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.
The impedance of the ground conductor can be seen as the pathway that a faulted current will have to take to reach the earth ground system. When insulation fails, a short circuit occurs. Protective devices like fuses or breakers open to stop the fault current, but before these devices can act, the ground conductor must carry the fault current to the ground rod in the earth.
The effective ground resistance needs to be as low as possible to quickly and safely dissipate the fault current for two reasons. First, the fault current has to quickly exceed the rating of the protective device, or exposed metal will be energized and potential for catastrophe exists. If the fault current does not exceed the rating of the protective device, the current will continue to leak on the ground continuously, until a complete failure occurs. Second, the fault current and resistance of the ground system can be multiplied together to calculate the effective voltage at the ground. Imagine that a ground system has an effective resistance of 50W. A person standing in mud on a job site may have a lower pathway to ground, say 30W. If that person is unlucky enough to touch the bare ground conductor when a five amp fault current is present, the five amp current at 150 volts may well pass through his body rather than the ground itself, since he has a lower effective resistance than the earth ground. If the effective resistance of the earth ground were at the NEC recommended 25W, virtually all of the fault current would flow through the intended pathway to ground, not through the person’s wet feet. This is a good reason to treat exposed grounds as if they are always energized. You do not want to become a ground conductor.
A similar condition occurs in a building when splices or bonds on ground conductors are not low resistance, or when the neutral and ground is connected at a sub-panel. A high resistance bond on the ground will produce high impedance to fault current. This naturally causes heat as well as increasing the likelihood that the fault current will find a path to ground other than the ground conductor. Here again, someone unfortunate enough to become a lower pathway to ground could suffer the consequence.
This high resistance bond can be a source of power quality problems as well. Modern digital electronics work at five-volt levels or less, switching, communicating and controlling our automated industrial processes. Imagine the problems fault currents cause when they produce voltage on the groundside of solid-state circuits. This common problem can be resolved completely by providing low resistance pathways for fault current to follow to earth ground.
Another related power quality issue is stray voltage. Commonly caused by connecting the ground and neutral conductor in a sub-panel, stray voltage can energize all exposed metal and building steel. Stray voltage in dairy farms causes cows to eventually stop producing milk and in hospitals will cause many problems with high-tech diagnostic equipment and patient-connected equipment. In our modern electrical environment, non-linear loads cause high neutral currents.
The neutral conductor can carry substantial current back to the earth ground system. The ground conductor is not considered an electrical conductor and is present to provide a low resistance pathway for fault current. The neutral must be carried back to the service entrance and can only be bonded to the ground conductor at the main neutral buss, where a large copper conductor carries all the return and faulted current back to the earth. Sometimes through error or ignorance, the neutral and ground are connected upstream from the service entrance. This is called a false or bootleg ground. If the neutral and ground are connected anywhere else in the building, all grounded metal becomes part of the neutral conductor, constantly energized and creating various voltage potentials on electronic equipment. This causes many nuisance problems with automated equipment and computers, but can also create a hazardous and expensive electrical environment.
The solution to these problems is to include complete ground pathway testing as part of the standard procedures in your facility and to choose test equipment which will help you locate and identify high resistance ground paths.
Questions to Ask
- Determine how the building is grounded. Is it bond-grounded through structural steel?
- Is conduit used as a ground or is a grounding conductor used in all instances?
- Is grounding of the neutral done correctly, i.e., at the service entry panel, at all separately derived devices (transformers, generators, and UPS only and nowhere else), i.e., in outlet boxes or in power panels?
- Where is the grounding system located and what gauge wire leaves the service entry panel?
- What is the primary method used? (Connection to a cold water pipe is normally the primary method.)
- What is the secondary method used? Is it buried below the permanent frost line? (Secondary methods normally consist of a counterpoise system, ring around the building, single or multiple grounding rods, or rods forming an equilateral triangle.)
- What is the condition of the electrical grounding system?
- When was the system last tested?
Glossary of Terms
Air Terminals—Also referred to as lightning rods, these copper or aluminum rods are vertically mounted on a structure’s roof or top at various high points. Positioned to protect above the roofline, the rods are designed to intercept lightning strikes.
Main Conductors—Made of copper or aluminum, these cables connect air terminals to grounds. Conductors are coursed inside the framing spaces during construction of the building, hidden from view and protected from corrosion. On existing buildings conductors may be coursed behind down spouts or other parts of the building.
Grounds—Main conductors are attached to metal ground rods that are set at least 10 feet deep in the earth. Special grounding requirements are sometimes necessary in sandy or rocky soil.
Bonds—The bonding connects grounded metal objects to the main conductor cable and prevents side flashes (lightning jumping between two objects).