Performance testing low-voltage ground-fault protection

Abstract

The National Electrical Code (NEC) requires ground-fault protection of equipment by devices intended to detect and interrupt low-current arcing phase-to-ground faults on many solidly grounded low-voltage systems having current ratings of 1000 A or greater. The NEC also dictates field performance testing of ground-fault protection, but does not prescribe specific techniques to be used. This paper reviews the types of ground-fault protection equipment and systems in use today, ranging from simple bolted-pressure switch and relay combinations to complex multiple-source systems and zone-selective-interlocked systems. Conditions that can cause misoperation of ground-fault protection are reviewed, including a summary of problems identified during field testing of actual projects. Available testing techniques are reviewed and primary current injection is recommended as the most reliable means of field verification for most ground-fault protection systems. Required test equipment setup for each system configuration is described, and cautions to be observed during this type of testing are presented. This paper will benefit engineers and technicians responsible for acceptance testing of electrical equipment.