Earthing Disconnection time

Note Some oil companies specify a lower disconnection time tdis than 5.0 seconds, e.g., 1.0 second. This significantly increases the disconnection current by a factor of about 3.0 times. This ensures a much lower permissible limit to Zioopf, and thereby making it more necessary to use an earth leakage circuit breaker. Indirectly this reduction in time should be accompanied by ensuring that the earth return impedance Z r (and Zer) is kept very low i.e. as far below... 

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. 

The object of the test is to ensure that the CPC is correctly connected, is electrically sound and has a total resistance which is low enough to permit the overcurrent protective device to operate within the disconnection time requirements of Regulation 411.4.6, should an earth fault occur. Every protective conductor must be separately tested from the consumer s main protective earthing terminal to verify that it is electrically soimd and correctly connected, including the protective equipotential bonding conductors and supplementary bonding conductors. 

The object of this test is to verify that the impedance of the whole earth fault current loop line to earth is low enough to allow the overcurrent protective device to operate within the disconnection time requirements of Regulations 411.3.2.2 and 411.4.6 and 7, should a fault occur. 

The increased risks arising from the harsh environment are recognised by reductions in the required disconnection times for TN systems, set out in Table 604A. For example, for 230 V systems, the normal 0.4 s disconnection time is reduced to 0.2 s. It may not be possible to achieve these disconnection times because of earth loop impedance restrictions, in which case RCD protection will normally need to be specified. Note also that the 50 V touch voltage value used in Section 413 is reduced to 25 V. 

The first of these measures is the most widely used, and its requirements are generally understood by the electrically qualified, except perhaps for the bonding of some extraneous metalwork such as metal framed windows where it is often a matter of risk assessment and judgement see section 7.2. The relation between disconnection times and degree of risk is explained, including the 5 s llOV centre tapped to earth (CTE) system used on construction sites. 

PME supplies are not considered to be acceptable, with a TN-S supply being acceptable if one can be made available. Failing that, TT supplies will be most common. However, the disconnection times are lowered for the TN systems, with consequentially lower earth loop impedance values. All final... 

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. 

Radial circuits are tested by connecting the phase and protective conductors together at the distribution board and then applying the test between the phase and earth terminals at each outlet point. A higher than expected reading should be investigated and will probably be due to a defective connection. Where the disconnection times are found to exceed those of Table 41A of BS 7671, it will be necessary to measure the protective conductor resistance on its own. 

Protection Against Overload Current. In order to protect against overload or excess current, the overloaded circuit, or part of the circuit, must be disconnected quickly. The most simple and common form of overload protection is the fuse. The fuse will offer close protection as it should melt within 4 hours if current of 1 5 times its rating flows (e.g. fuses to BS 88). The fuse should be located in the live side of the circuit only, and not in the earthed neutral side, because if the neutral fuse melts first, the apparatus will cease to operate and will appear dead , while in fact still being dangerously live. Circuit-breakers are also used for overload protection as an alternative to fuses. A circuit-breaker is an automatic switching device, which operates instantaneously when a significant overload current (typically 1.5 times rated current) is detected. Circuit-breakers are easily reset after a trip, but are more expensive than fuses. 

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