• • • TRANSFORMERS & SWITCHGEAR • • •
Reliability and performance in medical transformers
Steve Hughes, managing director of REO UK, explains the key considerations when specifying a medical transformer
M
edical requirements for the design of electronic systems can generally be considered among the most rigorous.
Because these standards, such as the IEC 60601-1 and the EU guideline 93/42/EWG, are especially demanding of products that will be used on or by patients whose health is already compromised, virtually all components of medical electronic products — as well as the end products themselves — must be designed with safety in mind. There are many considerations when specifying
a transformer for medical applications. For example, there are several types of insulation system temperature ratings for transformers, used for determining the overload capability and life expectancy. The insulation system is the highest internal temperature that a transformer can withstand, with a life expectancy of around 20–25 years when working within the limits. The IEC 60601-1 standard requires medical
transformers to pass a high potential (HIPOT) test – a stress test of the insulation of a device beyond what it would encounter during normal use. Medical equipment with applied parts that have direct contact with a patient is tested at much higher parameters than the norm, testing at 4,000 volts.
Meeting standards IEC 60601-1 outlines far more than the requirements for medical device insulation. It is part one in a group of standards that encompasses the essential performance, electromagnetic
compatibility and safety of medical equipment and systems that have an applied part of transferring energy in a patient’s care or detecting that energy. This includes a wide range of devices including patient monitors, infant incubators and cardiac defibrillators. While IEC 60101-1 is the basic general standard
of devices that will always apply if the device falls under the general scope, it is up to design engineers to understand which collateral and particular standards apply. IEC 60601-1-x is the collateral standard — the
primary standard that has several specific standards directly related to the safety of medical equipment. For example, 60601-1-3 relates to the collateral standard of radiation protection in diagnostic X-ray equipment, ensuring that the irradiation of the patient, operator and staff can be kept as low as reasonably achievable, without jeopardising the benefit of the radiological procedure. There is also the 60601-1-2 collateral standard,
described as the basic safety and essential performance of medical equipment and systems in the presence of electromagnetic disturbances and electromagnetic disturbances emitted by that equipment and systems. This standard addresses a concern affecting both the medical device in question and other devices in the vicinity. The reason for introducing this new standard is due to the growing use of devices that connect wirelessly using a cellular network, like smartphones or laptops.
IEC 60101-2-x is the set of standards specific to
different types of medical equipment, providing additional information to the collateral standards. IEC 60601-2-8, for example, encompasses the particular requirements for the basic safety and essential performance of therapeutic X-ray equipment operating in the range of 10kV to MV. When these standards are not followed, the use
of mains-powered diagnostic, measurement and treatment equipment — as well as the presence of electrically-conductive bodily fluids, gels, and other solutions used in a patient’s care — potentially exposes the patient or caregiver to the risk of injury directly due to leakage current resulting from improper grounding and electrical isolation. This means that virtually every piece of
equipment destined for use in a medical environment needs isolation from the mains. Most medical equipment is manufactured and tested to incorporate an isolation transformer — dependant on the class of the equipment (Class 1 or 2) — offering improved safety, noise reduction and better power quality. To ensure this for more medical manufacturers, REO has focussed attention on developing a range of isolation transformers that are specially designed to meet the exacting requirements of the medical sector. REOMED II is the latest generation of medical
isolation transformers to allow most Class 1 equipment to pass the ‘electrical’ side of the IEC 60601-1 standard, using polyurethane coated toroidal cores, which are more efficient and less likely to radiate an electromagnetic field than other designs.
Specifications Leakage current is one of the most important specifications for medical grade transformers. Medical systems must safely meet the leakage current requirement specified – if several devices are switched together, the overall leakage current increases accordingly. Isolating transformers, such as the TÜV-certified REOMED products, are proven and reliable equipment, limiting the leakage current and thus helping to ensure patient and staff safety. REOMED isolation transformers are
characterised by their high level of safety and very low magnetic stray field, whilst also providing high efficiency and easy connectivity. In addition to the standard range, custom transformers can be enhanced by adding an electronic starting current limiter, surge protector and mains filter. While the requirements of medical electronic
systems are rigorous, working with the transformer manufacturer can set design engineers on the best path to fulfilling these standards.
www.reo.co.uk 36 ELECTRICAL ENGINEERING • OCTOBER 2021
electricalengineeringmagazine.co.uk
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