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What to look for when replacing an uninterruptible power supply



David Bond, chairman, Centiel UK


A UPS will probably be installed and running for 10 or more years so there is more than just purchase price to consider when buying a new one. These other, arguably more important considerations to best ensure business continuity, are discussed below. 


Speed of Repair

By their very nature, most UPS can be considered reliable and are certainly more reliable than the raw mains. High-reliability is often considered to be the most important attribute for any UPS solution, and to an extent, this is fair. However, it’s maximum availability that is the most important. This is because reliability is a measure of how often something will fail while availability is a measure of how often something is available to work. They sound similar but are two very different things. Consider this:

Availability = MTBF / (MTBF + MTTR)

Where reliability is expressed as Mean Time Between Failure (MTBF) and the speed of repair is expressed as Mean Time To Repair (MTTR).

From the above equation it is clear that for any given MTBF the lower the MTTR (ie the faster the repair time) the closer the system availability gets to the desired 100%. So, speed of repair should be a key purchase consideration.


Almost all non-modular UPS need to be switched to manual bypass while being maintained or repaired. This places the critical load at risk of raw mains power related problems because the UPS is not working.

Correctly sized modular UPS can have each individual module maintained without the need to manually bypass the whole UPS system. ‘Safe swap’ UPS modules are an upgraded version of hot swap modules that eliminate human error and make the removal and insertion of UPS modules safe to both the operator and the critical load. 

The latest modular UPS is fitted with ‘safe swap’ modules that can be repaired by module replacement, without risk to the critical load, in less than 3 minutes. The typical repair time for a non-modular UPS is just under 6 hours. This is why availability of 9 nines (99.9999999%) is achieved by the latest three-phase modular UPS systems. 

Total Cost of Ownership (TCO):


Most UPS that are installed are oversized for the load they supply. This is because the existing load is known but the future load may not be, so an oversized one is fitted ‘just in case’. The result, a UPS that is too big and costs more to purchase, maintain and operate. 

The scalable power nature of modular UPS allows you to ‘pay as you grow’ by installing a UPS frame fitted with only the number of UPS modules needed to support the existing load. If the load increases in the future another UPS module can be quickly and easily added.


A UPS will be operating 24 hours/day, 365 days/year for all its working life. If it is inefficient it will be wasting expensive power and creating heat that must be cooled. To put this into context, if a 100kW UPS has an efficiency that is only 1% worse than a more efficient system it is wasting 1kW every hour. At 50.897p/kWH (accurate at the time of writing) this UPS is wasting £4,458p/a or £44,585 over its typical 10-year useful working life.

The latest generation of modular UPS are over 97% efficient and, because of their scalability, can be sized to ensure that they operate at an optimum point on the efficiency curve. It may cost a little more to install a scalable, highly-efficient modular UPS, but the ongoing OPEX savings will far outweigh any savings at the purchase price stage.  

In recent years three-phase modular UPS systems have revolutionised the power protection industry because, when properly configured, they simultaneously maximise load availability and system efficiency. However, not all modular UPS are true modular UPS that provide the availability, maintainability, scalability and efficiency benefits discussed above, so beware.

If a UPS is replaced with a modular UPS that has:

  • ‘9 nines’ availability (99.9999999%) because its MTTR is less than 3 minutes
  • the lowest TCO because it is 97% efficient and incorporates long life components that don’t need regular replacement  
  • the ability to grow as the critical load grows so it is not outgrown
  • maintainability designed in so it can be fully maintained without switching off

Then it could probably be considered a very good UPS replacement. 

At Centiel, our design team has been working with data centres for many years at the forefront of technological development.  We are the trusted advisors to some of the world’s leading institutions and our modular UPS product range only contains true modular UPS. Our CumulusPower three-phase modular UPS offers the industry-leading availability of 99.9999999% (9 nines), provides the lowest total cost of ownership (TCO) because of its very high level of operating efficiency and low maintenance costs, and is available in a single cabinet of 900kW capacity.  


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