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Theory of Operation 21
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 system to the
bypass source are:
a. System Overload: overcurrent condition in excess of the overload rating of the System Control Cabinet
(SCC).
b. Output Overload: overcurrent condition in excess of the current-versus-time overload capacity of UPS
modules ON LINE.
c. Over/Under Voltage (OV/UV): critical bus voltage is outside the allowable tolerance.
d. Non-redundant UPS Module Inoperative: an essential UPS module is taken OFF LINE for one of the
reasons below.
1. Battery discharged to the shutdown voltage.
2. Inverter overload capacity exceeded.
3. Inverter or rectifier fault condition (power, logic, or over-temperature) present or imminent.
4. Failure of UPS module logic or logic power.
e. SCC Logic:
1. Emergency Module Off (EMO) circuit activated.
2. Failure of UPS system logic or logic power.
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 shall be inhibited if any of the following conditions exist:
a. Bypass frequency deviates ±0.5 Hz from the nominal.
b. UPS system to bypass voltage difference ( V) exceeds a predetermined percentage (normally 5%).
c. Static switch disconnects (manual switches) open.
d. 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 has since dropped below 100% of the rated SCC current.
d. Enough UPS modules are ON LINE to supply the critical load.
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.
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 ±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.