Because negative or neutral electrode usually is grounded, but neutral isolation sometimes is mandatory.
Neutral protection according to 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;