Classification and Types of UPS – Part Four

In the previous article “Classification and Types of UPS – Part Two”, we stated that according to the Form factor/ configurations, the UPS Systems have five famous configurations, which are:

1- “N” System Configuration

2- “N+1” System Configuration, which includes:

  • Isolated Redundant Configuration (N +1)
  • Parallel Redundant Configuration (1+1)
  • Parallel Redundant Configuration (N +1)
  • Parallel Redundant Configuration (N +2) and so on

3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1)

4- Parallel Redundant with STS Configuration

  • Parallel Redundant Configuration (1+1) + STS
  • Parallel Redundant Configuration (N+1) + STS

5- System plus System 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N


We explained the first three UPS System Configurations and today, we will continue explaining the other configurations of UPS Systems.



4- Parallel Redundant with STS Configuration




Parallel Redundant with STS Configuration, sometimes called Distributed Redundant Configuration, has many sub-Configurations included under it like:

  1. Parallel Redundant Configuration (1+1) + STS
  2. Parallel Redundant Configuration (N+1) + STS




Important definitions:


1- Single Point of Failure:

An element of the electrical distribution system that at some point will cause downtime, if a means to bypass it is not developed in the system. An N configuration system is essentially comprised of a series of single points of failure. Eliminating these from a design is a key component of redundancy.


2- Static Transfer Switch (STS):

An STS has two inputs and one output. It typically accepts power from two different UPS systems, and provides the load with conditioned power from one of them. Upon a failure of its primary UPS feeders the STS will transfer the load to its secondary UPS feeder in about 4 milliseconds, and thus keep the load on protected power at all times. This technology was developed in the early 1990’s, and is commonly used in distributed redundant configurations.




1- Parallel Redundant Configuration (1+1) + STS


The distributed redundant configuration (1+1), see Fig-1, utilizes two standalone configured UPS systems in conjunction with two stand-alone Static transfer switches.


Fig.1- Parallel Redundant Configuration (1+1) + STS

In normal operation, both units are designed to carry 50% of the critical load, and have 50% reserve capability to support the load on the other bus in the event the UPS feeding the other load bus encounters an operational problem. This allows complete independence and total isolation of the two UPS units from each other, facilitates separate output load buses, and eliminates the possibilities of single point failures either due to faults on the load side or because of faults within the two units. The result is a configuration that allows both critical load buses to be automatically fed from either the dedicated bus unit or the redundant unit.


The failure of UPS #1 would cause External Static Switch #1 to feed that system’s load from UPS #2. Similarly, the failure of UPS #2 would cause External Static Switch #2 to feed that systems load from UPS #1. Note, a common bypass feed, i.e. from the same bypass source, is necessary to ensure that the output of the two systems will be synchronized, and to allow availability of unrestricted fault clearing power.







This system configuration eliminates the potential for single point failures associated with single output bus failures that the other configurations are susceptible too.



From a complexity standpoint this system ranks slightly below the hot standby and slightly above parallel redundant configuration.



System configuration allows for either UPS systems to be maintained while the critical load is being fed from a protected power source.



Protects the load from all types of Power Quality issues

Redundancy Level







Important definitions:


1- Concurrent Maintenance:

The ability to completely shut down any particular electrical component, or subset of components, for maintenance or routine testing without requiring that the load be transferred to the utility source.

2- Single-Corded Loads

When the environment consists of single-corded equipment, each piece of IT equipment can only be fed from a single STS or rack mount transfer switch. Bringing the switch closer to the load is a prerequisite for high availability in redundant architectures as demonstrated in APC White Paper #48. Placing hundreds of single-corded devices on a single large STS is an elevated risk factor. Deploying multiple smaller switches feeding smaller percentages of the loads would mitigate this concern. In addition, distributed rack-mount transfer switches do not exhibit the failure modes that propagate faults upstream to multiple UPS system as is the case with larger STS. For this reason, the use of rack-based transfer switches is becoming more common, particularly when only a fraction of the load is single-corded. APC White Paper #62, “Powering Single-Corded Equipment in a Dual Path Environment” discusses the differences between STS and rack mount transfer switches in greater detail.

3- Dual-Corded Loads:

Dual-corded loads are becoming more the standard as time progresses, therefore the use of an STS is not necessary. The loads can simply be connected to two separate PDUs which are fed from separate UPS systems.

