Installing earthing and equipotential bonding

Clarify whether the installation is supplied as TN-S (separate PE all the way), TN-C-S (PEN to one point, then separate PE/N) or TT (local earth electrode). TN-C with a combined PEN conductor is not permitted in new installations. The system determines the requirements for disconnection and protective devices.

Install a main earthing terminal (main earthing bar) as the central connection point for the protective conductor, the earth electrode and the bonding conductors. All earth connections in the installation refer to it.

Connect extraneous conductive parts — typically incoming metallic water and heating pipes and load-bearing metal structures — to the main earthing terminal with protective bonding conductors. This keeps them at nearly the same potential and reduces the touch voltage in the event of a fault.

In a TT system, the exposed parts of the installation are earthed via a local earth electrode. Measure the earth electrode resistance R_A — it determines how large a fault current can flow, and thus the requirement for the RCD's rated residual operating current.

In TT, a circuit breaker alone can rarely disconnect an earth fault, so an RCD is required. Choose the rated residual operating current so that I_ΔN ≤ 50 V / R_A, keeping the touch voltage below 50 V. The touch voltage with the chosen RCD is U_B = R_A · I_ΔN.

In rooms with increased risk — e.g. bathrooms — supplementary protective equipotential bonding may be required, connecting all exposed and extraneous conductive parts locally. Follow the relevant part of the DS/HD 60364-7 series for the room in question.

Verify that faults are disconnected within the requirement (TN ≤ 0.4 s at 230 V, TT ≤ 0.2 s) by measuring loop impedance and testing the RCD. Record earth electrode resistance, continuity and disconnection times in the verification report.