Neutral protection is related the earthing system.
The neutral must be broken by a multi-pole device:
- in TT and TN, if neutral cross-section is less than phase cross-section;
- in terminal distribution in view of the Neutral/Phase reversal risk.
The neutral must be protected and broken:
- in IT for intervention of the protection device on the double fault, with one of the faults possibly on the neutral;
- in TT and TN-S if neutral cross-section is less than phase cross-section;
For all earthing systems if the installation generates harmonic currents of rank 3 and multiples (especially if neutral cross-section is reduced).
In TN-C the neutral, which is also the PE, cannot be broken which is dangerous as a result of its potential variations, due to load currents and insulation fault currents.
To prevent risks, a local equipotentiality and an earth connection must be provided for each zone/consumer.
Below figure shows which types of circuit-breaker should be used for which earthing system.