Problèmes de qualité de l’alimentation électrique dans les établissements médicaux – Dysfonctionnement de l’électrocardiogramme
| Facilité | Hospital emergency room — location not disclosed |
| Problème | EKG machine producing unreliable diagnostic results |
| Cause première | Harmonic distortion from facility distribution system inductively coupled into EKG signal wiring routed parallel to power cord |
| Waveform observed | 60 Hz sine wave with flat-topped distortion at EKG signal output |
| Utility compliance | Fully compliant at the meter — disturbance originated entirely inside the facility |
| Solution | Re-route signal wiring away from power cord — eliminate inductive coupling path |
| Leçon clé | ~75% of PQ issues in medical facilities are wiring and grounding problems — not utility supply quality |
01 Problem Description
A hospital field engineer received a call from an emergency room nurse: an electrocardiogram (EKG) machine was producing unexpected and unreliable diagnostic results. The equipment had not been physically damaged, had not been moved, and had no recent maintenance history that would explain a sudden change in behaviour. The fault was intermittent — which immediately suggested a power system interaction rather than a component failure.
Upon inspection, the engineer noticed that the EKG signal output displayed a 60 Hz sine wave with a slightly flat-topped waveform — the characteristic signature of low-order harmonic distortion superimposed on the fundamental. A flat-topped waveform at the signal output of a medical instrument is not a trivial cosmetic anomaly: in the context of an EKG, it can distort the recorded cardiac waveform, producing results that could mislead clinical interpretation.
In most industrial settings, power quality problems cause process disruptions and equipment damage — serious, but recoverable. In a hospital emergency room, a malfunctioning diagnostic instrument can delay or misdirect life-critical clinical decisions. The engineering urgency is categorically different, even when the root cause is the same.
02 Mesures
A closer examination of the physical installation revealed the root cause: the EKG signal wires had been routed parallel to the AC power cord serving the machine. This created an inductive coupling path — the time-varying magnetic field surrounding the power conductor was inducing a voltage into the adjacent signal wires, superimposing the power system waveform (with its harmonic content) directly onto the EKG signal circuit.
This is not a failure of the medical equipment, the building wiring system, or the utility supply. The utility supply at the meter was fully compliant. The flat-topped distortion characteristic of third and fifth harmonic content from internal facility loads — lighting ballasts, HVAC motor drives, UPS systems — was present in the power distribution wiring, and simple physical proximity between signal and power conductors provided the coupling mechanism.
Test instruments used in power quality troubleshooting in medical environments should be safety-rated at CAT IV-600 V or CAT III-1000 V for measurements at the service entrance and on high-energy power circuits.[1] Instruments with recording capability, waveform display, and specialised measurements — harmonics, sags and swells, transient capture, and high-frequency noise — are essential. A single-reading voltmeter is insufficient: many medical facility PQ problems are intermittent and only reveal themselves under specific load conditions or times of day.
Approximately 75% of power quality issues in medical facilities are related to wiring and grounding problems rather than to deficiencies in the utility supply quality.[1] This statistic is counterintuitive to most hospital facilities managers, who naturally look outward to the utility when equipment malfunctions. The field evidence consistently points inward.
03 Théorie et analyse
The internal electromagnetic environment of a hospital
Medical facilities present a uniquely demanding electromagnetic environment. The loads are highly sensitive — diagnostic equipment, patient monitoring systems, imaging loads, programmable infusion pumps — and the tolerance for signal interference is far lower than in any industrial setting. À la fois, the facilities themselves generate significant internal electromagnetic disturbances:
- Motor-driven HVAC and refrigeration equipment — inrush currents, voltage notching from variable speed drives
- Electronic lighting ballasts — third harmonic injection, neutral current loading
- Uninterruptible power supplies (UPS) — harmonic currents on input side, high-frequency switching on output side
- Medical imaging equipment (MRI, CT) — large intermittent reactive power demands, radio-frequency emissions
- Sterilisation and surgical equipment — arc-generating loads producing broadband electromagnetic noise
These sources share the same distribution system with the sensitive diagnostic equipment they serve. The physical separation between power and signal wiring is the primary engineering control that prevents the internal electromagnetic environment from interfering with sensitive measurements.
The compliance gap — why utility standards do not protect medical equipment
The utility supply at the hospital’s point of common coupling (PCC) was fully compliant with applicable power quality standards. IEEE 519, EN 50160, et CSA C235 régissent toutes la limite entre le réseau de services publics et l’entrée de service du client.. Aucun d’entre eux ne régit ce qui se passe à l’intérieur de l’établissement. Un hôpital peut disposer d'une alimentation électrique parfaitement conforme tout en conservant un environnement électromagnétique interne incompatible avec ses équipements les plus sensibles, car l'incompatibilité provient de charges à l'intérieur du même bâtiment., sur le même système de distribution, parfois sur le même circuit de dérivation.
