Achieving power redundancy - UPS configurations explained

Power Control
22 Jan 2020

Over the years UPS designers have come up with ways to harness maximum power capacity with redundancy in a finite space. This is achieved by scaling the number of UPS systems in a configuration that sustains a load amount whilst reducing the number of single points of failure, preferably to zero. With that said, a configuration with zero points of failure is not always achievable or necessary and the customers’ budget, existing infrastructure, risk tolerance and level of criticality must also be considered.

In addition, redundancy can also help businesses expand without further strain to existing electrical infrastructure. Any business with future growth plans should consider installing a capacity (N) configuration.

Over the years, multiple design configuration ideas have taken on varying naming conventions, making it somewhat confusing for end-users to compare like for like redundancy concepts from different suppliers. For simplicity, there is an unwritten rule of the 5 principle redundancy configurations all stemming from the common nomenclature of ‘N’.

So, to further explain each of the 5 configurations, the concept of ‘N’ must be understood.


‘N’ can simply define the redundancy of a given system. It identifies the need of the critical load; the minimum requirement for the system to operate.

Also referred to as ‘power parallel’, It comprises a single standalone UPS module or a paralleled set of modules with a matched capacity to the anticipated critical load. One small single phase UPS system in a home office is an ‘N’ configuration. Likewise, two 400kW UPS units in parallel supporting an 800kW load would also be ‘N’.

Installing an ‘N’ configuration will keep initial costs to a minimum, however, consistently running a UPS at 100% load does not allow for any changes in mains power. This could be a load inrush surge, a common occurrence when multiple applications are booted up at the same time, nor does it allow for future business expansion.


An isolated redundant configuration is sometimes referred to as an N+1 system, however, it is considerably different from a parallel redundant configuration, which is also referred to as N+1. The main reason being that the load is not equally shared between UPS systems when placed in an isolated configuration.

Isolated Redundant (N+1): When placed in a parallel configuration (N+1) both units must be capable of supporting the full load for the full required autonomy in order to have a truly redundant system. The load is supported by a single UPS (UPS 1), the second UPS (UPS 2) would support the load in the event of UPS 1 failing. N can be used to either represent the load or the initial UPS value with the following number being the number of redundant UPS Systems connected in parallel with the first.

In this instance, the parallel bypass switch is rated for the capacity of 1 UPS system but provides enough UPS Input/UPS Output feeds for the total number of UPS connected in parallel. All UPS have a limitation as to how many systems can be connected.

Parallel Redundant (N+1) As it is not advised to consistently run a UPS at over 50% load capacity, a parallel redundant, or ‘N+1’, configuration consists of one UPS (‘N’) sharing the critical load evenly with another UPS system (‘+1’). Both UPS systems are either part of a common output bus meaning they are synchronised with one another or they have a function embedded within the module itself. The number of UPS systems that can be paralleled into a common bus is often left to the discretion of the UPS manufacturer. It is important to note, however, that the UPS systems used in a parallel redundant configuration must be the same model and capacity, and from the same manufacturer.

Arguably, when a supplier talks about N+1 they are more likely to be referring to the more widely used, parallel configuration. Whether it be two UPS systems in parallel or ten, the increased fault clearing capabilities of a parallel redundant configuration ensures that short circuits are cleared twice as efficiently without having to transfer the load to the bypass, avoiding unnecessary switching or tripping of supply switchgear.

The simplicity of the design and the ease of expansion allows for uncomplicated maintenance and makes it a cost-effective and efficient total cost of ownership (TCO) UPS solution.


Distributed Redundant System (2N): Before the 1990s there was no affordable way of achieving complete redundancy. That was until an engineering firm designed a distributed redundant configuration (2N) using three or more UPS systems within independent input and output feeders to provide redundant power paths to the critical load. As the load requirement (N) increases, so does the quantity of UPS systems. This configuration is often used for large, complex data centre installations. These systems can be designed in such a way that every conceivable single point of failure is eradicated, however, this comes at a  higher price. The more single points of failure eliminated, the higher the cost of implementation.

System + System 2(N+1): With increased reliability comes increased cost, a system + system configuration is the most reliable and the most expensive design in the industry. Depending on the requirements of the end user, the system can be very simple or incredibly complex. The fundamental concept behind the reliability of this configuration is that every piece of electrical equipment can fail without the need to transfer to utility power.

There are many variables affecting the availability or uptime of a system, for example; human error, reliability of components, maintenance schedules and recovery time. The impact that each of these variables has on the overall availability is determined by the configuration chosen. When choosing which design configuration is best suited to the successful operation of a facilities equipment, the advantages and limitations, as well as the suitability of size and scale should be considered. By thoroughly understanding the facilities budget, load requirement and capacity requirement, an informed decision can be made.

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