UPS 101 - Knowledgebase
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Cascade: UPS or Inverters in a Cascade redundant configuration entail Unit A's output feeding Unit B's
bypass input as shown in the drawing. In the event that the primary unit should malfunction, the secondary
unit is always in a hot-standby position. In the extremely rare event that both units fail, the load will be
transferred to a bypass source to maintain power to critical loads.
Pros: The transfer is natural to the design of the UPS. Provides very high reliability as 4 power sources are available to
the critical loads. 1.) AC Main Input source, 2.) UPS A battery bank, 3.) UPS B battery bank 4.) Bypass source.
Cons: Each unit must be correctly sized to handle the entire load.
Parallel: UPS or Inverters in a Parallel redundant configuration require both units' outputs to be connected
together. This is typically done by using a paralleling cabinet which consists of two inputs and a single
output, as well as inductors that allow for load sharing during parallel operation.
Pros: Can be used to double the output of a UPS and can be a cost effective method when paralleling for capacity.
Ex: Two 50kva UPS can be paralleled for a total output capacity of 100kva.
Cons: Not a very reliable means of redundancy. In an event that one unit unexpectedly transfers to a bypass source, the
remaining unit can potentially be back-fed by the bypass source resulting in cleared fusing or internal damage. This is
because the bypass source is typically a stiffer source of commercial A/C power.
Modular Systems in Parallel: (Also known as an N+1) Modular systems are designed to provide a low cost
solution with the reliability of a redundant system, however this design falls short. Their inherent design
doesn't contain the same type of de-rated components as an independent or stand alone UPS. The low
reliability of each individual module requires them to be designed in parallel configurations in case of a
single failure. This in turn, reduces their overall MTBF, and while the initial cost of a modular system may be
less, the cost of ownership and risk of premature failure is much higher.
bypass input as shown in the drawing. In the event that the primary unit should malfunction, the secondary
unit is always in a hot-standby position. In the extremely rare event that both units fail, the load will be
transferred to a bypass source to maintain power to critical loads.
Pros: The transfer is natural to the design of the UPS. Provides very high reliability as 4 power sources are available to
the critical loads. 1.) AC Main Input source, 2.) UPS A battery bank, 3.) UPS B battery bank 4.) Bypass source.
Cons: Each unit must be correctly sized to handle the entire load.
Parallel: UPS or Inverters in a Parallel redundant configuration require both units' outputs to be connected
together. This is typically done by using a paralleling cabinet which consists of two inputs and a single
output, as well as inductors that allow for load sharing during parallel operation.
Pros: Can be used to double the output of a UPS and can be a cost effective method when paralleling for capacity.
Ex: Two 50kva UPS can be paralleled for a total output capacity of 100kva.
Cons: Not a very reliable means of redundancy. In an event that one unit unexpectedly transfers to a bypass source, the
remaining unit can potentially be back-fed by the bypass source resulting in cleared fusing or internal damage. This is
because the bypass source is typically a stiffer source of commercial A/C power.
Modular Systems in Parallel: (Also known as an N+1) Modular systems are designed to provide a low cost
solution with the reliability of a redundant system, however this design falls short. Their inherent design
doesn't contain the same type of de-rated components as an independent or stand alone UPS. The low
reliability of each individual module requires them to be designed in parallel configurations in case of a
single failure. This in turn, reduces their overall MTBF, and while the initial cost of a modular system may be
less, the cost of ownership and risk of premature failure is much higher.
Cascade Redundant vs. Parallel Redundant
