Part P Earthing & Bonding 2 — Questions and Answers
Question 1: What is supplementary equipotential bonding?
- Local bonding between simultaneously accessible exposed and extraneous conductive parts in special locations (e.g., bathrooms) where the risk is elevated (Correct answer)
- Main bonding between incoming services
- Bonding between different circuits
- Bonding between the consumer unit and the meter
Correct answer: Local bonding between simultaneously accessible exposed and extraneous conductive parts in special locations (e.g., bathrooms) where the risk is elevated
Supplementary bonding locally connects metalwork that can be simultaneously touched (e.g., a bath and a radiator) in locations with increased shock risk; its purpose is to eliminate potential differences between simultaneously accessible parts.
Question 2: Under BS 7671 18th Edition, when is supplementary bonding in a bathroom NOT required?
- When all circuits serving the bathroom are protected by a 30 mA RCD (which provides equivalent protection) (Correct answer)
- Supplementary bonding is always required in bathrooms
- Only if no metalwork is present
- Only for bathrooms above ground floor
Correct answer: When all circuits serving the bathroom are protected by a 30 mA RCD (which provides equivalent protection)
The 17th and 18th editions introduced a relaxation: supplementary bonding is not required in bathrooms if all circuits in the room are protected by a 30 mA RCD, as the RCD disconnects quickly enough to prevent dangerous shock.
Question 3: What is a 'touch voltage'?
- The voltage appearing between simultaneously touchable parts (e.g., a faulty appliance casing and earth) during an earth fault (Correct answer)
- The voltage required to start an appliance
- The rated supply voltage
- The voltage drop across a cable under load
Correct answer: The voltage appearing between simultaneously touchable parts (e.g., a faulty appliance casing and earth) during an earth fault
Touch voltage is the voltage that appears between simultaneously touchable conductive parts when a fault occurs; it is dangerous because current passes through the human body if a person contacts both parts simultaneously.
Question 4: What is the permitted maximum touch voltage in AC installations under BS 7671?
- 50 V AC (or 120 V DC ripple-free) — above this level rapid disconnection or other measures are required (Correct answer)
- 230 V
- 110 V
- 25 V always
Correct answer: 50 V AC (or 120 V DC ripple-free) — above this level rapid disconnection or other measures are required
BS 7671 (and IEC 60364) sets the conventional touch voltage limit at 50 V AC (UL); above this voltage, protective measures must ensure disconnection within the required time to prevent prolonged dangerous shock current.
Question 5: What is the significance of the 'Ze' value measured at the origin of an installation?
- It is the external earth fault loop impedance from the supply — the impedance outside the installation. Adding R1+R2 (circuit impedance) gives Zs for each circuit (Correct answer)
- It is the total Zs including circuit cables
- It is the insulation resistance of the supply cable
- It is the phase voltage divided by the maximum current
Correct answer: It is the external earth fault loop impedance from the supply — the impedance outside the installation. Adding R1+R2 (circuit impedance) gives Zs for each circuit
Ze is the external earth fault loop impedance, measured at the origin (supply terminals) before any installation cabling. It is the starting point for calculating Zs for all circuits: Zs = Ze + (R1 + R2).
Question 6: What colour is the main earthing conductor identified by?
- Green and yellow stripes (Correct answer)
- Solid green
- Blue
- Brown
Correct answer: Green and yellow stripes
The main earthing conductor and all CPCs (circuit protective conductors) are identified by green and yellow striped insulation (or sleeving where the conductor is not insulated from new).
Part P Earthing & Bonding 2
What is supplementary equipotential bonding?