Grounding and bonding are crucial safety measures in electrical systems. They protect people and equipment from electric shock, fire, and damage by preventing voltage differences between conductive surfaces and facilitating the operation of overcurrent protection devices.
Proper grounding connects electrical equipment to earth, limiting voltage rises and directing fault currents safely. Bonding connects conductive parts to maintain equal electrical potential, reducing shock risk. Together, they form a comprehensive safety system for electrical installations.
Importance of grounding and bonding
- Grounding and bonding are critical safety measures in electrical systems that protect people and equipment from electric shock, fire, and damage
- Proper grounding and bonding help prevent voltage differences between conductive surfaces, reducing the risk of dangerous touch potentials
- Grounding and bonding also facilitate the operation of overcurrent protection devices, such as fuses and circuit breakers, by providing a low-impedance path for fault currents
Grounding vs bonding
Grounding for safety
- Grounding connects electrical equipment or systems to the earth to limit voltage rises and stabilize voltage levels during normal operation
- Grounding systems direct fault currents to the earth, allowing overcurrent protection devices to operate quickly and safely
- Proper grounding reduces the risk of electric shock by maintaining exposed conductive surfaces at or near earth potential
Bonding for potential equalization
- Bonding connects conductive parts of electrical systems together to maintain an equal electrical potential between them
- Bonding prevents voltage differences between conductive surfaces, reducing the risk of electric shock or arcing
- Bonding is essential for metal piping systems, structural steel, and other conductive materials that may become energized during a fault condition
Grounding electrode system
Grounding electrode conductor
- The grounding electrode conductor (GEC) is a conductor that connects the grounded conductor of an electrical system to the grounding electrode
- GECs must be sized according to NEC Table 250.66 based on the largest ungrounded service-entrance conductor or equivalent area for parallel conductors
- GECs are typically bare copper, aluminum, or copper-clad aluminum and should be installed in a straight line with minimal bends
Grounding electrodes
- Grounding electrodes are conductive elements intentionally connected to the earth to establish a low-impedance path for fault currents
- Common grounding electrodes include ground rods, metal underground water pipes, metal building frames, concrete-encased electrodes, and ground rings
- Grounding electrodes must be installed per NEC requirements, considering factors such as soil resistivity, electrode type, and spacing
Made vs natural electrodes
- Made electrodes are intentionally installed for grounding purposes (ground rods, ground plates, or ground rings)
- Natural electrodes are conductive elements that are part of the building structure or infrastructure (metal water pipes, building steel, or concrete-encased electrodes)
- NEC 250.52 specifies the requirements for both made and natural electrodes, including minimum sizes, materials, and installation methods
Equipment grounding
Equipment grounding conductor
- The equipment grounding conductor (EGC) is a conductive path that connects non-current-carrying metal parts of electrical equipment to the system grounded conductor or grounding electrode conductor
- EGCs help prevent electric shock by maintaining exposed metal surfaces at or near ground potential during fault conditions
- EGCs are sized according to NEC Table 250.122 based on the overcurrent device rating protecting the circuit conductors
Grounded vs ungrounded systems
- In a grounded system, one of the current-carrying conductors (usually the neutral) is intentionally connected to the earth through a grounding electrode system
- Ungrounded systems have no intentional connection between the current-carrying conductors and the earth
- Grounded systems are more common in low-voltage applications (under 600V), while ungrounded systems are often used in medium-voltage or high-voltage applications for continuous operation during a single ground fault
Bonding methods
Exothermic welding
- Exothermic welding is a method of bonding copper conductors or ground rods using a high-temperature, exothermic reaction between copper oxide and aluminum
- This process creates a permanent, low-resistance connection that is suitable for direct burial and resistant to corrosion
- Exothermic welding is often used for critical bonding connections, such as those in grounding electrode systems or cathodic protection applications
Compression connectors
- Compression connectors are mechanical devices that join two or more conductors by deforming the connector and conductors under high pressure
- Common types of compression connectors include crimp lugs, split-bolt connectors, and tap connectors
- Compression connectors offer a secure, low-resistance connection and are suitable for most bonding applications when properly sized and installed
Bonding jumpers
- Bonding jumpers are short lengths of conductor used to connect two or more conductive surfaces to establish electrical continuity and maintain equal potential
- Bonding jumpers are commonly used to bond metal piping systems, structural steel, and equipment enclosures
