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4. BACKFEED PROTECTION (MBS ARRANGEMENT ONLY)
In some of the possible combinations of switch positions for the MBS, the output of the
UPS could be connected to the input to the system. If the UPS is in Normal mode (the load
is being supplied power from the inverter), this could cause damage to the UPS. Further, if
the input to the system is not powered, the inverter could run on battery and supply voltage
to the input of the system. Service personnel working on the line feeding the input to the
system (or the line feeding that line, etc.) could be subjected to hazardous conditions.
Imagine an electrician opening the supply breaker feeding the MBS and expecting that line
to be safe to work on. A requirement for UPS design is that backfeed must be prevented.
Since the Transformer Cabinet with MBS arrangement is external to the UPS, the UPS
cannot provide backfeed protection without receiving information concerning the position of
the switches of the MBS. An additional requirement is that backfeed protection be provided
with the failure of any one component. That means that the protection must have
redundancy in the method.
The redundant methods implemented in the MBS are as follows:
1. If the UPS Output Breaker (Breaker #3) and the Bypass Breaker (Breaker #2) are
both closed, the UPS is commanded to go to Bypass mode, no matter what. If the
UPS was operating on battery due to an input source failure, the load will lose power,
but it will be impossible for the UPS to cause backfeed. This method requires a
signal from the MBS control to the UPS control via a pair of wires.
2. Unless the UPS is On Bypass OR the UPS Output Breaker (Breaker #3) is open, the
trip mechanism of the Bypass Breaker (Breaker #2) will be activated, preventing it
from closing. This requires a signal from the UPS control to the MBS control via a
pair of wires.
As is explained in
Section 5. Operation (MBS Arrangement Only)
, the proper procedure
for putting the MBS into the Bypassed mode requires that the user first put the UPS in
"Manual" Bypass Mode. Thus the signal that the UPS is On Bypass will be present and the
Bypass Breaker in the MBS can be closed. Further, closing the Bypass Breaker will assert
the command to the UPS to go to bypass, but this will have no effect as the UPS is already
in bypass. That is, if the user follows the correct procedures for operating the MBS, these
backfeed prevention schemes will be completely transparent.
4.1 Remote EPO (Emergency Power Off) (MBS Arrangement Only)
Some applications (notably, data centers) require that there is a provision to remotely shut
down power to the equipment powered by the UPS. Typically, this "remote" control is a red
button switch located next to the exit and wired to the UPS. The Transformer Cabinet with
MBS arrangement is equipped with a Remote EPO input that causes both the Bypass
Breaker (Breaker #2) and the UPS Output Breaker (Breaker #3) to trip when a REPO
switch activation is sensed. This way, power is interrupted regardless of which breaker
was actually supplying current to the load. It may be desirable to also shut down the UPS
upon activation of the EPO function. This requires a separate switch contact (NC) wired in
accordance with the restrictions in the UPS manual.
Note that the UPS has a front panel "EPO" switch. Pressing this switch will cause the UPS
to shut off immediately, but its action is only local. That is, it affects the UPS, but there is
no way for this switch to have a direct operating effect on the Transformer Cabinet with
