POWER SUPPLY
Harmonic distortion and the impact on critical care
Across the UK and internationally, a recurring requirement is appearing within critical medical equipment specifications: a maximum supply of 3 per cent voltage total harmonic distortion (THDv). This limit is consistently referenced in advanced MRI systems, radiotherapy linear accelerators, digital imaging platforms, and other precision clinical technologies. Understanding this limit – and its implications – is vital for safe, efficient equipment operation. Here, John Mitchell, global sales and marketing director at CP Automation, and independent power management consultant Steve Young MIET, explain why, in healthcare and medical facilities, 3 per cent THDv matters.
At first glance, 3 per cent may appear conservative. EN 50160 permits voltage THDv up to 8 per cent under defined public network conditions, and UK planning levels set out in Engineering Recommendation G5/5 typically allow around 5 per cent at low voltage. However, these are compatibility limits for public supply networks, not performance guarantees for sensitive clinical equipment. It may sound obvious, but there is a fundamental
difference between what a distribution network is permitted to deliver and what precision medical systems require to operate safely, accurately, and reliably. In critical healthcare environments, that difference is increasingly centred on THDv limits.
Where the 3 per cent is required The 3 per cent THDv limit is not an isolated recommendation. It is repeatedly specified within installation guidance and site preparation documentation from major global manufacturers of medical equipment. MRI systems supplied by organisations such as Siemens
Healthineers, GE HealthCare, and Philips Healthcare often reference a maximum total harmonic voltage distortion of 3 per cent at the equipment terminals. These systems rely upon highly stable gradient amplifiers, radio frequency drive chains, and precision timing circuits. Even marginal exceedance of the 3 per cent threshold can introduce image artefacts, background noise, banding, or calibration drift.
Similarly, radiotherapy linear accelerators supplied
by Varian and Elekta reference a 3 per cent THDv requirement. These platforms depend on stable high- voltage modulation systems and beam control electronics. Distortion beyond recommended limits has been associated with nuisance interlocks, sporadic beam hold events, and treatment interruptions. Advanced digital radiography and fluoroscopy systems, cardiac catheterisation laboratories, hybrid theatre imaging suites, and robotic surgical platforms have also demonstrated sensitivity to waveform distortion. In these environments, waveform integrity directly influences image clarity, dose accuracy, and control system stability. The repeated appearance of a 3 per cent THDv limit
across independent manufacturers and technologies is not coincidental. It reflects a shared engineering reality: modern medical platforms are converter-dense, microprocessor-controlled, and electrically sensitive.
Why marginal exceedance matters The most important point about the 3 per cent threshold is not simply that it exists, but what happens when supply conditions move from compliant to marginally non- compliant. In precision-driven medical facilities, this distinction can materially affect equipment performance. Modern medical equipment incorporates switched- mode power supplies, digital signal processors, FPGA-based control systems, and precision analogue
Medical equipment in a hospital.
April 2026 Health Estate Journal 63
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