4- Multiple Source Synchronization:

When STS units are employed in a data center, it is important for the two UPS feeds to be in synchronization. Without synchronization control it is possible for UPS modules to be out of phase, especially when they are running on battery.





2- Parallel Redundant Configuration (N+1) + STS


The distributed redundant configuration (N+1), see Fig-2, utilizes three or more UPS modules with independent input and output feeders. The independent output buses are connected to the critical load via multiple PDUs and STS.


Fig.2- Distributed Redundant Catcher UPS Configuration

From the utility service entrance to the UPS, a distributed redundant design and a system plus system design (discussed in the next section) are quite similar. Both provide for concurrent maintenance, and minimize single points of failure. The major difference is in the quantity of UPS modules that are required in order to provide redundant power paths to the critical load, and the organization of the distribution from the UPS to the critical load. As the load requirement, “N”, grows the savings in quantity of UPS modules also increases.

Figures 2 and 3 illustrate a 300 kW load with two different distributed redundant design concepts.



Fig.2 uses three UPS modules in a distributed redundant design that could also be termed a “catcher system”. In this configuration, module 3 is connected to the secondary input on each STS, and would “catch” the load upon the failure of either primary UPS module. In this catcher system, module 3 is typically unloaded.



Fig.3 shows a distributed redundant design with three STS and the load evenly distributed across the three modules in normal operation. The failure on any one module would force the STS to transfer the load to the UPS module feeding its alternate source.

Evident in both of these one lines is the difference between distributing power to dual-corded loads and single-corded loads. The dual-corded loads can be fed from two STS units while the single-corded loads can only be fed from a single STS. For the single-corded loads the STS becomes a single point of failure.


Fig.3- Distributed Redundant UPS Configuration

As the quantity of single-corded loads in data centers today are becoming fewer and fewer it is becoming more practical, and less costly to apply multiple, small, point of use transfer switches close to the single corded loads. In cases with 100% dual-corded loads this configuration could be designed without STS units.

Distributed redundant systems are usually chosen for large complex installations where concurrent maintenance is a requirement and many or most loads are single corded. Savings over 2N also drive this configuration. Other industry factors that drive distributed redundant configurations are as follows:

The primary weakness of this design is the use of static transfer switches. These devices are very complex and have some unexpected failure modes, the worst of which is that they can fail in a way that causes the two inputs to short circuit to each other. In this type of event the STS can become a single point of failure since it can cause two UPS to drop the load simultaneously. The failure of a STS can propagate upstream and affect the entire system operation. For this reason the system + system design described in the following section has greater fundamental availability, particularly if the load devices have dual cord redundant powering capability.

There are many options in an STS configuration and several grades of STS reliability on the market to consider. In this configuration, the STS is ahead of the PDU (on the 480 volt side). This is a common application. Many engineers believe, justifiably so, that placing the STS on the 208 volt side of two PDUs is a more reliable application. This is a much more expensive application than the 480 volt STS.

A solution to prevent an out of phase transfer is to install a synchronization unit between the two UPS systems, allowing them to synchronize their AC output. This is especially critical when the UPS modules have lost input power and are on battery operation. The synchronization unit makes sure that all UPS systems are in sync at all times, so during a transfer in the STS, the power will be 100% in phase, thus preventing an out of phase transfer and possible damage to downstream equipment. Of course, adding a synchronization unit between independent UPS systems allows for the possibility of a common mode failure, or failure that can simultaneously drop all UPS systems.



  • Allows for concurrent maintenance of all components if all loads are dual-corded
  • Cost savings versus a 2(N+1) design due to fewer UPS modules
  • Two separate power paths from any given dual-corded load’s perspective provide redundancy from the service entrance
  • UPS modules, switchgear, and other distribution equipment can be maintained without transferring the load to bypass mode, which would expose the load to unconditioned power. Many distributed redundant designs do not have a maintenance bypass circuit.



  • Relatively high cost solution due to the extensive use of switchgear compared to previous configurations
  • Design relies on the proper operation of the STS equipment which represents single points of failure and complex failure modes
  • Complex configuration; In large installations that have many UPS modules and many static transfer switches and PDUs, it can become a management challenge to keep systems evenly loaded and know which systems are feeding which loads.
  • Unexpected operating modes: the system has many operating modes and many possible transitions between them. It is difficult to test all of these modes under anticipated and fault conditions to verify the proper operation of the control strategy and of the fault clearing devices.
  • UPS inefficiencies exist due to less than full load normal operation






5- System plus System Redundant





System plus System, Multiple Parallel Bus, Double-Ended, 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N are all nomenclatures that refer to variations of this configuration.

