Undercurrent Protection Power System: A Novel Ground-Fault Protection Relay Scheme

THE NATIONAL ELECTRICAL CODE (NEC) [1] requires that ground-fault protection of equipment be provided when the building service disconnecting rating becomes 1,000 A or higher. To meet such a requirement, generally, the main service breaker is specified with ground-fault protection capability. This type of breaker provides ground-fault protection for faults occurring on its load side but not for faults occurring on the line side. Such ground faults may be on the building transformer secondary windings, including the secondary feeder cable terminations at both ends. This article describes a ground-fault protection relay scheme that can provide low-level arcing, ground-fault current protection of the building transformer secondary, including secondary feeder cables. A discussion is included on meeting the design requirements of NEC 705.12 for the building's 480Y/277V main switchboard (MSB) bus rating when a photovoltaic (PV) ac inverter connection is implemented using a dedicated circuit breaker inside the switchboard.