Grounding, Bonding, and Ground-Fault Protection
Earthing system behaviour
The Grounding, Bonding, and Ground-Fault Protection domain tests whether you understand the return path for fault current and the touch voltages that appear during a fault. The earthing system type determines the protection method: TN systems rely on a metallic return path and automatic disconnection, TT systems rely on an earth electrode plus RCD, and IT systems require insulation monitoring and careful first-fault management.
Core rules
- TN-S has separate neutral and protective conductors from the source. TN-C-S combines them in the supply and separates them at the origin. TT has an installation electrode independent of the supply neutral. IT is unearthed or earthed through a high impedance.
- Protective conductors must be sized for fault current and mechanical strength. Main protective bonding connects incoming metallic services, structural metal and exposed conductive parts to the main earthing terminal. Supplementary bonding reduces touch voltage in locations such as bathrooms and swimming pools.
- In a TT system, the disconnection condition is Ra x I delta n is less than or equal to 50 V. With a 30 mA RCD, the theoretical maximum Ra is 1667 ohm, but a practical target of 100 ohm or less is common to allow for seasonal variation.
- RCD types must suit the waveform: Type AC for sinusoidal AC, Type A for pulsating DC, Type B for smooth DC and three-phase equipment. A 30 mA RCD is used for additional protection, not as the sole means of fault protection in a TN system.
Worked example
A TT installation is protected by a 100 mA Type S RCD for fault protection. The measured electrode resistance is 420 ohm. Check the disconnection condition: 420 x 0.10 = 42 V, which is below 50 V, so the condition is met. If the electrode resistance rose to 600 ohm, the product would be 60 V, which exceeds the limit, so the electrode would need improvement or the RCD rating changed. The reasoning shows why TT electrode resistance is tied to RCD sensitivity, not to the current rating of the final circuit.
Traps that cost marks
Using a water pipe as the only earth electrode; bonding to plastic pipe; mixing neutral and protective functions downstream of the origin; and assuming a 30 mA RCD removes the need for a compliant protective conductor.