Medical Electronics
Ensuring quality for electrical medical devices
The use of electrical medical diagnostic, measurement and treatment equipment potentially exposes patients and caregivers to the risk of harm caused by leakage current resulting from improper grounding and electrical isolation. Here Steve Hughes, managing director of medical transformer specialist REO UK, discusses the main points to consider for the successful design, implementation and use of medical devices in the fi eld
T
he world around us is increasingly filled with
electromagnetic noise, which we cannot, usually, see or hear. This often manifests itself in multiple ways, such as inconsistent performance, unwanted product interactions or even device failure. Typically, in normal life, this is an inconvenience, but in high- consequence environments like the medical sector, outcomes can be much more serious. EMC expert Keith Armstrong’s book, The First Five Hundred "Banana Skins", details many of these serious events, including several in the medical sector. Typically, an operating theatre will
contain high-powered lights, computers, diagnostic and treatment equipment that all use high frequency components for communication, processing and power systems. Add the communication devices for each of the medical personnel to these and there is a high potential for errors and failures. These could have catastrophic consequences for a patient, who is already medically compromised, or even a medical practitioner.
Therefore, it is crucial that foreseeable worst-case situations are evaluated over a device’s anticipated lifecycle to ensure the design fully addresses any potential safety concerns. Any design, manufacturing or testing measures should be augmented if deemed required.
Safety standards
Most countries either use or base their own electrical safety standards on the medical device standard IEC/ EN 60601, which was developed by The International Electrotechnical Commission (IEC). This standard defi nes medical equipment electrical safety conditions necessary to protect patients, operators and the surroundings. IEC/EN 60601 was last updated in 2019
18 December/January 2023 Components in Electronics
and is due to be updated again in 2024. This is then supplemented by other, more specifi c, collateral standards such as IEC 60601-1-x, which deals with electromagnetic compatibility (EMC), X-ray protection and programmable electrical medical systems. In the standards, there are three key considerations for the successful design, implementation and use of medical devices in the field. These are basic safety, essential performance and functional safety.
The first point to consider is basic safety. This should ensure that a device is free from unacceptable risks when used in normal circumstances or in a single fault condition. The next consideration is essential performance. This measures a device’s actual clinical function when performing the
task it was designed to do. For example, the ability of a refrigerator to keep blood at the correct temperature and keep any fluctuations inside of acceptable limits. The final point to consider is functional safety. The safety of medical devices for use requires comprehensive risk management and any risks related to design and function must be identified and reduced to acceptable levels. This can be done either by using suitable control measures or by using appropriate countermeasures. The ISO 14971 standard, application of risk management to medical devices, provides guidance for this process.
Clearance and creepage
Within basic safety, there are the concepts of creepage and clearance distance, which are particularly important. For these, it is essential to understand the difference between a standard toroidal transformer and one specifically manufactured to achieve and go beyond the requirements of EN 60601. Clearance distance is the shortest distance between two contacts outside the solid insulation and fails as soon as the voltage becomes too high for the distance. For example, if there is a higher than desired applied voltage, this can result in arcing and potential damage, negatively impacting patient health.
Creepage is the shortest distance between the path of two conductive parts of a medical device and is measured along the surface of insulation. Creepage will always be a larger distance than clearance as it is indirect, following the shape of the insulation, whereas clearance is direct point-to-point distance through the air. This is usually given as 8 mm for a maximum working voltage of 300 V and with a test voltage of 4 kV. IEC/EN 60601-1 requires that two means of protection for both patients and operators from electric shock must be implemented for electrical medical equipment and systems and this is usually achieved by doubling the clearance and creepage distances between conductors. A toroidal transformer uses magnetic cores with a toroidal ring shape around which the insulation is wound. Because the windings are relatively short and wound in a closed magnetic field, toroidal transformers have lower secondary impedance than traditional transformers, which leads to increased efficiency. REO’s medical transformers, which are designed to exceed the specifications outlined by IEC 60601, use a unique method of layering and overlapping the interlayer insulation to ensure that the desired 8 (16) mm distance is achieved, and actually surpassed by a factor of three.
www.reo.co.uk/medical-transformers www.cieonline.co.uk
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