TECHNICAL
Why is it necessary to determine earth fault loop impedance? This article explains why it is necessary to determine the values of earth fault loop impedance (Zs) for new installations and for those in service that are being inspected and tested to establish their condition. It also discusses the use of calculation as an alternative to loop impedance measurement, as permitted by BS 7671.
R
egulation 313.1 requires a number of characteristics of the supply to an installation to be determined, including the nominal voltage to earth (U0), the earth loop impedance of that part of the system external to the installation (Ze), and the prospective short-circuit current at the origin of the installation. External earth loop impedance (Ze) The value of external earth loop impedance (Ze) measured or otherwise determined in accordance with Regulation 313.1 may differ from the typical maximum value declared by the electricity distributor, which is usually: • 0.8 Ω for TN-S system • 0.35 Ω for a TN-C-S system • 21 Ω plus the resistance of the installation earth electrode for a TT system. The resistance of the installation earth electrode in a TT system must be measured, as required by Regulation 612.7. When using this measured value (such as in the value of RA, mentioned later in this article) it must be remembered that this resistance is liable to
66 Autumn 2010 NICEIC Connections
p66-67 Loopimpedancetest.2.indd Sec2:66
change with the varying weather conditions, the value being highest when the electrode is subjected to drying or freezing. In all cases – TN and TT – the presence of an effective means of earthing for the installation needs to be confirmed. The prospective earth fault current at the origin can then be determined using Ohm’s Law, by dividing the nominal voltage (U0) by Ze. Earth loop impedance of final circuits and distribution circuits For each final circuit and distribution circuit, it must be confirmed that the value of line-earth loop impedance (Zs) is low enough to achieve automatic disconnection
of supply to the circuit within the relevant maximum time specified in Regulation Group 411.3.2 in the event of an earth fault. Table 1 of this article gives the maximum disconnection times for final circuits and distribution circuits in TN and TT systems at a nominal voltage to earth (U0) of 230 V. When checking that the value of Zs is sufficiently low to achieve the required disconnection time, account must be taken of characteristics of the protective device used for automatic disconnection. For commonly-used overcurrent devices, this is usually done by checking that the measured value of Zs at the electrically most remote part of the circuit is not more than 80 % of the applicable maximum value given in Tables 41.2, 41.3 and 41.4 of BS 7671. For overcurrent devices not covered by those tables, another reliable source of information on the limiting values of Zs must be consulted, such as the manufacturer’s data. Where the protective device is a non-delayed RCD used in a final circuit rated at up to 32 A, the maximum value of Zs can be found from Table 41.5 of BS 7671. The Zs values in that table are intended for a TT system but may also be applied to a TN system. These Zs values not only meet the disconnection time requirements of BS 7671, they also meet the condition RA x IΔn ≤ 50 V given in Regulation 411.5.3 (ii) for a TT system. (RA is the sum of the resistances of the earth electrode (to Earth) and the protective conductor connecting it to the exposed-conductive-part. IΔn is the rated residual operating current of the RCD.) For RCDs not covered by Table 41.5, the maximum value of
Table 1 – Maximum disconnection permitted by Regulation Group 411.3.2 at a nominal voltage (U0) of 230 V a.c Type of circuit Maximum disconnection time (seconds) TN system TT system Final circuit rated at 32 A or less 0.4 0.2 Final circuit rated at more than 5.0 1.0 32 A Distribution circuit 5.0 1.0 Note: In a TT system where disconnection is achieved by an overcurrent protective device and protective equipotential bonding is connected to all the extraneous-conductive-parts in accordance with Regulation 411.3.1.2, the maximum disconnection times for a TN system may be used.
www.niceic.com
23/9/10 12:43:55