Rencontre IEEE 519 au compteur utilitaire ne dit rien sur ce qu'un appareil ECG voit à ses bornes d'entrée de signal. La norme mesure ce que l'installation injecte dans le réseau électrique. The EKG machine is affected by what the facility’s own loads inject into the internal distribution system — and that is governed by internal wiring practice, equipment placement, and electromagnetic compatibility engineering, not by utility power quality standards.
Inductive coupling — the mechanism
When a current-carrying conductor (the power cord) is routed parallel to a signal conductor (the EKG signal wire), the time-varying magnetic field of the power current induces a voltage in the signal conductor according to Faraday’s law. The induced voltage is proportional to the rate of change of the magnetic flux linkage — meaning harmonic frequency components (which change faster than the fundamental) induce proportionally larger voltages in the signal circuit than the fundamental frequency current alone would predict. A power system with 20% third harmonic current content will induce a signal interference voltage that has a significant 180 Hz component — well within the frequency range of EKG signal processing.
The physical separation between conductors and the length of parallel routing both determine the magnitude of coupling. Even a few centimetres of separation, if maintained consistently over a run of several metres, can dramatically reduce the induced voltage.
04 Solution
The EKG signal wiring was re-routed away from the power cord, eliminating the inductive coupling path. The machine returned to normal operation immediately. No component replacement, no equipment modification, no utility intervention was required.
A broader review of signal and power wiring separation throughout the affected clinical area was recommended as a preventive measure — not because other instruments were known to be affected, but because the same installation practice (signal and power cables bundled together for convenience) was likely present elsewhere in the department.
- Route signal cables and power cables in separate conduit or cable trays with physical separation
- Where signal and power cables must cross, cross at 90° rather than running parallel
- Use shielded signal cable with the shield grounded at one end only (single-point grounding)
- Maintain separation from motor leads and VFD output cables — these carry high dV/dt switching transients in addition to fundamental and harmonic currents
- CEI 60364-5-52 and NFPA 99 both address wiring separation requirements in sensitive environments
Power quality issues in medical facilities do not always cause immediate equipment failure. Failures and diagnostic errors frequently occur well after the disturbance event, making correlation with the power system difficult without continuous monitoring. An intermittent wiring coupling problem — like this one — may produce errors only when specific loads are energised on the same circuit, making it appear random and frustrating to trace without systematic PQ measurement.
05 Perspective de la qualité de l'énergie
Du point de vue de la qualité de l’énergie électrique, this case illustrates a distinction that is worth stating explicitly: power quality at the meter and electromagnetic compatibility inside the facility are two different engineering problems. Utility PQ standards address the first. EMC engineering — wiring practice, blindage, échouage, equipment placement — addresses the second.
In most industrial facilities, the gap between these two problems is managed implicitly: the loads are robust, the signals are high-level, and the consequences of interference are production disruptions rather than diagnostic errors. In a hospital, the gap is explicit and consequential. The diagnostic instruments are designed to measure millivolt-level physiological signals in the presence of a building electrical system that may carry hundreds of amperes of distorted current in adjacent wiring.
This is precisely the gap that an internal EMC audit addresses. Measuring only at the service entrance — which is what a standard PQ survey does — would have found nothing wrong. The problem existed entirely within the facility wiring, and required looking at the electromagnetic environment at the point of use, not at the point of supply.
The field statistic cited by Fluke — 75% of medical facility PQ problems are wiring and grounding issues — aligns with field experience from industrial sites as well. The utility is rarely the primary problem source in internal equipment compatibility issues. The EMC audit that looks inside the facility, at actual equipment terminals, under actual operating conditions, consistently reveals problems that a PCC measurement cannot find. The payback on finding and correcting these issues before they cause diagnostic errors — or production losses, or drive failures — is rapid.
Références
- Fluke Corporation. Power Quality Issues in Medical Facilities — Case Study: When Power Quality is Life or Death. Fluke Learning Center, 2019. Disponible à: www.fluke.com
- NFPA 99-2021. Health Care Facilities Code, Chapitre 6 — Electrical Systems. National Fire Protection Association, Quincy, MA, 2021.
- CEI 60364-5-52:2009+AMD1:2017. Low-voltage electrical installations — Selection and erection of electrical equipment — Wiring systems. CEI, Genève.
This case study is based on material originally published by Fluke Corporation:
Power Quality Issues in Medical Facilities — Case Study: When Power Quality is Life or Death.
Fluke Learning Center, 2019. Read the original article at fluke.com →
This case study is presented in summary and commentary form for educational purposes. All original technical content is attributed to Fluke Corporation. La section Perspective PQ (Section 5) represents IPQDF editorial commentary by Denis Ruest, M.Sc.. (Appliqué), P.Eng. (ret.). IPQDF does not claim authorship of the original case material.