Redundancy Tier Structure

Arguably there is no “best” architecture in absolute terms. Each application has often unique and specific requirements needing the involvement of a specialist to conduct a Needs Analysis and propose a solution on a case-by-case basis. Understandably, the more redundancy incorporated into your system, the more reliable it’s expected to be; however this is directly proportionate to higher costs in both initial investment and ongoing maintenance.

The Telecommunications Industry Association is a trade association accredited by ANSI (American National Standards Institute) and in 2005 it published ANSI/TIA-942, Telecommunications Infrastructure Standard for Data Centres, which defined four levels of data centers in a thorough, quantifiable manner. TIA-942:Data Center Standards Overview describes the requirements for the data centre infrastructure. The simplest is a Level 1 data centre, which is basically a server room, following basic guidelines for the installation of computer systems. The most stringent level is a Level 4 data centre, which is designed to host the most mission critical computer systems, with fully redundant subsystems, the ability to continuously operate for an indefinite period of time during primary power outages. The Uptime Institute, a data centre research and professional-services organization based in Seattle, WA defined what is commonly referred to today as “Tiers” or more accurately, the “Tier Standard”. Uptime’s Tier Standard levels describe the availability of data processing from the hardware at a location. The higher the Tier level, the greater the expected availability. Other classifications exist as well. For instance, the German Datacentre Star Audit program uses an auditing process to certify five levels of “gratification” that affect data centre criticality.(Wikipedia)

Regardless of the variations in terminology, the basis for infrastructure design is the same and adheres to the following guidelines:


Referred to as an “N system”, simply stated, this is a system comprised of a single UPS Unit, or a paralleled set of modules whose capacity is matched to the critical load requirement. This type of system is by far the most common of the configurations in the UPS industry. It’s the small UPS under an office desk or two 200kW UPS modules paralleled onto a common bus supplying a computer room load of 400kW. An N configuration can be looked at as the minimum requirement to provide protection for any critical loads.

Advantages of an “N” DESIGN
  • Cost effective
  • Easy to implement
  • Small footprint
  • Uncomplicated
Disadvantages of an “N” design
  • Limited availability in the event of a UPS failure, as the load will be transferred to bypass operation, exposing it to unprotected Mains power
  • During maintenance of the UPS, batteries or down-stream equipment the load is exposed to unprotected Mains power
  • Lack of redundancy limits the load’s protection against UPS failures
  • Has many single points of failure, which means the system is only as reliable as its weakest point

A Tier II configuration can be achieved through an Isolated Redundant or Parallel Redundant configuration.

The Isolated Redundant configuration is sometimes referred to as a “Hot Standby” system. Its design concept does not require a paralleling bus, nor does it require that the UPS units have to be of the same capacity, or even from the same manufacturer. In this configuration, there is a “primary” UPS that normally feeds the load. The “secondary” UPS feeds the static bypass of the main UPS unit. This configuration requires that the primary UPS has a separate input for the static bypass circuit. This is a way to achieve a level of redundancy for a previously non-redundant configuration without completely replacing the existing UPS. In a normal operating scenario the primary UPS will be carrying the full critical load, and the secondary UPS will be completely unloaded. Upon any event where the primary UPS load is transferred to static bypass, the secondary UPS would accept the full load of the primary UPS instantaneously. The secondary UPS has to be chosen carefully to ensure that it is capable of assuming the load this rapidly. If it is not, it may, itself, transfer to static bypass and thus defeat the additional protection provided by this configuration.

Advantages of a Hot Standby design
  • Flexible product choice, products can be mixed with any make or model
  • Provides UPS fault tolerance
  • No synchronizing needed
  • Relatively cost effective for a two-UPS system
Disadvantages of an isolated redundant design
  • Requires that both UPS’ static bypass must operate properly to supply currents in excess of the inverter’s capability
  • The secondary UPS has to be able to handle a sudden load step when the primary UPS transfers to bypass.
  • Switchgear becomes complex and costly
  • Higher operating cost due to a 0% load on the secondary UPS, which draws power to keep it running
  • A two UPS system (one primary, one secondary) requires at least one additional circuit breaker to permit choosing between the utility and the other UPS as the bypass source. This is more complex than a system with a common load bus and further increases the risk of human error.
  • Single load bus per system, a single point of failure

Parallel redundant configurations allow for the failure of a single UPS module without requiring that the critical load be transferred to the Mains utility source. The intent of any UPS is to protect the critical load from the variations and outages in the utility source. A parallel redundant configuration consists of paralleling multiple, same size UPS units onto a common output bus. The system is N+1 redundant if the “spare” amount of power is at least equal to the capacity of one system UPS; the system would be N+2 redundant if the spare power is equal to two system UPS’; and so on. Parallel redundant systems require UPS units identical in capacity and model. The output of the modules is synchronized using an external paralleling board or in some cases this function is embedded within the UPS unit itself.

Advantages of an “N+1” design
  • Higher level of availability than capacity configurations because of the extra capacity that can be utilized if one of the UPS units breaks down
  • Lower probability of failure compared to isolated redundant because there are less breakers and because UPS’ are online all the time (no step loads)
  • Expandable if the power requirement grows. It is possible to configure multiple units in the same installation
  • The hardware arrangement is conceptually simple, and cost effective
Disadvantages of an “N+1” design
  • Both UPS’ must be of the same design, same manufacturer, same rating, same technology and configuration
  • Still single points of failure upstream and downstream of the UPS system
  • The load may be exposed to unprotected power during maintenance if the service extends beyond a single UPS module, or its batteries. If service is required in the parallel board or the parallel controls or down-stream equipment, the load will be exposed to un-protected power. Lower operating efficiencies because no single unit is being utilized 100%
  • Single load bus per system, a single point of failure

Also known as a Distributed Redundant system, the basis of this design uses three or more UPS units with independent input and output feeds. The independent output buses are connected to the critical load via multiple PDUs. In some cases STS are also used in this architecture.

Advantages of a distributed redundant design
  • Allows for concurrent maintenance of all components if all loads are dual-fed
  • Cost savings versus a TIER IV design due to fewer UPS units
  • Two separate power paths from any given dual-fed load’s perspective provide redundancy from the service entrance
  • UPS units, 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.
Disadvantages of a distributed redundant design
  • 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
  • Complex configuration; in large installations that have many UPS units 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

“System plus system”, “isolated parallel”, “multiple parallel bus”, “double-ended”, “2(N+1)”, “2N+2”, “[(N+1) + (N+1)]”, and “2N” are all names 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 eliminate 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 centre equipment space, or even larger. 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. The 2(N+1) variation of this configuration, as illustrated below, 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 centre and the cost of downtime to operations. Many of the world’s largest organizations have chosen this configuration to protect their critical load.

Advantages of a system plus system design
  • Two separate power paths allow 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 units, 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.
Disadvantages of a system plus system design
  • 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
With all these options, how then does one choose the configuration that’s right for your Infrastructure?

Give our expertly trained sales team a call and we’ll conduct a needs analysis with you covering all the following considerations:

• Cost vs impact of downtime
• Budget
• Available Resources
• Facility Availability
• Types of loads
• Types of IT architecture
• Risk tolerance
• Availability performance
• Reliability performance
• Maintainability performance
• Maintainability support performance

    I'm Interested in:

    My Application is:

    Share this Post