309A Grounding and Bonding 3 — Questions and Answers
Question 1: What is a concrete-encased grounding electrode (Ufer ground)?
- A ground rod encased in a concrete pad
- A minimum 6 m of bare No. 4 AWG copper conductor encased in concrete foundation footing (Correct answer)
- A copper plate buried under a concrete slab
- A grounding ring around a concrete foundation
Correct answer: A minimum 6 m of bare No. 4 AWG copper conductor encased in concrete foundation footing
A concrete-encased electrode (Ufer ground) consists of at least 6 m of bare No. 4 AWG copper conductor or 6 m of rebar encased in at least 50 mm of concrete in a building footing, providing excellent ground contact.
Question 2: What is the purpose of a grounding ring around a building?
- To provide lightning protection only
- To establish a continuous grounding electrode encircling the building at the foundation level (Correct answer)
- To prevent stray voltage in the parking area
- To bond rebar in the foundation
Correct answer: To establish a continuous grounding electrode encircling the building at the foundation level
A grounding ring consists of a bare copper conductor (minimum No. 2 AWG) buried at least 750 mm deep encircling the building, providing a comprehensive grounding electrode with low resistance.
Question 3: What is the danger of a floating (ungrounded) neutral in a single-phase service?
- The breakers will trip
- Voltages become unbalanced, potentially causing overvoltage on lightly loaded circuits (Correct answer)
- The power consumption increases
- The meter stops reading
Correct answer: Voltages become unbalanced, potentially causing overvoltage on lightly loaded circuits
A floating neutral causes the voltage to divide unevenly between the two legs of a 120/240 V service based on load. The lightly loaded leg receives excessive voltage that can destroy appliances and cause fires.
Question 4: According to the CEC, when must a bonding conductor be installed with non-metallic conduit?
- Never — PVC does not require bonding
- Always — a separate bonding conductor must be pulled through the conduit (Correct answer)
- Only in wet locations
- Only for circuits over 100 A
Correct answer: Always — a separate bonding conductor must be pulled through the conduit
Since non-metallic conduit (PVC) cannot serve as an equipment bonding path, a separate bonding conductor must always be installed within the conduit to provide fault current return path.
Question 5: What is the purpose of exothermic welding (Cadweld) for grounding connections?
- To speed up installation
- To create a permanent, molecularly bonded connection that will not loosen or corrode (Correct answer)
- To allow easy disconnection for testing
- To reduce material costs
Correct answer: To create a permanent, molecularly bonded connection that will not loosen or corrode
Exothermic welding creates a molecular bond between conductors that is permanent, has lower resistance than the conductors themselves, and will not loosen or corrode, making it ideal for critical grounding connections.
Question 6: What is the CEC requirement for bonding around non-metallic insulating joints in metal piping systems?
- No bonding is required
- A bonding jumper must be installed around the insulating joint (Correct answer)
- The pipe section must be grounded separately
- The insulating joint must be removed
Correct answer: A bonding jumper must be installed around the insulating joint
CEC Rule 10-406 requires a bonding jumper to be installed around any insulating joint in a metal piping system to maintain electrical continuity of the bonding path throughout the piping system.
309A Grounding and Bonding 3
What is a concrete-encased grounding electrode (Ufer ground)?