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Switchgear Riddle No.3 - Four pole circuit breakers
When and why are the four pole circuit breakers used as outgoing feeder in switchgears?
Author : Arash - From: Iran
Sat, August 15th, 2009 - 14:44
Generally the decision about neutral breaking depends on the way of earthing system execution. 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.

For details of various earthing system you can refer to IEC 60364 and below figures:

Author : Hamid - From: Iran
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