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Electrical Riddle No.33 - 3P vs 4P CB
Author : manirul - From: India
Tue, March 3, 2009 - 19:12
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.

Author : Hamid - From: IRAN
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