POWER SUPPLY
EN 50160 defines the characteristics of voltage supplied by public distribution systems under normal operating conditions. Engineering Recommendation G5/5 provides planning levels intended to prevent excessive harmonic propagation within networks. Neither document guarantees that a particular medical device will perform optimally at those limits. A hospital may be fully compliant with public
A heart rate monitor.
measurement stages. These elements assume a relatively pure sinusoidal voltage waveform. When harmonic voltage distortion increases, subtle but consequential behaviours can emerge.
Image artefacts or banding may appear in MRI or
CT systems, calibration drift may occur intermittently, unexplained background noise can develop in imaging chains, and sporadic beam interlocks may arise in radiotherapy. Control system resets are often initially attributed to software rather than electrical conditions, while elevated thermal stress can develop within power electronic assemblies. The earliest warning signs are rarely catastrophic failures. Instead, they are subtle indicators that clinical systems are operating outside optimal electrical conditions. In imaging environments, this may present as intermittent artefacts, unexplained background noise, minor calibration drift, or nuisance interlocks during peak demand. In radiotherapy environments, it can manifest as
sporadic beam hold events or unexplained control resets that are initially attributed to software or operational factors rather than supply quality. From an engineering perspective, rising background
THDv, increasing neutral currents, transformer temperature elevation, voltage unbalance, or unexplained reactive power variation frequently precede visible equipment alarms. If waveform quality is not permanently monitored, these electrical precursors can remain invisible.
Public network compliance vs equipment performance
A common misconception in project delivery is that compliance with public network standards ensures compatibility with all connected equipment. This is rarely the case.
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.
64 Health Estate Journal April 2026
Harmonic distortion in hospitals Clinical technology is becoming increasingly converter- dependent and electrically dense. Non-linear loads are widespread and include variable speed drives in HVAC systems, chiller compressors and air handling plant, UPS systems supporting clinical and IT infrastructure, LED lighting, and switched-mode power supplies in IT and medical equipment. Each of these contributes harmonic noise to the network. Voltage distortion at a given point is determined by
the interaction between harmonic current and system impedance. As background harmonic currents increase, voltage THDv can rise accordingly. It is important to note that distortion fluctuates throughout the day. In a modern acute hospital, variation of 1 to 1.5 per cent between low demand and peak operational periods is not uncommon. Imaging schedules, theatre utilisation, HVAC cycling, and generator operation all influence harmonic behaviour.
Why periodic testing is not enough Traditional power quality assessment typically involves monitoring temporarily during commissioning or in response to a problem. While usual, this approach can only capture a snapshot – a defined time window – and may not coincide with peak distortion conditions. As a result, it cannot reliably identify gradual upward trends, nor can it provide continuous assurance that a 3 per cent THDv threshold is being respected under all modes of operation.
In critical clinical environments, reactive investigation following equipment malfunction is no longer acceptable as a primary strategy. If waveform integrity is a design requirement, it must be managed as an ongoing engineering parameter rather than an occasional measurement exercise.
The importance of continuous oversight Permanent monitoring transforms power quality from an assumption into a measurable, time-based parameter. Continuous THDv measurement at relevant distribution points enables high-resolution trending, identification of repeatable peak distortion windows, and regression analysis to detect gradual background increases. It also allows correlation between electrical behaviour and clinical events.
network requirements while simultaneously operating sensitive equipment outside its manufacturer-defined performance envelope. This misalignment often originates during early project design and procurement, when programme pressure and capital constraints dominate decision-making. Power quality may be assumed to be acceptable by default, without direct comparison between measured supply conditions and equipment terminal requirements. When high-value imaging or treatment platforms are subsequently installed, any shortfall becomes difficult and costly to address. Retrofitted mitigation, transformer replacement, or impedance modification can introduce delay, contractual complexity, and operational disruption. Therefore, aligning infrastructure capability with equipment specification at an early stage is essential.
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