Earth fault loop impedence

BS7430 (1998), sub-section 3.13, defines the earth fault loop impedance Zioop in relation to the various types of earthing systems, as follows. 

Therefore if the earth fault loop impedance is low enough to allow at least 30 A to flow in the circuit under fault conditions, the protective device will operate within the time required by lET Regulation 411. 

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. 

The total earth fault loop impedance is therefore ... 

Table 3.2 Maximum earth fault loop impedance Zg MCB Type B maximum measured earth fault loop impedance (n) when overcurrent protective device is MCB Type B to BS EN 60898...

The CPC forms an integral part of the total earth fault loop impedance, so it is necessary to check that the cross-section of this conductor is adequate. If the cross-section of the CPC complies with Table 54.7 of the lET Regulations, there is no need to carry out further checks. Where line and protective conductors are made from the same material. Table 54.7 tells us that ... 

A 230V ring main circuit of socket outlets is wired in 2.5 mm single PVC copper cables in a plastic conduit with a separate 1.5 mm CPC. An earth fault loop impedance test identifies Zg as 1.15Verify that the 1.5 mm CPC meets the requirements of lET Regulation 543.1.3 when the protective device is a 30A semi-enclosed fuse. 

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 ... 

The protective conductor resistance values calculated by this method can only be an approximation since the length of the CPC can only be estimated. Therefore, in this case, a satisfactory test result would be obtained if the resistance of the protective conductor was about 0.6 f2. A more precise result is indicated by the earth fault-loop impedance test which is carried out later in the sequence of tests. 

In order that an overcurrent protective device can operate successfully, meeting the required disconnection times, of Regulations 411.3.2.2, that is, final circuits not exceeding 32 A shall have a disconnection time not exceeding 0.4 s. To achieve this, the earth fault loop impedance value measured in ohms must be less than those values given in Appendix 2 of the On Site Guide and Tables 41.2 and 41.3 of the lEE Regulations. The value of the earth fault loop impedance may be verified by means of an earth fault loop impedance test as described in Chapter 14 of this book. The formula is ... 

The maximum permitted value given in Table 2A of the On S/feGu/c/e for a 20 A MCB protecting a socket outlet is 2.3 Has shown by Table 12.2. The circuit earth fault loop impedance is less than this value and therefore the protective device will operate within the required disconnection time of 0.4s. 

When calculating the earth loop impedance, Zs, that part of it which consists of metalwork used as a protective conductor, e.g. conduit, trunking and switchboard metalwork, is normally ignored because its resistance is usually negligible. Where the earth fault loop impedance exceeds the values... 

For TT systems, the minimum sizes of buried earthing conductors are given in Table 54A where a buried electrode is used. For other electrodes, such as a building metal frame and for TN-S systems, the size may be selected from Table 54G or calculated from the formula in section 543-01-03. For TT systems, however, it may not be possible to calculate because the earth fault loop impedance is not likely to be known until it can be measured. 

Appendix A Maximum permissible measured earth fault loop impedance is a rewrite of Appendices 7 and 8, and Appendix B - Resistance of copper and aluminium conductors under fault conditions - is a repeat of Appendix 17 of the 15th edition. Appendix C - Minimum separation distances between electricity supply cables and telecommunication cables - tabulates the separation specified for external and internal cables. Appendix D - Permitted leakage currents - gives the figures culled from the relevant British Standards for fixed and portable equipment. 

To obtain an interrupting time of 50 ms, it will usually be necessary to use an RCD. The earth fault current Ip is the phase voltage, normally 230 V, divided by the earth loop impedance Uq/Zs. The disconnecting time, t, for the earth fault current Ip is found from the time/current characteristics for the relevant fuse or circuit breaker, obtainable from the maker. However, in BS 7671, section 604, the maximum earth fault loop impedances are given for disconnection times of 0.2 s for a range of fuses and MCBs, and the time/ current curves are in Appendix 3. 

Depending on the protective system specified it may be also necessary to execute earth fault loop impedance tests. 

Test the earth fault loop impedance where the protective measures used require a knowledge of earth fault loop impedance. 

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