Electrical Power System Grounding and Ground Resistance Measurements

Introduction

System grounding has been used since electrical power systems began. However, many companies and industrial plants have used system grounding methods differently. The problem of whether a system neutral should be grounded, and how it should be grounded, has many times been misunderstood completely. Therefore, grounding of many systems has been based upon past experience rather than engineering analysis.

This chapter provides applicable information for grounding, such as definitions, reasons for having a system ground, the most desirable ground- ing method, and so on, and how to measure ground resistance in order to maintain the grounding system.

The definition of grounding is commonly used for both, system grounding and equipment grounding. The National Electrical Code (NEC) defines system ground as a connection to ground from one of the current-carrying conductors of an electrical power system or of an interior wiring system, whereas an equipment ground is defined as a connection to ground from one or more of the noncurrent-carrying metal parts of a wiring system or equipment connected to the system.

The Institute of Electrical and Electronic Engineers (IEEE) and American National Standard Institute (ANSI) standard 142-2007, “IEEE recommended practice for grounding of industrial and commercial power systems” covers system grounding. Many of the following definitions are found in this standard in describing power system grounding.

System neutral ground: A connection to ground from the neutral point or points of a circuit, transformer, motor, generator, or system.

Grounded system: A system of conductors in which at least one conductor or point is intentionally grounded.

Ungrounded system: A system of conductors in which there is no intentional connection to ground.

Solidly grounded: A system in which there is no intentional impedance in ground connection; in such a system the line to ground fault currents may equal three-phase fault current.

Resistance grounded: A system grounded through a resistance the value of which can be such as to provide either a low- or high-resistance ground system. The low-resistance ground system can have from 25 to several thousand amperes depending upon the value of the resistance. The high- resistance ground system usually has a value less than 25 A but greater than the value given by XCO/3, where XCO is the charging capacitance of the system.

Reactance grounded: A system grounded through a reactance.

Resonant grounded: The system grounding reactance value is such that the rated frequency fault current flowing through it is substantially equal to the current flowing between the conductors and the earth (charging current of the system).

Ground-fault neutralizer: A grounding device that provides an inductive component of current in a ground fault that is substantially equal to, and therefore neutralizes, the rated frequency capacitive component of the ground fault current.

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