With this design, it now becomes possible to create UPS systems that may never require the load to be transferred to the utility power source. These systems can be designed to wring out every conceivable single point of failure. However, the more single points of failure that are eliminated, the more expensive this design will cost to implement.

Most large system plus system installations are located in standalone, specially designed buildings. It is not uncommon for the infrastructure support spaces (UPS, battery, cooling, generator, utility, and electrical distribution rooms) to be equal in size to the data center equipment space.

This is the most reliable, and most expensive, design in the industry. It can be very simple or very complex depending on the engineer’s vision and the requirements of the owner. Although a name has been given to this configuration, the details of the design can vary greatly and this, again, is in the vision and knowledge of the design engineer responsible for the job.


Fig.4- System plus System Redundant

The 2(N+1) variation of this configuration, as illustrated in Fig.4, revolves around the duplication of parallel redundant UPS systems. Optimally these UPS systems would be fed from separate switchboards, and even from separate utility services and possibly separate generator systems. The extreme cost of building this type of facility has been justified by the importance of what is happening within the walls of the data center and the cost of downtime to operations. Many of the world’s largest organizations have chosen this configuration to protect their critical load.


The cost of this configuration is affected by how “deep and wide” the design engineer deems is necessary to take the system duplication efforts to meet the needs of the client. The fundamental concept behind this configuration requires that each piece of electrical equipment can fail or be turned off manually without requiring that the critical load be transferred to utility power. Common in 2(N+1) design are bypass circuits that will allow sections of the system to be shut down and bypassed to an alternate source that will maintain the redundant integrity of the installation.


An example of this can be seen in Fig.4: the tie circuit between the UPS input panelboards will allow one of the utility service entrances to be shut down without requiring one of the UPS systems to be shut down. In a 2(N+1) design, a single UPS module failure will simply result in that UPS module being removed from the circuit, and its parallel modules assuming additional load.


In this example illustrated in Fig.4, the critical load is 300 kW, therefore the design requires that four 300 kW UPS modules be provided, two each on two separate parallel buses. Each bus feeds the necessary distribution to feed two separate paths directly to the dual-corded loads. The single-corded load, illustrated in Figure 6, shows how a transfer switch can bring redundancy close to the load. However, Tier IV power architectures require that all loads to be dual-corded.


Companies that choose system plus system configurations are generally more concerned about high availability then the cost of achieving it. These companies also have a large percentage of dual-corded loads. In addition to the factors discussed in the distributed redundant section, other factors that drive this design configuration are as follows:


1- Hardening:

Designing a system, and a building, that is immune to the ravages of nature, and is immune to the types of cascading failures that can occur in electrical systems. The ability to isolate and contain a failure; for example, the two UPS systems would not reside in the same room, and the batteries would not be in the same room with the UPS modules. Circuit breaker coordination becomes a critical component of these designs. Proper Circuit breaker coordination can prevent short circuits from affecting large portions of the building.

Hardening a building can also mean making it more immune to events such as hurricanes, tornadoes, and floods, as might be necessary depending on where the building is. For example designing the buildings away from 100 year flood plains, avoiding flight paths overhead, specifying thick walls and no windows all help to create this immunity.

2- Static Transfer Switch (STS):

With the advent of dual-cord capable IT equipment, these devices along with their undesirable failure modes can be eliminated with a significant increase in system availability.


3- Single-Corded Loads:

To take full advantage of the redundancy benefits of system plus system designs, single-corded loads should be connected to transfer switches at the rack level.



  • Two separate power paths allows for no single points of failure; Very fault tolerant
  • The configuration offers complete redundancy from the service entrance all the way to the critical loads
  • In 2(N+1) designs, UPS redundancy still exists, even during concurrent maintenance
  • UPS modules, switchgear, and other distribution equipment can be maintained without transferring the load to bypass mode, which would expose the load to unconditioned power
  • Easier to keep systems evenly loaded and know which systems are feeding which loads.



