Understanding the HTM: UPS design requirements

Power Control
28 Apr 2021

Rapidly advancing healthcare technology is transforming the way patients are being diagnosed and treated. From sensitive medical imaging equipment to connected devices, data monitoring and lab tests, the future of healthcare lies in working hand-in–hand with technology and with that, comes a greater need for 24/7 regulated power.  

Without the convergence of a secondary and tertiary backup power supply – generator and UPS system, this level of power continuity would not be possible. After determining the required medical groups and locations needing tertiary UPS backup, consideration must then be focussed on the design and resilience of the backup systems. 

UPS Ratings 

UPS systems must not only be rated to support the design load but thought should also be given to the type of load and fault clearance design. 

Design loads may have a nominal rating, however, some medical loads such as an MRI, PET, CAT and X-Ray can all present a peak load during capture or image acquisition. Important consultation is required between the UPS and equipment design teams to ensure high peak inrush currents and line impedances are met by the UPS and infrastructure design.  

Additionally, section 11.43 of the HTM references that UPS should be capable of clearing downstream circuit faults similarly to other distribution boards. This means designers should consider the effect of overloads and short circuit fault conditions.  

Short circuits in the UPS load are isolated either by a downstream protective device, via the UPS static switch, or by the UPS inverter. Following BS 7671, where a UPS is used as a ‘life safety system’, it must clear the protective devices when on battery power.  

When the mains power is unavailable, the UPS inverter output has limited overload capabilities at 120%. Some UPS will allow a maximum of 150% and up to 200% for a short period. 

With this in mind, the UPS inverter electronics are more sensitive than the mains or static switch so careful calculations are required to ensure the inverter can clear protective devices. Actual overload characteristics vary from UPS to UPS so designers should verify the coordination of fault conditions when selecting the UPS type. 

Redundancy and Resilience 

Monolithic (standalone) UPS systems can be configured using a single unit (N) or multiple units working concurrently in parallel (N+1) to provide varying levels of redundancy and mitigate single points of failure. Modular UPS also provide redundancy and due to their modular architecture, this redundancy is achievable within the UPS system. Multiple modular units would eliminate additional points of failure but are often not cost–effective for healthcare facilities.  

If the UPS are installed as multiple units in redundant UPS arrangements, each UPS should be sized to fully support the entire load.  

Both UPS topologies can be installed into a variety of electrical design arrangements shown in section 11 of the HTM. The design configuration can be, single, N+1 and N+N, serving the critical medical locations via dual UPS output board and/or changeover devices within the Isolated Power System (IPS) or distribution boards.  

The configuration should be selected based on the application it’s supporting and the medical locations. In many cases, it may be desirable to have redundancy within the UPS configuration, having distribution configuration via changeover switches, or having dual feeds into the medical location. 

The HTM examples are for guidance only and state that the designer should demonstrate that any solution provided is the optimum solution to maintain both patient safety and their associated systems where the loss of power will affect the normal operation of the healthcare facility. 

Consultation is therefore needed between the designers and UPS specialists at early stages.  


The configuration of battery strings in a UPS installation is critical to eliminating single points of failure. Batteries can be arranged in a single string or dual parallel strings. For simplicity, and to keep costs to a minimum, UPS battery subsystems are typically configured as single strings. However, during a mains failure, this leaves the system vulnerable due to multiple single points of failure.  

In line with section 11.9 of the HTM, the use of parallel or split battery banks should instead be specified. This configuration allows the UPS to remain online even while half the battery system is being serviced.  

Read our blog on Understanding the HTM: Section 11.4 UPS Batteries, for more information on battery configuration for healthcare facilities.  

Transformer Arrangements 

Advice in the HTM stipulates that designers may wish to consider the use of zigzag transformers on the input, bypass line and output. In most cases, this is to provide a grounded neutral supply for the UPS and load. Additionally, such transformers provide a local earth and installation point. This provides a grounded neutral and adequate grounding for fault currents. 

Under BS 7671 where a UPS is used as an electrical source for life safety systems, it should be able to operate distribution circuit protective devices.   

Due to the added complexity, cost and space requirements, consultation between the designers and UPS specialists is required at the early stages of a project.  

More can be found on transformer and neutral switching arrangements in our Understanding the HTM: Section 11.45 Isolation in a UPS system blog. 

Dual UPS Supplies 

UPS can be fed from a single mains supply. This single supply is usually from the LV (low voltage) distribution board within a hospital. This LV distribution may be supplied from the hospital’s primary electrical and secondary power supply from either an alternative energy plant, combined heat and power (CHP) or standby power system like a diesel genset. 

Additionally, many UPS can be fed from a dual supply. Both feeds can come from the same LV distribution arrangement but via separate circuits or separate LV sources.  

Firstly, the advantage of dual supplies and the UPS selects between them. When a UPS is supplied from a single supply inside the UPS, it’s split into one that feeds the rectifier (charger) and the other feeding the bypass line. When the UPS transfers to its internal bypass line (manually or failsafe) the load is fed from the mains supply via the UPS internal bypass, not from the UPS output inverter. 

With a dual supply, the UPS bypass line can be fed from the secondary supply. The advantage of this is that once the UPS has depleted its batteries, it won’t switch off. Instead, it has the potential to transfer the load to the service line if this is available. Therefore, if a main supply protective device fails or trips, the UPS will have the better source and reverse supply.  

Secondly, if the UPS faults and trips the incoming protective device, the load can be automatically transferred to the reserve supply with minimal interruption.  

Careful thought is needed if supplies are fed via separate LV sources. Why? Because if the hospital has two primary supplies from separate building transformers, then each supply has its own earthing and neutral arrangements. In most cases, it’s critical for the designer to keep the two sources separate and not to bond or create multiple neutral paths.  

The problem is that while many UPS can take dual inputs, there’s only one UPS output, resulting in most UPS systems internally bonding both neutrals together. While this UPS type is generally OK with dual supplies from the same source, they’re not compatible with sources from separate neutrals because they connect both paths. 

For separate sourced dual supplies, compatible UPS are required that won’t bond the neutral internally. These UPS usually have an internal inverter transformer. Whilst taking dual supplies, they will use a single point as a reference and therefore provide further resilience without affecting the electrical earth and neutral design arrangements.  

Consultation between the designers and UPS specialist is again required at the early stages of a project.   

Power Control has been specifying, installing, and maintaining tertiary power systems in healthcare estates for over 27 years. Every UPS system, both monolithic and modular, suitable for healthcare applications are available with either VRLA or lithium-ion batteries and meet all relevant medical regulations including the IEC 60601, HTM 06-01, BS 7671 and BS 6290-4. Contact us to find out more, 01246 431 431, power@powercontrol.co.uk