Overview of Parallel System

MIB Mains input breaker
BIB Bypass input breaker
UIB Unit input switch
SSIB Static switch input switch
UOB Unit output switch
Ext. UOB External unit output breaker
MBB Maintenance bypass switch
Ext. MBB External maintenance bypass breaker
SIB System isolation breaker
BB Battery breaker
NOTE: In parallel systems with an external maintenance bypass breaker Ext. MBB, the maintenance bypass breakers/switches MBB must be padlocked in the open (OFF) position.

UPSs for External Batteries

UPSs for Internal Batteries

The impedance of the bypass paths need to be controlled in a parallel UPS system. When operating in bypass mode, the parallel load sharing is determined by the total impedance of the bypass path comprising cables, switchgear, static bypass switch, and cable formation.

Risk of equipment damage
To ensure correct load sharing in bypass operation in a parallel system, the following recommendations apply:
  • The bypass cables must be the same length for all UPSs.
  • The output cables must be the same length for all UPSs.
  • The input cables must be the same length for all UPSs in a single mains system.
  • Cable formation recommendations must be followed.
  • The reactance of busbar layout in the bypass/input and output switchgear must be the same for all UPSs.
If the above recommendations are not followed the result can be uneven load sharing in bypass and overload of individual UPSs.
Failure to follow these instructions can result in equipment damage.
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