68 Electric Wiring: Domestic semi-enclosed rewirable fuse. A 6.0 mm2 twin cable has a 2.5 mm2 cpc. We will also assume that the external loop impedance Ze is measured at 0.27 Ω. Will there be a shock risk if a phase-to-earth fault occurs? The total loop impedance Zs Ze R1 R2 and we are given Ze 0.27 Ω. For a 6.0 mm2 line conductor with a 2.5 mm2 cpc, R1 R2 is 10.49 mΩ/m. Hence, with a multiplier of 1.2 for 70° C PVC, the total R1 R2 18 10.49 1.2/1000 0.23 Ω. Therefore, Zs 0.27 0.23 0.53 Ω. This is less than the 1.09 Ω maximum given in the IEE Regulations for a 30 A BS 3036 fuse. Consequently, the protection will disconnect the circuit in less than 0.4 s.
RESIDUAL CURRENT DEVICES We have seen how very important the total earth loop impedance Zs is in the reduction of shock risk. However, in TT systems where the mass of earth is part of the fault path, the maximum values of Zs given in the IEE Regulations may be hard to satisfy. Added to this, climatic conditions will alter the resistance of the earth in such a way that Ze may be satisfactory in wet weather, but not in very dry. The regulations recommend, therefore, that the preferred method of earth fault protection for installations in TT systems be achieved by RCDs, such that the product of its residual operating current and the loop impedance will not exceed a figure of 50 V. Residual current breakers (RCBs), residual current circuit breakers (RCCBs) and RCDs are one and the same thing. Modern developments in CB (circuit breaker), RCD and consumer unit design now make it easy to protect any individual circuit with a combined CB/RCD (RCBO), making the use of split-load boards unnecessary. In domestic premises the use of 30 mA RCDs is required for the protection of all socket outlets rated at not more than 20 A, for all circuits in a bath or shower room and for cables embedded in