Understanding the HTM: Section 11.45 isolation in a UPS system

Power Control
13 Apr 2021

Isolation in a UPS system forms the second of a four-part series of blogs looking at section 11 of the HTM 06-01. Section 11 directly relates to UPS systems and this blog aims to provide simplicity on section 11.45, which forms a part of the segment relating to ‘UPS fault condition design.’

There are generally three types of isolation in a UPS:

  • Galvanic isolation between input and output
  • Input isolation between mains and battery
  • Isolation between the battery and UPS output

It is important to distinguish between these types of isolation to correctly specify or interpret the requirements especially regarding the HTM 06-01 section 11.45.

Galvanic Isolation

It’s a common misconception that transformer-based UPS also provides galvanic isolation. In a transformer-based UPS, the transformer is used to step the voltage up from the inverter. However, the neutral line still passes through the bypass line and therefore no galvanic isolation between input and output is provided.

If full galvanic isolation is required between the mains and load then the transformer should be on the UPS input. The transformer can also go on the UPS output, however, this will only provide galvanic isolation between UPS and load.

When considering this option, the design of UPS and the number of input supplies needs to be thought through.

Input Isolation

In the past, when open lead acid batteries were typically the battery of choice in UPS solutions, input isolation was frequently used to isolate the batteries from the mains supply for safety reasons. Now that modern UPS use sealed lead acid or maintenance free battery technology, input isolation is generally no longer required for this purpose.

However, to provide a fixed neutral point for the UP input, input isolation transformers are still a common option for large 3phase UPS systems for reasons explained in the Input Transformer section.

Output Isolation – Transformer Based UPS

As mentioned previously, in transformer based UPS, the transformer is used to step up the voltage at the output of the inverter to supply the required regulated voltage.

Additionally, the transformer isolates the DC components and hence the load is isolated from the inverter and DC circuit. Although the inverter converts DC to AC, the DC can pass through to the load due to IGBT or thyristor failure. In the event of this type of fault, a DC component will be generated. In the transformer based UPS, the inverter output transformer will isolate the DC from the load.

Another benefit of transformer based UPS comes as a side effect of the UPS design. Usually, most transformer based UPS don’t require a neural supply to the rectifier. The benefits and considerations of this will be covered later.

UPS Systems and Neutral Earthing Philosophies

The output neutral of a UPS is referenced to the input neutral, so in normal operation, the downstream supply is fully referenced. Problems can occur when the input neutral is switched or broken.

In this instant, the UPS loses its reference point. This may result in neutral to ground or line to ground over voltages during neutral switch or losses. In turn, the phase and neutral conductors could be at different potentials than normally expected or required by the electrical installation design standards.

Possible Reasons for Loss of UPS Neutral Supply

Generally, there are 4 reasons for the loss of UPS neutral supply:

  • 4 devices tripping
  • 4 pole isolation
  • Generator or secondary supply change over arrangement
  • Bad connection


To eliminate the risk of unintentional isolation, switching or isolation of supply, all protective devices supplying the UPS should be 3-pole with a fixed neutral. If the neutral needs to be disconnected to isolated for maintenance purposes, an isolator on the neutral line could be installed.

There are several solutions to prevent the neutral from being lost to the UPS. Not all are suitable for every project and they each have pros and cons:

  • Input isolation transformer
  • Neutral to earth contactor
  • Neutral to earth link

Input Transformer

As mentioned previously, input isolation transformers are an option on larger 3phase UPS systems to provide a fixed neutral point for the UPS input. Technically this is the next best solution to 3pole switching as it offers a permanent neutral reference fixed to the earth.

For example, a delta zigzag (Dzn0) isolation transformer on the UPS input will supply a fixed neutral reference to earth on the secondary UPS supply side.

This solution can be retrofitted however, the downside is the initial capital cost and space required. There could also be an implication to the overall system efficiency if the transformer supplies the UPS rectifier rather than just the bypass line supply.

Neutral to Earth Contactor

The device detects when the neutral load is on the UPS input and connects the input neutral to earth via a contactor.

This is a less expensive option than that of an input transformer and can be installed within the switchgear. However, there is a remote possibility of component failure with this option unlike when using contactor devices.

Can the neutral and earth be bonded?

Permanent Bond

A permanent bond solution is similar to the neutral to earth contactor but provides a permanent bond between the Neutral and Earth.

This can either be achieved on the UPS or locally. However, an important consideration is needed to the electrical design characteristic to avoid multiple Neutral – Earth bonds and keep in compliance with BS7671.

This is also a less expensive option than that of an input transformer or contactor.

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