Grounding Essentials
Electrical system grounding has three distinct purposes: to cause the operation of overcurrent protection devices in the event of a fault condition; to provide zero reference for the building electrical system ; and to equalize potential differences in the system.
The National Electrical Code (NFPA 70) stipulates that grounding must occur at the building (premises wiring system) service entrance and at each separately derived source?in most cases, a transformer. At the service entrance, the ground and neutral are bonded together; then, the grounding conductor is taken from the neutral bus to ground rod(s), switchgear enclosure, building steel, an underground cold water pipe or other available electrodes (NEC 250.30, 250.52) .
Creating a neutral-to-ground bond at the service entrance creates a line-to-ground voltage reference for the electrical system. This zero reference establishes a convenient frame of reference for line-to-ground voltage measurements. The neutral-to-ground bond also creates an effective grounding system and minimizing the voltage to ground and can limit overvoltage stresses on conductors to electrical equipment. This allows for intended equipment performance by isolating potential fault.
Grounding at each separately derived system is also of benefit under a fault condition, because electrons emanating from a source?transformer, generator or inverters?will attempt to return to the source. Under a phase-to-ground fault condition, the current will travel back along the ground wire or ground path?such as conduits and equipment enclosures?to the source. The source will provide current on the phase conductor(s) to meet the requirements of the short, thus causing the overcurrent device to trip. The purpose of the ground wire in this case is to provide a low impedance path back to the source.
Note that the ground wire is not returning the current to ground. In this sense, "ground wire" is a bit of a misnomer. Many times, this is called an "equipment" or "safety" ground, the latter being the most appropriate term, because it is meant to provide personnel safety by isolating the fault in the system.