According to the common practice that was followed years ago, many electrical installations were not grounded. Perhaps the need for adequate grounding was not fully recognized, understood, or considered necessary. Today, electrical grounding is considered important to most, but not all, electrical installations.
Yet, the importance of proper electrical grounding is often neglected. In fact, the U.S. Office of Safety and Health Administration reports that the most common safety violation is the improper grounding of equipment or circuits.
The fundamental purpose of electrical grounding is to both protect personnel as well as property. An electrical ground prevents conductor voltages from exceeding the rating of the respective conductor insulation.
Equipment grounding will prevent equipment as cabinet enclosures and motor cases from becoming charged to a potential that could be harmful to personnel or equipment.
Whereas grounding is considered necessary to most electrical circuits, there are many circuits that, for a variety of reasons, are definitely not to be grounded. Specifically, the NEC requires that (for safety reasons) the following circuits are to be ungrounded:
- Health-care facilities (some restrictions)
- Electrolytic cells
In a system that is specifically not grounded, the first inadvertent grounding becomes the ground point for the circuit.
A feature of an ungrounded system is that the first, inadvertent grounding will not trip the overcurrent protective devices. This characteristic of an ungrounded system is sometimes important to a process that would be harmed by the immediate deactivation of an electrical system.
An ungrounded system also minimizes the chances of a serious arc flash that might otherwise result from a grounded system.
Ungrounded systems are often fitted with a ground fault detector that will provide notification of a ground fault condition and, in some instances, may simultaneously initiate a trip of the circuit.
The two common intended functions for the grounding of an electrical system are as a non-current-carrying function or as a current-carrying function.
Symbols commonly used to indicate an electrical ground are shown in Figure 1.
Besides the need for adequate grounding of an electrical system, most facilities also require grounding for the purpose of minimizing the effects of lightning.
Figure 1: Symbols for an electrical ground.
- Non-Current-Carrying Ground
A typical non-current-carrying ground includes the “equipment ground conductor” (as defined by the NEC) that connects equipment to a ground. A typical application is a metallic cabinet that houses electrical equipment.
The primary purpose for a grounding of the cabinet would be to avert an electrical shock to personnel in the event that a live conductor of an electrical circuit would inadvertently contact the metallic cabinet.
If a live conductor would come in contact with metallic components connected to the equipment ground conductor, the circuit protective device would be tripped thereby avoiding a condition potentially hazardous to personnel.
In short, the purpose of the equipment ground is primarily personnel safety.
- Current-Carrying Ground
Unlike non-current-carrying conductors that conduct current only under abnormal circumstances, current-carrying ground conductors conduct currents under normal operation of a circuit and are necessary for proper functioning of an electrical circuit. A neutral conductor is a typical current- carrying conductor.
- Lightning Grounds
Grounding for lightning protection of buildings and facilities is necessary for several reasons. A facility lacking adequate lightning protection could be vulnerable to powerful lightning strikes that could cause damage in a number of ways. First, there is always the potential for a fire resulting from a strike due to the high currents that are sometimes involved. Lightning can also damage a number of support systems as CATV, fire protection, security systems, and electronic controls. Under some circumstances, there is also the risk of damage to the insulation of electrical systems.
Lightning occurs as a result of cloud masses passing over structures on earth. A moving cloud mass can be at a very high electrical potential with respect to a building or structure. When the potential rises to a value that can jump the gap between the earth and the cloud, a lightning strike can occur.
Lightning Protection System
A properly installed lightning grounding system will permit moderate electrical current flow between the clouds and the protected system in advance of a strike.
A lightning protection system will include strike termination devices that are usually mounted high and are connected to the grounding system. The result can be a large reduction in the potential for damage that might otherwise result from a subsequent lightning strike.
A properly installed lightning protection system will interconnect the grounds of different structures of a facility so as to eliminate any potential difference between the buildings.
An important characteristic of the electrical grounding connections of a lighting system is that the path to ground must be through conductors of a relatively low resistance. The grounding conductors must connect to a ground matt or ground electrodes.
If the path to ground is of a high resistance, the high current of a lightning strike will generate high temperatures because of the I2R effect and possibly cause a fire. NFPA Code 780, Standard for the Installation of Lightning Protection Systems, sets forth the requirements for a properly installed lightning protection system.
Generally, a facility should have a single ground matt that connects to both the electrical ground system and the lightning ground system. An integral part of a properly installed lightning protection system will include adequately sized surge arrestors that divert the high potentials of a lightning strike away from the conductors of an electrical system and to ground.
High-tension electrical transmission lines usually have at least one bare ground conductor positioned above the current-carrying conductors for the purpose of absorbing at least a part of a lightning strike.
A lightning strike can be at an extremely high potential and can travel along elevated transmission lines to cause damage to electrical gear not capable of withstanding the high voltages and currents.
Large transformers are almost always fitted with lightning arrestors designed to divert the high potential of a lightning strike to a ground conductor.
Unlike an electrical system that is usually grounded at a single point, lightning grounding conductors provided over transmission lines may connect to ground at numerous locations.