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Operating Instructions
30
Maintenance Procedures - Tie Control Logic and Power Components
Refer to the procedures in 4.2.3 - Maintenance Procedures - Isolating Each UPS System to iso-
late the UPS’s load circuit breaker and the associated downstream feeders for service.
By using downstream switchover devices for the individual critical loads, such as ATSs or dual input
PDUs, the load feeder components to be maintained are de-energized. All critical load operations are
transferred to the feeder which will power the combined loads.
There are three approaches to maintaining the Tie circuit breaker and its associated circuit compo-
nents depending on the convenience and isolation desired.
1. Single fixed-mount Tie circuit breaker
Complete UPS system and critical loads must be placed on a reliable maintenance bypass source
(e.g., standby engine-generator) to de-energize the Tie circuit breaker.
Using appropriate safety equipment and procedures, the Tie circuit breaker can be serviced while
remaining energized.
2. Single drawout Tie circuit breaker
Drawout mechanism permits servicing the Tie circuit breaker and associated circuitry while the
Power-Tie system is in its normal mode of operation.
To de-energize the drawout carriage plug-in stubs and buswork, however, one feeder at a time
must be isolated for service. Refer to 4.2.3 - Maintenance Procedures - Isolating Each UPS
System.
Using appropriate safety equipment and procedures, these components can be serviced while
remaining energized.
3. Two Tie circuit breakers in series - Fixed-Mount or Drawout
By using two Tie circuit breakers in series, each circuit breaker and its associated buswork can be
conveniently and completely de-energized for service. Refer to 4.2.3 - Maintenance Procedures
- Isolating Each UPS System.
Unless the controls are in a continuous parallel mode or currently in the process of transferring a
load, it is possible to service the Tie controls by taking a few precautions. Since all control signals are
isolated, they can be safely de-energized for maintenance without affecting the UPS systems.
Abnormal Operations
Improper Transfer
Transfers cannot be performed unless the OK TO TRANSFER LOAD (#) TO SYSTEM (#) light turns
on. This makes it difficult to begin an improper transfer.
The source feeding the UPS modules does not matter because the UPS inverter output is isolated
from the rectifier.
The logic permits transfers to and from a UPS system with the UPS modules running on Utility, Bat-
tery, or Engine-generator so long as the system accepting the load is running on inverter output.
Synchronization is verified by forcing the system to meet three independent criteria. First, all on-line
module oscillators must be synchronized. Second, the voltage zero crossings must be within tolerance.
Third, the instantaneous voltage differential between UPS system voltages must be within tolerance.
But, regardless of synchronization status, a critical load will not be allowed to transfer to a system
running on its bypass source. Transfers from a bypass source to a system on UPS is permitted.
Circuit Breaker Failure
If a “Close” or “Open” command to a circuit breaker fails to return a confirmation of completion within
an acceptable time period the Power-Tie logic initiates corrective actions based on the system’s initial
state and current state.
Loss of Tie Controls
Should a failure occur while the UPS systems are tied, the Loads will be transferred to the selected
Primary Bypass. And, the Load breaker to the other UPS system will be opened to prevent backfeed
from the Primary Bypass. A failure in the Tie controls while the systems are separated has no effect
on either UPS system since all control signals are isolated.