Introduction to Grounding

Priorities in Grounding:

Safety First - Personnel

Secondary - Equipment Protection

• The primary goal of the grounding system throughout any facility is SAFETY. Secondary are effective lightning protection, diminishing electromagnetic coupling (EMC), and the protection against electromagnetic pulses (EMP).
• Grounding is implemented to ensure rapid clearing of faults and to prevent hazardous voltage, which in turn reduces the risk of fires and personnel injuries. Grounding serves the primary functions of referencing the AC systems and providing a means to ensure fault clearing.
• 99.5% survival threshold –
  • 116 mA for one (1) second.
  • 367 mA for zero point one (0.1) second.
• What constitutes a good ground.
• A frequently quoted criteria is the establishment of a one (1) ohm resistance to earth. A large number of equipment manufacturers have this in their installation guides. The NEC requires only twenty-five (25) ohms of resistance for made electrodes, while the ANSI/IEEE Standard 141 (Red Book) and ANSI/IEEE 142 (Green Book) specifies a ground resistance of one (1) to five (5) ohms.
• External changes in the grounding system (environment) may affect the ultimate functionality of the entire electrical system.
• Frequency matters in very complex grounding systems. Leakage currents of equipment do not return to the earth; high frequency leakage currents return to the equipment which generated them, while power frequency leakage currents return to the derived source.
• The impedance of the system is viewed from the perspective of power frequencies and immediate harmonics (i.e., 60Hz and its associated harmonics).
• Generally accepted electrical wiring practices are not good ground system wiring practices (i.e., no sharp bends or turns).
• Grounding systems are not meant to last forever. The best grounding systems need the most attention as they will corrode the quickest.
  
  

  Typical of all Grounding Systems:

  Detect Fault

  Clear Fault - Relay (Protection)

  
  
  

Standard Industrial Grounding Methods and Types of Grounding

Ungrounded System:

The ungrounded system is one that has no intentional connection between the neutral or any phase and ground. Please note that an ungrounded system is grounded through the concept of capacitive coupling. The neutral potential of an ungrounded system, with balanced loading will be close to ground potential due to the capacitance between each phase conductor and ground.
• Low ground fault current.
• Very high voltages to ground potential on unfaulted phases.
• Sustained faults lead to system line-to-line voltages on unfaulted line.
• Insulation failure.
• Arcing Ground Faults.
• Failure due to restrike ground faults.
• Continued operation of facility.

Solidly Grounded System:

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 system will result in a large magnitude of current to flow (Aids in coordination), but has no increase in voltage on unfaulted phases.
• Low initial cost to install and implement, but stray currents then become a possible consequence.
• Common in low-voltage distribution systems, such as overhead lines.
• Typically feeds to transformer primary with high side fuse protection.
• Not the preferred grounding scheme for industrial or commercial facilities due to high magnitude fault currents.

Low Resistance Grounded System:

The low resistance grounded system is one that has the neutral connected to ground through a small resistance that limits the fault current. The size of the grounding resistor is selected to detect and clear the faulted circuit.
• The resistor can limit ground currents to a desired level based on coordination requirement or relay limitations.
• Limits transient overvoltages during ground faults.
• Low resistance grounding is not recommended for low voltage systems due to the limited ground fault current. This reduced fault current can be insufficient to positively operate fuses and/or series trip units.
• Ground fault current typically in the 100 – 600 Amp range.

High Resistance Grounded System:

The high resistance grounded system is one that has the neutral connected to ground through a resistive impedance whose resistance is selected to allow a ground fault current through the resistor equal to or slightly more that the capacitive charging current of the system.
• The resistor can limit ground currents to a desired level based on coordination requirement or relay limitations.
• Limits transient overvoltages during ground faults.
• Physically large resistor banks.
• Very low ground fault current, typically under 10 Amps.
• Special relaying methods utilized to detect and remove ground faults.
• High resistance grounding is typically applied to situations where it is essential to prevent unplanned outages.
• Recent trend has been to utilize high resistance grounding methods on 600 volt systems and lower.

Grounding System and Design Considerations

1. Equipment Grounds: An equipment ground is the physical connection to earth of non-current carrying metal parts. This type grounding is done so that all metal part of equipment that personnel can come into contact with are always at or near zero (0) volts with respect to ground. All metal parts must be interconnected and grounded by a conductor in such away as to ensure a path of lowest impedance for flow of ground fault current. Typical items (equipment) to be grounded are; electrical motor frames, outlet boxes, breaker panels, metal conduit, support structures, cable tray, to name a few.
2. Static Grounds: A static ground is a connection 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. This type grounding system is utilized in dry materials handling, flammable liquid pumps and delivery equipment, plastic piping, and explosive storage facilities.
3. System Grounds: 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. This ensures longer insulation life of motors, transformers and other system components.
4. Maintenance Grounds: This type ground is utilized for safe work practices, and is a temporary ground.
5. 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. Grounding systems for the modern electronics installation must be able to provide effective grounding and bonding functions well into the high frequency megahertz range.
6. Lightning Protection: Lightning protection grounding requirements are dependent upon the structure, equipment to be protected, and the level of lightning protection required or desired.

• The area available for installation of the grounding system. This could lead to the requirement and utilization of chemical rods, or wells.
• Water table and seasonal changes to it.
• Soil condition and resistivity, please see chart of typical results. Also elevation above sea level and hard rocky soil are concerns that would need to be addressed.
• Available fault currents (i.e., three (3) phase, line-to-ground and line-to-line-to ground, etc.).
• NEC and ANSI/IEEE requirements. Also include here the requirements of the process equipment to be installed.
• Consideration to the number of lightning strikes and thunder storm days per year.
• Utility ties and/or service entrance voltage levels.
• Utilization of area were ground system is to be installed, (i.e., do not install under paved parking lot).

Soil Resistivities

1. Low resistivity soils are highly influenced by the presence of moisture.
2. Low resistivity soils are more corrosive than high resistivity soils.
3. Crumbly soil composed mostly of clay with a high limestone content.
4. Metamorphic rock formed by recrystallization of granite, separated into bands.
5. Metamorphic rock much coarser than gneiss.

Grounding Diagram Zones of equipment with localized transformers to isolate the equipment and control leakage current. Reference grids in all computer, data processing and information technology rooms. Perimeter ground ring bonded to the service entrance. Intentional continuity of structural steel. Bonding of all communication cables to structural steel. Architectural steel treatment for lightning protection. Ufer ground treatment per NEC for all main vertical steel footers. Grounding grid below moisture barrier. Bonding horizontal steel pans to structural steel.

Sample Images

Grounding Reports