Understanding the HTM: Modular UPS

Power Control
24 May 2021

When installing a tertiary power system, there are numerous considerations and a myriad of decisions to be made. These decisions are of utmost importance and could be the difference between an electrical infrastructure that can support all the functions of a healthcare facility and one that buckles in the face of a power failure. After exploring and understanding the power requirement, the next decision to be made is the UPS topology.  

Traditionally tower UPS (also known as monolithic, static or single UPS) are used in either a single (N) arrangement or parallel configuration. Single arrangements provide the most cost effective arrangement while paralleling 2 tower systems in an N+1 or N+N arrangement provides greater resilience.  

As part of the 2017 update of the HTM 06-01, section 11.58 includes guidance on how modular UPS can now be integrated into the design. In this blog, we’ll look at both advantages and disadvantages of installing tower and modular topologies into healthcare estates.  

Modular UPS Overview 

When it comes to UPS investment, modular UPS solutions offer a flexible and scalable approach with reduced operating costs and easier overall maintenance. UPS engineering works can be quickly undertaken due to the hot swappable modules within the frame, allowing them to be removed or replaced without disturbance to the load. This in turn can lead to a more reliable solution over the lifespan.   

Great consideration should be given to the rationale behind choosing a modular UPS architecture over other topologies before the decision is made. For example, some key questions to consider are: 

  • Do I require redundancy within the frame or within the infrastructure? 
  • If in the frame, how many modules are required to cover the design load N (this could be theatre sockets, IPS loads etc)? 
  • How many redundant modules are required?  
  • If redundancy is required within the infrastructure (switchgear, dual feed load, etc.) then would a tower, or parallel tower solution be more suitable? 

Understanding these key points will ensure the design, consultancy and product procurement process runs smoothly.  

Modular Redundancy 

UPS redundancy is central to healthcare installations as it provides resilience. Various methods of achieving resilience are discussed in section 11 of the HTM 06-01 and modular UPS is a topology that satisfies the needs of those specific requirements. Modular UPS provides redundancy within the UPS frame using integrated multiple power modules. Additional modules can be installed to increase capacity from the day one amount, to provide the required power rating (N) and additional redundancy (+1). The initial N can be made up of one or more modules.  

For example, 100kW can be built using four 25kW modules and a fifth added to provide redundancy, hence N+1. However, as well as providing 100kW N+1, the five 25kW modules may be set up to supply 125kW total load.  

In addition to the power modules being redundant, other critical parts of the UPS can also be modular. For example, the static switch, communication and driver cards, PSU cards, etc. This provides additional built-in resilience when compared with a tower system. 

Modular UPS Have A Reduced MTTR (Mean Time To Repair) 

The Mean Time to Repair is reduced due to the power modules and other components being modular. The fault finding process is also quicker as faults are quickly identified and isolated to a single module. Additionally, the physical time spent on sit can be reduced because in many cases the modules are hot swappable meaning the test and repair time required is reduced.  

In comparison, a traditional tower UPS requires full isolation from the load by locking off in bypass to carry out an in depth and sometimes time consuming fault finding process. Once on bypass, components and boards then need testing, repaired or swapped over. Testing of the new components will still be required with the load in bypass before transferring back to UPS support.   

Spare parts on site will also be easier to manage as parts will be complete modules rather than having to stock multiple components and PCBs. This provides peace of mind for the hospital with easier and quicker repair times, thus reducing downtimes of theatres.  

Smaller Footprint 

Modular UPS systems can be constructed within a smaller footprint than a traditional tower UPS. The power density and architecture mean the modular solution can reduce the footprint of the UPS by up to 50% with a reduced requirement for ventilation and maintenance access over an equivalent tower UPS. 


Modular UPS are available as either vertical or cross scalable configurations. However, within the HTM, UPS are specified to support the design load without the need for future expansion. This doesn’t mean it can’t be allowed for, but in most cases, the infrastructure is designed for a day one maximum potential load. 


As well as the topology providing internal redundancy, modular UPS by the nature of their construction uses mass-produced components in comparison to tower systems that are built in smaller numbers due to the array of power ranges requires. The smaller number of interchangeable modular components provides greater flexibility of power ranges and spare parts for multiple UPS systems.  

Although there are numerous benefits of using modular UPS systems, there are further considerations that should be given before concluding that a modular UPS design is the most suitable architecture. 


Although the TCO of a modular UPS may be lower for a modular solution, the initial outlay costs of installing a modular UPS are more expensive than tower UPS. The increased number of mechanical and power components adds to the complexity of design and construction which has a knock-on effect on the overall cost. Replacement parts can also be more expensive when full modules need replacing in comparison to individual parts in a tower system. 

Do Modular UPS provide true N+1 redundancy? 

A modular UPS provides redundancy within the frame. However, this will usually mean there is still single points of failure. These will be present in the input supply, input fuses, cable terminal and infrastructure supporting a single modular frame.  

To achieve true N+1 redundancy matching that of a tower system, multiple frames would need to be connected in parallel. Like a tower system, modular UPS can have additional dual input supplies or multiple battery strings to reduce points of failure. However, a single tower (modular or not) will not provide the level of resilience of multiple towers.  

When considering N+1 configuration, thought should be given to the whereabouts of the required redundancy in the design.  

N+1 could be: 

  • within the load, for example, dual fed sockets in an operating theatre 
  • within the switchgear or IPS (Automatic Transfer Switches, ATS) 
  • Within the infrastructure  
  • Bypass arrangement  
  • UPS 

Depending on these and other factors, a modular UPS may be more advantageous than a paralleled tower system or vice versa.  


Most decisions to install modular UPS are driven by the amount of redundancy achievable or the requirement of future expansion – which is why the maximum frame capacity is most likely larger than the initial design load (N). The ultimate question is, should the electrical infrastructure be designed and sized to the design load or the maximum UPS frame capacity? 

Unlike modular UPS which are only usually available in 2 to 3 frame sizes of 100kW, 200kW and 300kW, tower UPS are available in conveniently stepped power ratings such as, 100kW, 125kW, 150kW, 200kW, 300kW and so on. When sizing a modular UPS, the actual required capacity may be different from the frame capacity, i.e., 100kW N+1 modular (five 25kW modules) could require a 200kW frame.  

In most cases, the electrical infrastructure should be designed around the load requirement and distribution. Within a hospital, this could be the number of theatres or isolated power systems (IPS). Doing this ensures the UPS is then designed to support the load demand rather than the electrical infrastructure designed to cover the UPS maximum capacity. 

Oversizing the infrastructure to support the maximum frame capacity is sometimes preferred. Although, in this case, consideration should be given to the fault clearance capacity of the modular UPS. The fault clearance depends on the day one UPS capacity and type of static bypass switch. For example, a centralised modular UPS uses a single static switch sized at the frame capacity. Even though this is a single point of failure it will have greater fault clearance capacity than a decentralised system where the static switches are housed within the individual power modules.  

Consultation is required to ensure the UPS is specified correctly for the required clearance. However, as the modules are housed in one casing with single fuses, breakers and cable arrangement achieving true N+1 redundancy with a modular UPS may not be as simple or cost effective as using a Tower UPS.