作者:C.D. Hughes、E.C. Strycula
DOI:10.1109/2943.838039
日期:——
In solidly grounded power systems, where separate equipment grounding conductors are used inside conduit or other raceways, sizing the equipment grounding conductor (EGC) per the minimum specified in NEC Article 250-122, Table 25D-122, can result in an inadequate ground-fault circuit design. Depending upon the magnitude of the ground-fault current and the specific rating or settings of the overcurrent protective device, an EGC can be damaged or completely destroyed before an upstream overcurrent device can clear the fault. Maximum thermal damage to an EGC often results from low-level ground faults, where currents fall below the short delay pickup setting for the protective circuit breaker. It should be recognized that Table 250-122 of the NEC serves only as a guide in determining the minimum size of an EGC. This is emphasized by the note added beneath the cable, which states "Equipment Grounding Conductors may need to be sized larger than specified in this cable in order to comply with Section 250-2(d)." Section 250-2(d), states than an effective grounding path shall "have the capacity to conduct safely any fault current likely to be imposed on it." Unfortunately, Table 250-122 is often used as the only basis for selecting an EGC. Here we present an example that illustrates how selecting an EGC based only on Table 250-122 can result in a ground-fault circuit design that may violate Section 250-2(d).
在牢固接地的电力系统中,导管或其他滚道内使用了单独的设备接地导体,按照 NEC 第 250-122 条表 25D-122 中规定的最小值来确定设备接地导体 (EGC) 的尺寸,可能会导致接地故障电路设计不充分。根据接地故障电流的大小和过流保护装置的具体额定值或设置,EGC 可能会在上游过流保护装置清除故障之前被损坏或完全毁坏。低电平接地故障通常会对 EGC 造成最大程度的热损坏,此时电流低于保护断路器的短延时拾取设置。应该认识到,NEC 表 250-122 只是确定 EGC 最小尺寸的指南。电缆下方添加的注释强调了这一点,注释中写道:"设备接地导线的尺寸可能需要大于本电缆中规定的尺寸,以符合第 250-2(d)条的规定"。第 250-2(d)节规定,有效的接地路径应 "有能力安全地传导可能施加在其上的任何故障电流"。遗憾的是,表 250-122 经常被用作选择 EGC 的唯一依据。在此,我们举例说明仅根据表 250-122 选择 EGC 可能会导致接地故障电路设计违反第 250-2(d)条的规定。