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System Description
1
1.0 SYSTEM DESCRIPTION
1.1 Function
The Liebert Continuous Power-Tie™ system provides the capability to momentarily or continuously
parallel the output buses of two otherwise independent UPS Systems (See Figure 1). It also provides
manually initiated, uninterrupted transfers of a critical load bus between the two UPS Systems.
This topology permits one UPS and its associated distribution system to be shut down for mainte-
nance while the load continues to be supplied by another UPS, without the necessity of transferring
the load to bypass during shutdown or restart of the UPS being maintained.
1.2 System Flexibility
In the unlikely event that one or both UPS systems have only partial available capacity (e.g., loss of
one or more modules in an otherwise functional system), this partial capacity system can be continu-
ously tied to a second full (or partial) capacity system, powering simultaneously the now-tied two crit-
ical buses.
This results in a single, fully redundant UPS system powering two different distribution strings
through a single UPS critical bus.
When the off-line (whether for maintenance or repair) modules are brought on-line again, the two
UPS critical buses are again separated without the need to transfer to the bypass source. For continu-
ous-tied systems, module kVA and kW ratings must be identical.
1.3 Inter-System Isolation and Reliability
For multiple UPS installations at a single site, best total system reliability is achieved when system-
to-system independence is maximized. However, to transfer loads or parallel critical buses between
multiple UPS systems, a certain amount of cross-connected system-to-system power and signal con-
ductors are required. See Figure 2.
Proper implementation of power conductor runs and circuit breaker placement will minimize the risk
that a catastrophic critical bus fault (e.g., a dropped wrench) on one system will cause a failure in a
second system.
Liebert has minimized the risk of system-to-system electrical noise susceptibility and faults in sys-
tem-to-system copper wire control circuitry through the use of fiber-optic technology as well as trans-
former and impedance isolation techniques in critical control circuits.
The possibility of signal and ground loops through control interconnections has been eliminated by
isolating all control signals between each UPS system and the Tie Controls (See Figure 3). For exam-
ple, a water leak over one UPS could effectively short all the control conductors together, causing a
catastrophic fault in one system. Through the noted isolation techniques, this fault will not be propa-
gated through intersystem (e.g., load sharing control conductors) control wiring to the second system.