IEEE 1246:2011 pdf free download

IEEE 1246:2011 pdf free 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 XR ratios (such as close to a gcncrating plant sir a large transmission substation), the short duration (6 to 60 cycles) fusing current ratings of grounding cables calculated using Ondcrdonk’s equation as considered in ASTM F855 might no be consersative. The additional heating from (he dc current component reduces the cable currentcarrying capability, The cable symmetrical current-carrying capability for the six-cycle rating is reduced approximately 2X% when the ZR ratio is changed from XR 40 to XR 0 as shown in Table 2 and Tablc 5. respectively.
At or near large generating plants and transmission substations, a large ZR ratio is likely because the impedance of generators and transformers contains very little resistance. Whereas in extreme cases the ZR ratio can be as high as 50. under most circumstances, the ZR ratio does not exceed 40 within the s4ibstations. Sever-al miles away from the substation, the ZR ratio is dominated by the impedance of the line. The overall X/R ratio iii such cases can be determined from the line’s XIR ratio The typical range of ZR ratios for lines Ls from 2 to 20 depending on the conductor configuration. A single, small conductor line will have a low ZR ratio, whereas a bundled large conductor line will have a higher ZR ratio.
In addition to the effects on fusing current, the ZR ratio and de oflset can produce extremely high current peaks in the first few cycles relative to the niis current. Whereas the current peaks are proportional to the .VR ratio, the rate of decay is inversely proportional to the ZR ratio. The slowly decaying high current peaks, corresponding to higher ZR 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. IEC 61230 requires temporaiy grounding (earthing) devices to withstand a peak asymmetrical current of 2.6 times the ross current value for 60 Hz systems above I IsV.
4.3.2 Short-circuit duration including primary and backup relaying
The short-circuit duration is another critical factor to consider when sizing protective grounds. The short- circuit duration is the lime required to clear the short circuit by primary or backup relaying. The short. circuit clearing time is thc sum of relay and breaker operation times. Primary relaying is the tirst line of defense to clear a short circuit at high speed. Utilizing the primary relay short-circuit clearing time minimizes the grounding cable size; however, before relying on the primary relay operation to size the protective grounds, consider th reliability of the relays. Many circuits are protected by slower clearing fuses that can take many cycles or even seconds to interrupt the current.