Good practices in the design and installation of a facility ground system

Why do we need grounding?

Grounding is defined as a conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth. Simple as it is and defined a long time ago by IEEE.

Grounding is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground current to and from the earth (or the conducting body).

Based on this definition, the reasons for grounding can be identified as:

1. Personnel safety by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system
2. Personnel safety and control of electrostatic discharge (ESD) by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system and the
3. Earth fault isolation and equipment safety by providing a low-impedance fault return path to the power
   source to facilitate the operation of overcurrent devices during a ground fault

There are other reasons for grounding not implicit in the IEEE definition. Overvoltage control has long been a benefit of proper power-system grounding. With the increasing use of electronic computer systems, noise control has become associated with the subject of grounding.

Equipment Grounding

Personnel safety is achieved by interconnecting all noncurrent-carrying metal parts of an electrical distribution system and then connecting the interconnected metal parts to the earth. This process of interconnecting metal parts is called equipment grounding and is illustrated in Figure 1, where the equipment grounding conductor is used to interconnect the metal enclosures.

Equipment grounding insures that there is no difference of potential, and thus no shock hazard, between noncurrent-carrying metal parts anywhere in the electrical distribution system. Connecting the equipment grounding system to earth insures that there is no difference of potential between the earth and the equipment grounding system.

It also prevents static charge buildup.

System Grounding

System grounding, which is also illustrated in Figure 1, is the process of intentionally connecting one of the current-carrying conductors of the electrical distribution system to ground. The figure shows the neutral conductor intentionally connected to ground and the earth. This conductor is called the grounded conductor because it is intentionally grounded.

Fault isolation is achieved by providing a low-impedance return path from the load back to the source, which will ensure operation of overcurrent devices in the event of a ground fault. The system ground connection makes this possible by connecting the equipment grounding system to the low side of the voltage source.

Methods of system grounding include solidly grounded, ungrounded, and impedance-grounded.

Solidly grounded means that an intentional zero-impedance connection is made between a current-carrying conductor and ground. The single-phase system shown in Figure 1 is solidly grounded. A solidly grounded, three-phase, four-wire, wye system is illustrated in Figure 2. The
neutral is connected directly to ground with no impedance installed in the neutral circuit.

For low-level arcing ground faults, the application of sensitive, properly coordinated, ground-fault protection (GFP) devices is necessary to prevent equipment damage from arcing ground faults. The NEC requires arcing ground-fault protection at 480 Y/277 V services, and a maximum sensitivity limit of 1200 A is permitted.

Severe damage is less frequent at the lower voltage 208 V systems, where the arc may be self-extinguishing.