NRC Information No. 88-86: Operating with multiple grounds in direct current distribution systems

During recent NRC maintenance inspections at Quad Cities, Oconee, and D.C. Cook power reactor facilities, it was found that plants had been operating with multiple grounds in the dc distribution systems for extended periods. Specific examples are described. Most nuclear power plant dc systems are two-wire ungrounded, combination battery/charger systems equipped with ground detection. Typical ground detection system features include a remote annunciator and a local indicator and/or recorder. Ground detectors are incorporated in the dc system so that if a single ground point does occur, immediate steps can be taken to clear the ground fault from the system. Failure to respond to a single ground will mask subsequent grounds. Multiple grounds can cause the indiscriminate operation of equipment, which may have safety consequences. Grounds can cause control circuit fuses to fail and can render important safety equipment inoperable as previously described. Furthermore, batteries have a designed capacity to supply power during a station blackout condition, and this capacity can be affected by the presence of unanalyzed loads in the form of multiple grounds. It is recognized that troubleshooting and finding grounds on a dc system are difficult tasks that may affect plant operation. The licensees previously mentioned have reviewed their designs and conditions for potential impact on safety system operability and have taken corrective actions to minimize the effect of grounds