This Question of the Day, concerning isolation of the neutral, is answered by Eaton: (theme - Distribution Equipment):
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This question, and thousands of others, have been answered by our Voltimum Experts on a daily basis in the Technical Expertise area.
Question: What is the advantage, or purpose, of breaking the neutral on a switch?
Answer: Isolation of the neutral prevents possible contact with live conductors on the load side of a switch. If the neutral is connected to earth by a cable or busbar of low impedance, then the neutral should be virtually at earth potential and safe.
However, with highly inductive loads such as supplying the primary side of power supply transformers and out of balance three-phase loads, neutral currents can be considerable - instead of virtually zero as with a well-balanced resistive load. This results in voltage appearing on the neutral and is typical of supplies today. Therefore, isolation of the neutral, as well as the phase, renders the load side of the switch totally safe to work on.
Also, the neutral should make first, break last in any switching operation, as the neutral provides the voltage reference point for the three-phases. Without this, phase-to-phase voltage may drift from 400V to considerably greater across one pair of phases, with damage liable to 400V equipment on the load side.
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