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Arcing Fault Current and the Criteria for Setting Ground Fault Relays in Solidly-Grounded Low Voltage Systems
This paper provides a simplified technique to accurately estimate this arcing fault current, and discusses its effects on the settings of relays and interrupting devices utilized in low voltage systems. It suggests that the criteria for setting the ground fault sensing devices should be changed to accommodate the maximum allowable exposure of personnel to arc flashes that may occur in the system. This is a much stricter guideline than that currently required by the NEC. In the 1960's numerous damages were documented on 480V, 3-phase, 4-wire power systems due to arcing ground faults. To alleviate this problem, the National Electrical Code (NEC) now requires ground fault protection in low voltage switchgear with continuous current ratings above 1,000A and operating at more than 150V phase-to-ground on solidly-grounded systems. However, burn down damage and personal injury continues to occur.
The most common type of faults on low voltage 480V power systems are comprised of phase-to-ground with arcing. This switchboard was installed in compliance with the National Electrical Code (NEC) and was protected by a ground fault relay. However, the overall protection was inadequate to properly protect either the equipment or personnel. Judicial selection of proper protective devices and relay settings can greatly reduce the possibility of damage and injury caused by similar arcing faults.
This paper suggests a simplified method of calculating the fault current in circuits containing an electric arc and shows how this information should be used in selecting and setting protective devices. The present NEC requirements are inadequate and need to be changed to further protect against the type of damages still seen in 480V solidly-grounded system.
In low voltage systems, typically the 3-phase bolted arcing fault current (usually the highest possible level) is computed and, based upon this value, the system is designed. The protective devices must be capable of interrupting this current and the equipment must also be able to withstand this value during the time needed to interrupt. However, an arcing fault, in general, results in much lower levels of current. Fuses and relays designed to operate at 3-phase bolted levels may not properly respond to these lower values allowing an arcing fault to persist until severe burn down damage occurs.
|Arcing Fault Current and the Criteria for Setting Ground Fault Relays in Solidly-Grounded Low Voltage Systems|