Part P Earthing & Bonding 2

Question 1

What is supplementary equipotential bonding?

Accepted Answer

Local bonding between simultaneously accessible exposed and extraneous conductive parts in special locations (e.g., bathrooms) where the risk is elevated

Answer

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?

Accepted Answer

When all circuits serving the bathroom are protected by a 30 mA RCD (which provides equivalent protection)

Answer

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'?

Accepted Answer

The voltage appearing between simultaneously touchable parts (e.g., a faulty appliance casing and earth) during an earth fault

Answer

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?

Accepted Answer

50 V AC (or 120 V DC ripple-free) — above this level rapid disconnection or other measures are required

Answer

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?

Accepted 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

Answer

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).

Part P Electrical Practice Test

Part P Electrical Practice Test

Part P Earthing & Bonding 2