Few topics in the industry elicit as much controversy as grounding. Poor grounding practices cause ongoing equipment problems in a facility, whereas proper deployment ensures reliable and productive facility operation. Specialized grounding techniques have evolved to meet the perceived grounding requirements of electronic equipment performance but sometimes also violate the National Electrical Code (NEC). Terms such as single point, multiple point, isolated and equipotential reference grounding have special meaning and illustrate different approaches to grounding.
To the core
So what exactly does grounding entail? Grounding serves as the center point for applications where electrical currents are produced, in cases when an operations facility needs to have a particular source of power and in which the voltage of an electrical current has some sort of physical connection to the Earth. According to TEAMWORKnet Inc., Lakeland, Fla., grounding ensures rapid clearing of faults and prevents hazardous voltage, which in turn reduces the risks of fires and personnel injuries. Grounding serves the primary functions of referencing the AC systems and providing a means to ensure fault clearing. Ungrounded, solidly grounded systems and low-resistance make up the three basic types. Ungrounded are electrical power systems that are operated with no intentional connection to earth ground. The solidly grounded system is one that has the neutral connected to ground without an intentional impedance. In contrast to the ungrounded system, the solidly grounded one results in a large magnitude of current to flow (aids in coordination), but has no increase in voltage on unfaulted phases. The low resistance grounded system is one that has the neutral connected to ground through a small resistance that limits the fault current.
Most commonly used in electrical engineering, there are six grounding systems in use today:
An equipment ground is the physical connection to earth of non-current carrying metal parts.
In static grounds the connection is made between a piece of equipment and the earth for the purpose of draining off static electricity charges before a flash over potential is reached.
A system ground refers to the point in an electrical circuit that is connected to earth. This connection point is typically at the electrical neutral. The sole purpose of the system ground is to protect equipment. This type ground also provides a low impedance path for fault currents improving ground fault coordination.
Maintenance grounds are utilized for safe work practices and are temporary.
Electronic and computer grounds--grounding for electronic equipment is a special case in which the equipment ground and the system ground are combined and applied in unity. Electronic equipment grounding systems must not only provide a means of stabilizing input voltage levels, but also act as the zero (0) voltage reference point.
Lightning protection grounding requirements are dependent upon the structure, equipment to be protected and the level of lightning protection required of desired.
Here are some key points that must be considered when examining the type of grounding system you currently maintain or want to have:
• The difference between floated and grounded electrical systems.
• The adverse effects of grounding schemes-recognizing grounding as a simple, scientific practice rather then as magic or myth.
• Why isolated ground receptacles work in some cases and not in others.
• How ground plane technology prevents skin effect from defeating equipment grounding.
• Why and how telecommunications and cable TV systems must be grounded.
• How to preserve NEC® grounding compliance and still achieve effective grounding without sacrificing equipment performance.
Testing of the effective resistance of a ground is important, but a good testing program must include the ground conductors and its connections and splices. The electrical ground system ultimately includes the grounded device itself and the entire pathway back to the earth ground.