3. Low-Voltage Power System Basics · Page 2 of 6
TN, TT and IT Earthing Systems
System earthing changes SPD selection and N-PE behavior.
TN systems
In TN-S / TN-C-S, PE is solidly referenced. SPD connections to PE are straightforward, but you must still respect backup fuse / breaker coordination.
TT systems
TT systems rely on earth electrodes with higher impedance. Follow manufacturer guidance for SPD topology and residual current device interactions.
IT systems
IT systems may leave one point isolated or impedance-earthed. SPD selection needs special attention to avoid nuisance issues during first faults.