring contracts cite BS 7671 as the required standard. (Parts 1–4, slide/page 11; BS 7671 pp. 21, Regs 114.1 & 115.1)
What is an extraneous-conductive-part?
A conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation. The deck also notes that metallic pipes with an insulating section at their point of entry need not be bonded, and non-metallic pipes need not be bonded. (Parts 1–4, slides/pages 35–36; Regs 411.3.1.1 & 411.3.1.2)
How can we make sure that there will be enough current for ADS to operate the protective device in time?
By ensuring the earth fault loop impedance is low enough to allow sufficient fault current to flow. Use Zs × Ia ≤ Uo × Cmin, so the maximum permitted Zs is calculated from Uo, Ia and Cmin. The worked slide example uses 230 ÷ 100 = 2.3 Ω, then applies Cmin = 0.95 to give 2.19 Ω. (Parts 1–4, slides/pages 48–50; Regs 411.4.4–411.4.5)
How much current is required to disconnect the supply in 0.4 s using a rewireable protective device rated at 30 A?
210 A. (Parts 1–4, slide/page 51; BS 7671 p. 412, Reg 411.4, 3A2(a))
How much current is required to disconnect the supply in 5 s using a rewireable protective device rated at 30 A?
87 A. (Parts 1–4, slide/page 52; BS 7671 p. 412, Reg 411.4, 3A2(a))
How much current is required to disconnect the supply in 0.4 s or 5 s using a 32 A Type B (B32) circuit-breaker?
160 A. The slide shows the same current requirement for both 0.4 s and 5 s for a B32 circuit-breaker in this example. (Parts 1–4, slide/page 54; BS 7671 p. 417, Reg 411.4, Table 41.3, 3A4)
A PVC/PVC flat cable (T+CPC) is installed to feed a lighting circuit protected by a 6 A Type B circuit-breaker. What is the maximum measured value of Zs we can accept?
6.14 Ω. This is the commonly accepted measured maximum based on the tabulated Zs value for a B6 device of 7.67 Ω, adjusted by the 0.8 rule of thumb for measured values: 7.67 × 0.8 = 6.136 Ω ≈ 6.14 Ω. (Question appears on Parts 1–4, slide/page 60; the deck later notes the 0.8 rule of thumb for maximum measured values.)
Why might TT systems require a different configuration to TN systems?
Because TT systems usually cannot rely on an overcurrent protective device alone for ADS due to higher earth loop impedance. The MET is connected to an installation earth electrode, and ADS may be provided by an RCD or an OCPD, though in practice an RCD is often needed. (Parts 1–4, slides/pages 61–64; Regs 411.5.1–411.5.3)
What is an AFDD?
An Arc Fault Detection Device. The deck defines it as a circuit-breaker that operates (opens) when an arc fault is detected. It looks similar to an RCD, has a test button, and may be combined with a CB, RCD or RCBO. (Parts 1–4, slides/pages 97–98; Reg 421.1.7)
Why is it important not to disconnect the neutral with the line still connected?
Because if only the neutral is broken while the line conductors remain connected, loads can become connected directly across the line voltage of the supply transformer, creating danger. (Parts 1–4, slide/page 112; BS 7671 p. 94, Reg 431.3)
Where do we get ‘k’ from?
From Table 43.1. In the adiabatic equation, k is the factor based on conductor material, insulation and temperature limits. The slide points specifically to Table 43.1. (Parts 1–4, slide/page 120; BS 7671 p. 99, Table 43.1)
What is combined protective and functional earthing on about?
It refers to electronic equipment, control systems and communications equipment where the earth is required not just for protection, but also for signalling and/or electromagnetic compatibility (EMC). (Part 5, slide/page 75; Reg 543.5)
What is the issue with equipment or circuits that have high protective conductor current?
The risks include perception of current at about 1 mA, harmful effects around 9 mA, possible touch voltage if the protective conductor breaks or develops resistance, and nuisance tripping of 30 mA RCDs. The aim is to ensure that failure of the protective conductor does not lead to electric shock causing injury or death. (Part 5, slide/page 77; Reg 543.7)
Where might we install supplementary bonding?
Where supplementary bonding is required, including to fixed appliances. The following slide notes that for a fixed appliance the circuit CPC, and flex if applicable, can be used for the supplementary equipotential bonding. (Part 5, slides/pages 80–81; Reg 544.2)
