User talk:Bernie Hoyne

The number of electrodes required to achieve a satisfactory earth resistance is determined by the soil resistivity (SR) and the design criteria, therefore it is a dangerous practice to assume that one electrode will suffice everywhere for a SWER HV earth. e.g. with soil of one hundred ohms/cubic metre resistance,(pretty much average) a 4 metre electrode will have a resistance of 26.5 ohms, but if SR is 350 ohm/mtr, the resistance will rise to 92.7 ohms. To keep elec R below 10 ohms (ideal) would require 3 electrodes at least 3m apart & preferably in a looped earth design.IEEE Std 80-1986 Appendix H Table 1 refers to electrode resistance calculation based on soil resistivity. An insufficient number of electrodes will result in possibly fatal levels of step and touch potential occuring around the base of the substation, while too many electrodes will incur excessive costs. Bernie Hoyne (talk) 04:55, 30 December 2011 (UTC)