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Theory of Operation
15
Transfer and Retransfer Conditions
1. Automatic Transfers to Bypass: Critical bus conditions that will initiate an automatic transfer
of the critical load from the UPS inverter output to the bypass source are:
a. Output Overload: overcurrent condition in excess of the current versus time overload capacity
curve. See Figure 58.
b. Overvoltage/Undervoltage (OV/UV): critical bus voltage is outside the allowable tolerance.
c. Inverter Inoperative: inverter diagnostic circuitry senses an imminent inverter output OV/UV
condition:
1. Battery discharged to the shutdown voltage.
2. Inverter or rectifier fault condition (power, logic or overtemperature) present or
imminent.
3. Failure of system logic or logic power.
4. Emergency Module Off (EMO) circuit activated.
2. Manual Transfers: Manual transfers may be initiated at any time provided no transfer
inhibition conditions are present.
3. Transfer Inhibited: A manual transfer to the bypass source will be inhibited if any of the
following conditions exist:
a. UPS system to bypass voltage difference (∆V) exceeds a predetermined percentage
(normally ±5%).
b. Static switch disconnects open.
c. OK to Transfer signal from the control logic is not present.
4. Automatic Retransfers to UPS: Critical bus conditions that must be present to initiate an
automatic retransfer (Auto-Rexfer) of the critical load from the bypass source to the UPS system are:
a. The number of Auto-Rexfer Attempts selected must be greater than zero (0). If zero (0) is
selected, no automatic retransfer will occur.
b. Critical load was initially transferred to the bypass source due to a system overload only.
c. Overload condition has since been corrected (the load has dropped below 100% of the rated
load).
d. Both the Input and Battery (MBD) circuit breakers have remain closed since the overload
transfer.
e. OK to Transfer signal received from the control logic for at least 10 seconds, within 5 minutes
of the overload transfer. (A manually initiated retransfer from bypass is required for
overloads lasting 5 minutes or more.)
f. Cyclic-type system overloads, which occur up to five (select range is 0 to 5) times in
60 minutes, are automatically returned to the UPS system for each event including the Nth
overload. A manually initiated retransfer from bypass is required for the N+1 overload.
5. Manual Retransfers: Manual retransfers may be initiated at any time provided no retransfer
inhibition conditions are present.
6. Retransfer Inhibited: A retransfer (automatic or manual) from the bypass source to the UPS
system shall be inhibited if any of the following conditions exist:
a. Manual (and Automatic) Retransfer Inhibitions:
1. UPS system-to-bypass voltage difference (∆V) exceeds a predetermined percentage
(normally ±5%).
2. Bypass circuit breaker is inoperative.
3. OK to Transfer signal from the control logic is not present.
4. Inverter or rectifier fault.
b. Automatic Retransfer Inhibitions (in addition to those above):
1. The load transfer to bypass was not caused by an output overload.
2. Excessive cyclical overloads within a one-hour period.
3. Retransfer conditions are not satisfied within 5 minutes of the initial transfer.
NOTE
A load transfer to the bypass line will be completed whenever an automatic transfer to bypass
is initiated. If the Static Switch Unable alarm message is present for any reason (including a
greater than ±20° phase lock synchronization error), the automatic transfer will be interrupted
for 40 to 120 milliseconds. Because of the reliability of the UPS components, an interrupted
load transfer is a very unlikely occurrence.