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Theory of Operation
18
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. Overvoltage/Undervoltage (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 overtemperature) 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 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 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.
5. Manual Retransfers
Manual retransfers may be initiated at any time provided no retransfer inhibition conditions are
present.
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
40-120 milliseconds. Because of the reliability of the UPS components, an interrupted load
transfer is a very unlikely occurrence.