- Bonding jumpers must be sized according to NEC Table 250.102(C)(1) based on the rating of the overcurrent device protecting the circuit conductors
Ground fault protection
Ground fault circuit interrupters (GFCIs)
- GFCIs are devices designed to protect people from electric shock by quickly interrupting power when a ground fault is detected
- GFCIs constantly monitor the current flowing through the hot and neutral conductors of a circuit and trip when an imbalance exceeds 4-6 milliamps
- NEC requires GFCIs in specific locations where electrical equipment is near water or in damp environments, such as bathrooms, kitchens, and outdoor receptacles
GFCI operation and types
- GFCIs operate by comparing the current flowing through the hot and neutral conductors using a current transformer
- If an imbalance is detected, indicating current leakage to ground, the GFCI trips and opens the circuit within 25-40 milliseconds
- Types of GFCIs include receptacle-type, circuit breaker-type, and portable GFCIs, each suitable for different applications and installation requirements
Special grounding situations
Isolated grounds
- Isolated ground (IG) systems use a separate equipment grounding conductor that is insulated from the conduit or raceway to reduce electrical noise
- IG receptacles have an extra grounding terminal that connects directly to the grounding electrode system, bypassing any noise on the conduit or raceway
- Isolated grounds are often used in sensitive electronic equipment applications, such as computers or medical devices, to minimize electromagnetic interference (EMI)
Separately derived systems
- A separately derived system (SDS) is an electrical system that receives power from another system through a transformer, generator, or converter
- SDSs require their own grounding electrode system and bonding jumper to establish a local ground reference and prevent dangerous voltage differences
- Examples of SDSs include transformers, generators, and uninterruptible power supplies (UPS)
Hazardous locations
- Hazardous locations are areas where flammable or explosive materials may be present, requiring special grounding and bonding practices to minimize the risk of ignition
- NEC Article 500 classifies hazardous locations based on the type and concentration of the hazardous material and specifies grounding and bonding requirements for each class and division
- Intrinsically safe systems, explosion-proof enclosures, and increased safety equipment are examples of specialized grounding and bonding practices used in hazardous locations
Grounding and bonding best practices
Minimizing ground loops
- Ground loops occur when there are multiple paths for current to flow between two points in a grounding system, leading to noise, interference, or safety hazards
- To minimize ground loops, use a single-point grounding system, where all equipment is connected to a common grounding point
- Avoid running grounding conductors in parallel or creating multiple connections between grounded surfaces
Proper conductor sizing
- Grounding and bonding conductors must be sized according to NEC tables and requirements to ensure they can safely carry fault currents without overheating or failing
- Use NEC Table 250.66 for sizing grounding electrode conductors, Table 250.122 for equipment grounding conductors, and Table 250.102(C)(1) for bonding jumpers
- Consider factors such as conductor material, insulation temperature rating, and conduit fill when selecting and installing grounding and bonding conductors
Regular system maintenance
- Regularly inspect and maintain grounding and bonding systems to ensure they remain effective and compliant with NEC requirements
- Check for loose connections, corrosion, or damage to grounding electrodes, conductors, and bonding jumpers
- Perform periodic testing to verify the continuity and effectiveness of the grounding and bonding system, and address any deficiencies promptly
Testing and verification
Earth ground resistance testing
- Earth ground resistance testing measures the resistance between the grounding electrode system and the earth to ensure a low-impedance path for fault currents
- Use the fall-of-potential method, which involves placing auxiliary electrodes at specific distances from the grounding electrode and measuring the resistance using a specialized tester
- NEC 250.53(A)(2) requires a maximum resistance of 25 ohms for a single rod, pipe, or plate electrode, but lower values may be necessary for larger systems or critical applications
Continuity testing
- Continuity testing verifies the integrity and effectiveness of grounding and bonding connections by measuring the resistance between conductive surfaces
- Use a low-resistance ohmmeter or continuity tester to measure the resistance between equipment grounding conductors, bonding jumpers, and grounded surfaces
- Resistance values should be as close to zero as possible, typically less than 1 ohm for most applications
Periodic inspection requirements
- NEC 250.4(A)(1) requires regular inspections of grounding and bonding systems to ensure they remain in proper condition and comply with NEC requirements
- Inspect grounding and bonding systems during initial installation, after any modifications or repairs, and at least every five years for commercial or industrial facilities
- Document inspection findings and promptly address any deficiencies or non-compliant conditions to maintain a safe and effective grounding and bonding system