  • Highest cost solution due to the amount of redundant components
  • UPS inefficiencies exist due to less than full load normal operation
  • Typical buildings are not well suited for large highly available system plus system installations that require compartmentalizing of redundant components





Choosing the Right Configuration




Important definitions:

1- Reliability:

Evaluates a configuration’s capability to maintain conditioned power to the load.

2- Availability:

It is the estimated percentage of time that electrical power will be online and functioning properly to support the critical load.

3- Complexity:

Looks at the complexity of the configuration and the potential for single point failures.

4- Maintainability:

The system configuration must allow for concurrent maintenance of all power system components – supporting the load with part of the UPS system while other parts are being serviced.

5- Functionality:

The system configuration must be able to protect the critical load from a full range of power disturbances without transferring the critical load to external power sources, i.e. batteries or alternate power sources.

6- Tiers:

All UPS systems (and electrical distribution equipment) require regular intervals of maintenance. The availability of a system configuration is dependent on its level of immunity to equipment failure, and the inherent ability to perform normal maintenance, and routine testing while maintaining the critical load.





The considerations for selecting the appropriate configuration are:


1- Cost / Impact of downtime:

How much money is flowing through the company every minute, how long will it take to recover systems after a failure? The answer to this question will help drive a budget discussion. If the answer is $10,000,000 / minute versus $1,000,000 / hour the discussion will be different.

As the configuration goes higher on the scale of availability, the cost also increases. Table-1 shows the scale of availability and cost for different UPS Configurations.



2- Risk Tolerance:

Companies that have not experienced a major failure are typically more risk tolerant than companies that have not. Smart companies will learn from what companies in their industry are doing. This is called “Benchmarking” and it can be done in many ways.

The more risk intolerant a company is, the more internal drive their will be to have more reliable operations, and disaster recovery capabilities.

3- Availability requirements:

How much downtime can the company withstand in a typical year?

If the answer is none, then a high availability design should be in the budget. However, if the business can shut down every night after 10 PM, and on most weekends, then the UPS configuration wouldn’t need to go far beyond a parallel redundant design. Every UPS will, at some point, need maintenance, and UPS systems do fail periodically, and somewhat unpredictably. The less time that can be found in a yearly schedule to allow for maintenance the more a system needs the elements of a redundant design.

4- Types of loads (single vs. dual-corded):

Dual-corded loads provide a real opportunity for a design to leverage a redundant capability, but the system plus system design concept was created before dual-corded equipment existed. The computer manufacturing industry was definitely listening to their clients when they started making dual-corded loads. The nature of loads within the data center will help guide a design effort, but are much less a driving force than the issues stated above.

5- Budget:

The cost of implementing a 2(N+1) design is significantly more, in every respect, than a capacity design, a parallel redundant design, or even a distributed redundant. As an example of the cost difference in a large data center, a 2(N+1) design may require thirty 800 kW modules (five modules per parallel bus; six parallel busses). A distributed redundant design for this same facility requires only eighteen 800 kW modules, a huge cost savings.

The flowchart illustrated in Fig.5 is a useful starting point for selecting the right UPS system design configuration for a particular application. By following the questions in the flowchart, the appropriate system can be identified.



Fig.5- flowchart for selecting the right UPS system design configuration


For designs with no or little redundancy of components, periods of downtime for maintenance should be expected. If this downtime is unacceptable, then a design that allows for concurrent maintenance should be selected.



In the next Article, I will continue explaining other Classifications of UPS Systems like:

7- Topology,

8- Distribution Architecture,

9- Use of transformers.

So, please keep following.



Subject Of Pervious Article


Applicable Standards for UPS Systems

  • What is a UPS?
  • Why do we need a UPS?
  • UPS Rating
  • Classification of UPS:

  1. Voltage range,
  2. No. of phases,
  3. Mobility,
  4. Technological design,


Classification and Types of UPS – Part One

5- Physical Size/capacity,

6- Form factor/ configurations:

6.1- “N” System Configuration

Classification and Types of UPS – Part Two

6.2- “N+1” System Configuration, which includes:

  • Isolated Redundant Configuration (N +1)
  • Parallel Redundant Configuration (1+1)
  • Parallel Redundant Configuration (N +1)
  • Parallel Redundant Configuration (N +2) and so on

6.3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1)


Classification and Types of UPS – Part Three



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