Zs can be determined from Table 3A in Appendix 3 of BS 7671, by using the formula given on the same page as that table. In addition, for a TT system, Zs must be low enough to meet the condition RA x IΔn ≤ 50 V, mentioned above. What problems can arise when carrying out earth loop impedance tests? Earth loop impedance testing should present few operational problems during the initial verification process, as the installation will not have been put into service. However, for an installation that is in service, there may be serious consequences for the user of the premises if, for example, computer data is lost or corrupted as the result of an inadvertent interruption of supply during the test, such as might be caused by the unintended operation of an RCD. The number of instances where protection – in particular additional protection – by RCDs is required has increased significantly over the past three editions of BS 7671, so their presence is increasingly likely in all types of premises. Inadvertent disconnection of a circuit, group of circuits, distribution board or even a whole installation may occur if an RCD operates when an earth fault loop impedance test is carried out. As a result, a number of methods have been developed to minimise the likelihood of an RCD operating during such a test. One such method, calculation, is described below. Another method is to measure the earth loop impedance with a loop test instrument that supplies a test current sufficiently low not to trip the RCD, such as 15 mA. Determining earth loop impedance by calculation Regulation 612.9 permits earth loop impedance to be determined by means other than measurement. Therefore, where reliable measured values are available for the external earth loop impedance (Ze) and for the loop resistance of the line and protective conductors (R1 + R2) of the circuit, it is permissible to derive the
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p66-67 Loopimpedancetest.2.indd Sec2:67
Use of a proprietary plug-in adaptor
loop impedance of a circuit by using the following formula: Zs = Ze + (R1 + R2) However, using a previouslymeasured value of Ze, or a value of Ze that has been determined by enquiry made with the electricity distributer, does not provide confirmation that the intended means of earthing is present and of suitably low ohmic value. Such confirmation can only be obtained by measuring the value of Ze with a loop impedance test instrument whilst the installation is isolated from the supply, and the means of earthing is disconnected from the protective bonding conductors of the installation. In larger, more complex installations, the value of earth loop impedance measured at a distribution board (Zdb) from which a distribution circuit or final circuit originates may be substituted for Ze in the formula above. This method has the added benefit that it is unnecessary to carry out a loop test for each circuit in the installation, which reduces the risk of electric shock. Further, as the loop test is being performed on the supply side of any RCD protecting the circuit in
question, it should not, in most cases, cause the RCD to operate when the measurement of Ze or Zdb is taken. However, persons using this method should satisfy themselves that there is no other RCD upstream of the circuit under test, protecting for example a sub-main circuit. Sensible precautions The use of proprietary plug-in adaptors is recommended whenever high or low current loop impedance testing is to be carried out that would otherwise involve the use of test probes and partial dismantling (for example, to test lighting circuits in domestic premises). The use of probes to perform live tests can increase the risk of electric shock, and the dismantling or disturbance of an installation could introduce a wiring fault where one did not previously exist. In a future issue of Connections, we will consider how earth loop test instruments work and the various approaches that have been adopted by manufacturers over the years to limit the incidence of unwanted operation of RCDs when loop tests are carried out. The question of how accurate loop impedance test results need to be will also be discussed.
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