Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, concerning the 17th Edition, maximum Zs values and any correction values, is answered by the NICEIC, the ECA and by SELECT:
Answer by the NICEIC: This column should record the maximum permitted values of Zs, by reference to the limiting earth loop impedance values given in Chapter 41 of BS7671: 2008, not to other tabulated 'corrected' values used for comparison with measured values obtained at ambient temperature.
The maximum permitted value from Chapter 41, or from manufacturers data should be entered, unless the column states the maximum measured value.
Answer by the ECA: The maximum Zs is normally taken from Table 41.2, 41.3 or 41.4 from BS7671: 2008, then entered into the schedule of test results. The correction factors are applied to the measured values.
Answer by SELECT: The Zs inserted on most test sheets is the (on-site) measured value. The maximum Zs is normally found by referring to Table 41.3 from BS7671: 2008 and recording the figures. Some contact with the organisation or company producing the test sheets is often advised.
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