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.
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.
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