In addition to the normal rectifier/inverter path, there are two bypass
switches that place the critical load on raw utility power. The automatic
“static” bypass is a make-before-break overlap transfer switch. This switch
will activate in the event of a failure in the normal power path. The
manual bypass, also known as the maintenance bypass, manually places the
load on raw utility power for corrective or preventive maintenance.
A transient can gate the automatic (static)
bypass switch closed, transferring the surge directly to the critical load. While
most UPS systems specifications indicate that they do provide some transient
protection, there is no status indication. A UPS is not a surge protector. The user will not
know if the surge protection fails. Transients
have been known to damage the input harmonic filters of UPS systems causing
an entire system meltdown.
Some variations of the standard UPS shown in
the diagram include dual input from two utility sources into the same UPS
and external maintenance bypass, also using dual power supplies for the
connected load. Obviously, both of these power paths must be protected.
If a UPS system is in place, we can safely
assume that the connected load is of critical importance. A surge protector, placed on
the panel (or panels) that feed the UPS, is an inexpensive solution to a
If you have any questions or would like to
learn more about these systems and how to apply surge protection, feel free
to call 1-800-851-1508 or email Paul Moraff at
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