Electrical Specifier Link GE Consumer & Industrial Electrical Distribution
Issue VIII
System Neutral Resistance
Typical Ground Circuit

There are many neutral grounding options available for both Low and Medium voltage systems. The choice of system grounding should be selected to provide safety, reliability, and continuity of service.

Traditionally, LV systems have been solidly grounded. Ungrounded systems have also been used to avoid unscheduled shutdowns on the first ground fault. With the renewed concern over arc flash hazards it may be time to look at resistance grounding.

High resistance grounded LV systems along with high and low resistance grounded MV systems offer many safety and operational advantages for the user.

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Harmonic Design Considerations

Power quality can be defined as the comparison of voltage and current waveforms to their fundamental "clean" sinusoidal waveforms. Pure sine waves that are free of distortion are sometimes referred to as clean power. Harmonic distortion is one of the main causes of power quality problems. Many papers have been written on this subject, so only a brief review of the subject will be presented here.

lt is the main intent of this paper to discuss some design ideas that should be considered when trying to decide whether harmonic mitigation is necessary, and if so, what type should be implemented.

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Digital Relay Reference Guide

The process of designing any protection scheme requires a basic understanding of short-circuit analysis, selective coordination and zones of protection. The IEEE Protection and Coordination of Industrial and Commercial Power Systems is a good source as are the many available protective relay design books that can aid in computations.

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Application Considerations for Selective Coordination of LV Distribution Systems

Selective Coordination has been applied at the discretion of design professionals as a system performance standard in critical applications such as data centers and healthcare facilities for many years.

NFPA 110 - Standard for Emergency and Standby Power Systems 2005 Edition Chapter 6, Section 6.5 Protection, General 6.5.1 states: "The overcurrent devices in the EPSS (Emergency Power Supply System) shall be coordinated to optimize selective tripping of the circuit overcurrent protective devices when a short circuit appears." NFPA 110, Annex A - Explanatory Material A.6.5.1 states: "It is important that the various overcurrent devices be coordinated, as far as practicable, to isolate faulted circuits and to protect against cascading operation on short circuit faults. In many systems, however, full coordination is not practicable without using equipment that could be prohibitively costly or undesirable for other reasons. Primary consideration also should be given to prevent overloading of equipment by limiting the possibilities of large current inrushes due to instanteous reestablishment of connections to heavy loads."

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Information provided is subject to change without notice. Please verify all details with GE. All values are design or typical values when measured under laboratory conditions, and GE makes no warranty or guarantee, express or implied, that such performance will be obtained under end-use conditions.