UPS 101 - Knowledgebase
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Proper phasing is very important in the power flow of a UPS system.  Prior to installing a UPS with a
Make-Before-Break External Maintenance Bypass Switch, it is critical to confirm your bypass source will be
in phase with your UPS output. If the bypass source is not in phase with the UPS output at the time of the
manual transfer, a harmful phase shift occurs that can damage both the load, and the UPS.  This damage
occurs because the Maintenance Bypass Switch contacts will overlap for a short period of time, t
o allow for
a seamless transfer

Many problems can arise due to a phase difference being present between the UPS's output waveform and  
the Maintenance Bypass Switch's input source waveform.  The most common example is when two legs of
a 3-phase bypass source are used to feed a single phase load.  

The phase separation angles of a 3-phase  source are not the same as those of a true single phase
source.   When looking at the oscilloscope image below, you can see (2) of the (3) phases from a 3-phase
source.  They are separated by 120 degrees. The second image depicts two phases of a true single phase
source.  The third image shows the 4 phases overlaid onto each other to better illustrate the differences.  
During an out-of-phase transfer, the two different sets of phases will collide with each other violently,
possibly causing UPS and load damage.  

The most feasible solution is to install a 1PH bypass isolation transformer in-line with the 3PH bypass
source that will f
eed the Maintenance Bypass Switch (and the UPS, if it has a Bypass Input).  This isolation
transformer will use the bypass power source and convert it to a new useable 1PH source.  This new
separately derived source will provide the 180 degree phase separation that is compatible with the UPS's
true single phase output.
Feeding a True Single Phase Load with a Three Phase Bypass Source
L1 and L2 of a 3PH source (commonly 208V L-L)
are separated by 120 degrees.
L1 and L2 of a true 1PH source (commonly 240V L-L)
are separated 180 degrees.  
The 3PH source and the single phase sources are both
overlaid to show that the phases do not line up with each
other.  If a transfer were made at this time,  possible
damage could occur to the system and the loads.