Encyclopedia of Grounding (CA09040E)

WHY “H” RATED CLAMPS?

Table 2 has clamp ratings of 1H – 7H. Oneof themaindifferences in the two tables is that Table 2 focuses more on the mechanical stress on thegroundingequipment,whereasTables 1 focuses on the thermal stress. Becauseof theasymmetrical current and high X/R values associated with Table 2, the current waveform is “offset,” and the ground sets will see significantly higher peaks in the first several cycles. The ground sets must survive this initial burst of energy in order to provide worker protection. For example, aGrade5Hclamp is rated at 47kA but must survive a 1st cycle peak of 127kA (or 90% greater than the symmetrical peak) and a 15th cycle peak of 70kA during testing. The two waveforms below indicate the differences in a 31kA current at an X/R of 1.8 and an X/R of 30. You can see the significant increase in magnitude in the first several cycles with the X/R of 30. Utilities in the past have assumed a low X/R ra tio and a common symmetrical current during a ground fault. However, with higher loading on the system and protective grounding locations near generation and substations many utilities are discovering that these locations in fact have a higher X/R ratio. Remember, any X/R greater than 1.8 necessitates the use of ground sets rated to Table 2 (with the H rating). Grounding designs tested to Table 1 will not provide adequate worker protection at these higher X/R locations. All utili ties should do the necessary engineering analysis to determine the X/R and current values at their grounding locations.

Prior to2009, ASTMF855, StandardSpecifications for Temporary Protective Grounds to Be Used on De-energizedElectricPower Lines andEquipment only listed one table indicating the specifics to which ground set designs were to be tested. This “Table 1” is based on a near symmetrical current, limitingthecircuit inductive reactance toresistance (X/R) ratio to a maximum of approximately 1.8 (20% asymmetry). Table 1 specifies grounding component ratings of Grades 1-7. A rating can be achieved by passing the Withstand requirements or the Ultimate re quirements. Per this ASTM F855, the definition of Withstand is: a near symmetrical current which shall be con ducted without any component being damaged sufficiently topreventbeingoperableandreusable. The protective ground shall be capable of passing a second test at this current after being cooled to ambient temperature. The definition of Ultimate is: a current which it is calculatedthecomponent iscapableof conducting for the specified time. It is expected that compo nent damagemay result. The component shall not be reused, except in test situations. In 2009 ASTM 855 added “Table 2.” This table establishes the values to which grounding com ponents must be tested if they are to be used at grounding locations where the asymmetry is greater than 20% (X/R>1.8). The values in Table 2 represent an X/R ratio of 30 (90% asymmetry).

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ENCYCLOPEDIA OF GROUNDING