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The Merits And Flaws Of Modular Uninterruptible Power Supply (UPS) Systems
Modular UPS systems are lately gaining momentum due to an attractive concept of answering power protection needs as you grow. Acieved by increasing UPS capacity or redundancy when needed, by means of small, lightweight, compact, hot swappable, low cost modules, without allocating additional floor space.
The modular UPS approach has the following advantages over Standard UPS solutions: 
1.Lowest floor space.
Most current Three Phase UPS systems have scaling ability, which enables to add additional units in parallel for additional power or redundancy. Modular systems are based on a rack type enclosure that includes battery cabinet at its bottom, and small light weight swappable modules located one above the other, which can be added whenever needed. This vertical scaling approach takes about 25% of floor space, compared to standard UPS units with the same total power capacity.
2. Highest Power Availability.
Availability of UPS system is defined as the percentage of the time that conditioned power is available. If a UPS would never fail, its availability would be 100%. Unfortunately UPS systems or its backup batteries do fail, and during repair time no conditioned power is available to protect the load. The modular approach enables to decrease the repair time to a few hours needed for a technician to arrive and swap the faulty module, compared generally to 24 hours or more for a service engineer to arrive and fix the system.  
3. Higher efficiencies.
UPS efficiency reaches its maximum level when the load is at its maximum rating. The ability to increment UPS power by adding small modules when needed, while maintaining high ratio of total load to total UPS capacity, renders the most efficient solution.  
4. Lowest probability of UPS failure during maintenance. 
Reviews indicate that 30% of UPS failures are caused by human errors during UPS repair or maintenance. The modular approach enables to replace and isolate on site the faulty module, which is sent to the service center for repair, diminishing the probability of UPS failure during service.
The modular UPS approach has the following disadvantages over Standard UPS solutions:
1. Limited Output power. 
A modular approach calls for relatively high quantity of small, light weight parallel power modules. Most UPS systems are based on 10kVA or 20kVA modules, which can be handled by a single person, located in a 50 kVA or 100kVA Rack. For additional power, manufacturers recommend to add additional Racks in parallel. The probability of failure when more modules are connected reduces the Availability, advantage. In addition, the nuisance and high maintenance costs will also increase when number of module goes up, making the modular approach unpractical for systems exceeding 100kVA or 4 to 5 modules in parallel.  
2. High Single Point of Failure Probability
Unlike conventional stand alone UPS systems, each equipped with all the needed UPS function blocks, a single failure in certain functional components, if common to all modules in the rack, such as common Battery bank, Transfer switch, Control and Display units, may cause total output failure.  
3. Higher Initial Investment.
Generally, installation of a modular UPS includes the Rack and all common components increasing initial price. This investment may however be compensated in future due to higher efficiency and reduced running costs, as well as due to reduced financing costs associated  with the Grow as you Go approach. 
In conclusion, Modular UPS approach isn't always the best solution to all scalable UPS needs, but it should be thought over as a part of a general UPS strategy. In addition, in order to see the entire picture, selection of a modular UPS requires thorough knowledge of the particular UPS design including detailed information about common items, not always provided by UPS manufacturers.