Earthing, bonding, lightning protection and electrical safety
Domain 4: Earthing, bonding, lightning protection and electrical safety
This domain tests protective earthing and equipotential bonding as a system, not a single electrode. You must show that fault current has a reliable return path, touch voltages are controlled, and lightning and surge protection are coordinated.
Core rules
- Protective earthing provides a low-impedance fault path so protective devices operate within disconnection time.
- Main protective bonding connects extraneous conductive parts to the main earthing terminal; supplementary bonding reduces touch voltage in special locations.
- In TN systems the neutral is earthed at the source; in TT systems the installation uses an earth electrode and RCD protection.
- Verify earth-fault loop impedance and RCD operation; electrode resistance alone does not guarantee disconnection.
- Lightning protection follows a risk assessment and provides air termination, down conductors, earth termination and equipotential bonding.
- Coordinate surge protective devices (SPDs) between origin, sub-boards and sensitive equipment.
Worked example
A TT installation uses a 30 mA RCD for fault protection. For a 50 V touch-voltage limit, the maximum earth electrode resistance is Ra ≤ 50/0.03 = 1667 Ω. A measured 100 Ω electrode therefore appears acceptable for touch voltage, but the RCD must still trip within the required time. Record the test result and verify bonding, because a poor main bond can create a dangerous potential difference.
Traps
- Using the earth electrode as the only fault path in a TN installation.
- Omitting main bonding at water, gas and structural metalwork.
- Assuming a low electrode resistance proves disconnection.
- Installing SPDs without coordination or earthing.