Circuitry and Related Parts of BS 7671: 2008
design every final circuit from first principles. Standard circuit designs are usually used for common circuits, particularly domestic circuits. The designs in this Section C 9.3 can be used in commercial and industrial installation designs for final circuits where the eath fault loop impedance at the local distribution board does not exceed the standard values given below. The tables have been calculated using particular parameters as follows: External earth fault loop impedances Those used by the UK supply industry are as follows:
•• •
0.35 Ω for PME supplies; 0.80 Ω for TN-S supplies; 21 Ω for TT supplies.
Where RCDs are used, the tables in this section have been calculated so that the earth fault loop impedance values comply with the limiting values for overcurrent devices in Chapter 41. This gives maximum lengths, which are generally recommended, but considerably longer lengths are possible. Cable types and installation methods The standard circuit tables have assumed flat twin and earth cable to BS 6400 with reduced cpc size and installed as Table 4A2 of BS 7671 with current rating taken from Table 4D5 of BS 7671 (see Table C 9.1): Table C 9.1 Flat twin and earth cable installation methods, adapted from Table 4D5 of BS 7671. Reference method A
Examples
Description
Rating of 2.5 mm2 cable
Enclosed in conduit in an insulated wall
20
(Continued.)
75
C