[manirul]

In a switchboard some where I have seen 3P main breaker and some where 4P breaker, what is the technicality about breaking of neutral as well. please explain technical point of view.

Topic

[Hamid]

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]

Replies