Ask the experts/Technical
Fully charged Why the ‘limiting values of measured earth fault loop impedance’ have changed Contractors who purchase Amendment 3 compliant NICEIC/ELECSA test certificates may well have noticed that in the tables supplied with the test certificates there has been a reduction in the ‘Limiting values of measured earth fault loop impedances’ shown in Table 1. These reductions are due to the introduction, in Amendment 3, of a Minimum Voltage Factor which appears in the text of BS 7671 as the abbreviation Cmin.
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rior to Amendment 3 the values of Maximum earth fault loop impedance given in the tables found in Part 4 of BS 7671, were based on a nominal voltage (U0) of 230 V1. However, in practice, and as recognised by the Electricity Safety, Quality and Continuity Regulations 2002, the voltage at the terminals of the consumer intake will vary between the limits set by these aforementioned regulations. The Minimum Voltage Factor accounts for certain less typical operating conditions where the value of the voltage appearing between line and earth at the consumer’s intake may be less than U0 . The following issues can contribute to the decrease in supply voltage: 1 During periods of high demand there can be a volt drop along the distribution network operator’s distribution cable; this volt drop will depend upon both the distance between the sub-station transformer and the particular installation and loading on the system. 2 In order to keep the output voltage close to a set value many sub-station transformers have automatic tap-changers. However, the voltage appearing across the output of the final sub-station transformer supplying the distribution circuit may decrease with loading. Using the values of maximum earth fault loop impedance given in previous editions of BS 7671, in the event of a fault of negligible impedance to earth occurring when the voltage between line and earth is less than the nominal value (U0), then the resulting fault current may not be sufficient to operate the overcurrent protective device in the required disconnection time. Based on the content of various IEC technical reports and in line
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with the Electricity Safety, Quality and Continuity Regulations 2002, the third amendment to BS 7671: 2008 has adopted a Minimum Voltage Factor (Cmin) of 0.95. The adoption of this factor ensures that in situations when the supply voltage falls to 95% of U0 and a fault to earth occurs, the overcurrent protection device will operate within the required disconnection time. Regulation 411.4.5 now requires that in TN systems the characteristics of protective device and the circuit impedances shall fulfil the following requirement:
Zs × Ia ≤ U0 × Cmin Rearranging this formula gives
Maximum Zs = U0 × Cmin Ia
Where Zs is the earth fault loop impedance U0 is the nominal a.c. rms line voltage to Earth Ia is the current causing operation of the protective device within the specified operating time Cmin is the minimum voltage factor (0.95) The current required to cause the operation of a particular protective device within the required operating time can be determined from the table accompanying the device time/current characteristics shown in Appendix 3 of BS 7671. For example, using the table of current required to cause the device to operate that forms part of Fig 3A1, it can be seen that to cause a 32 A BS 88-3 fuse system C to operate within 0.4 sec requires a minimum fault current of 240 A. Substituting U0 = 230 V, Ia = 240 A, and Cmin = 0.95 in the formula for Zs results in a Maximum Fault Loop Impedance Zs of 0.91 Ω. This value concurs with that shown in Table 41.2 of BS 7671. Prior to the introduction of Amendment 3, due to Cmin not being included in the formula, the value would have been 0.96 Ω. The maximum Zs values given in Tables 41.2 to 41.4 are based upon the line conductors carrying load current and being at a temperature of 70 °C. However, when carrying out an earth fault loop impedance test on a circuit as part of an Initial Inspection and Test the line conductors are likely to be at ambient temperature and the measured value of Zs will be less than it would have been had the line conductor been at 70 °C. In order to take into account the change in the resistance of the line conductor with increasing operating temperature when carrying out a measurement of earth fault loop impedance Appendix 14 of BS 7671 1 It is recognised in the UK recommends that the formula for Maximum Zs is that when measured the typical voltage supplied to replaced by: single-phase installations is 240 V 50 Hz a.c.
Maximum Zs = 0.8 ×
U0 × Cmin Ia
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