IEEE Std 1246-2011 pdf download – IEEE Guide for Temporary Protective Grounding Systems Used in Substations.
Analytical studies indicate that when full dc offsets occur in the locations with high X/R ratios (such as close to a generating plant or a large transmission substation), the short duration (6 to 60 cycles) fusing current ratings of grounding cables calculated using Onderdonk’s equation as considered in ASTM F855 might not be conservative. The additional heating from the dc current component reduces the cable current- carrying capability. The cable symmetrical current-carrying capability for the six-cycle rating is reduced approximately 28% when the X/R ratio is changed from X/R = 40 to X/R = 0 as shown in Table 2 and Table 5, respectively. At or near large generating plants and transmission substations, a large X/R ratio is likely because the impedance of generators and transformers contains very little resistance. Whereas in extreme cases the X/R ratio can be as high as 50, under most circumstances, the X/R ratio does not exceed 40 within the substations. Several miles away from the substation, the X/R ratio is dominated by the impedance of the line. The overall X/R ratio in such cases can be determined from the line’s X/R ratio. The typical range of X/R ratios for lines is from 2 to 20 depending on the conductor configuration. A single, small conductor line will have a low X/R ratio, whereas a bundled large conductor line will have a higher X/R ratio. In addition to the effects on fusing current, the X/R ratio and dc offset can produce extremely high current peaks in the first few cycles relative to the rms current. Whereas the current peaks are proportional to the X/R ratio, the rate of decay is inversely proportional to the X/R ratio. The slowly decaying high current peaks, corresponding to higher X/R ratios, create the most severe electromechanical forces, which can destroy the TPG assembly long before it fails thermally. In such a case, the worker would be without protection for a longer duration before the short circuit clears.