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This document describes issues concerning the 3 Pole vs. 4 pole breakers and the correct application for an emergency power application.

Sometimes issues arise on the issue concerning the 3 pole vs. 4 pole breaker and the correct application for a emergency power application.  In a UPS installation we use a
3 pole breaker for the UPS input and a 
4 Pole for the Bypass & Output.
The reason for a 4 Pole for the Bypass & Output is that the current should always return via the same path, i.e. if online it should go through the inverter circuit and if on
Bypass through the utilities circuit.  The neutral current shouldn't be going back through the UPS.  Basically if there was only had 3 pole isolation at the output, some of
the current would go back through the UPS as the 
 N is straight through.  This becomes even more important when we have a dual supply.
This is not desirable as we want it to be completely current free.  If there was a problem on the N there could be a voltage on the N as well. 

Concerning the use of a 3 pole breaker for the UPS input, the reason that the Input breaker is 3 pole and doesn't break the Neutral is that, in a 3 Phase UPS, the Mid Point of the batteries is referenced to Neutral and that creates problems.

This is also something to be considered when a Generator is being used and this can be a complex issue as other applications in the building may have different requirements. 
Using a 3 pole switch-over means that there won’t be issues with the Mid Point of the batteries being referenced to Neutral, but it can create  other problems, like n
uisance tripping of
RCD’s on de-energized source.
Other options for such applications are the use of a 3-Pole Switching with Overlapping Neutral or the use of a Technical earth to the UPS.  However the options must be weighed up carefully as different applications will require different solutions.

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