Medical power supply requirements

Understanding the basics of the medical power supply safety standard IEC 60601-1

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Power supplies designed for use with medical and healthcare equipment need to conform to the internationally recognised safety standard IEC 60601-1-2:2015. This standard defines the safety criteria and specifications for any item of equipment connected to a mains power supply used to monitor, diagnose and treat a patient. In particular, the standard covers applications where the patient is physically attached to the equipment. Since clinical staff may be operating the connected equipment, they too are covered under the scope of the specification.

Examples of equipment that come under the remit of the IEC 60601-1 standard include heart and blood pressure monitors, surgical lasers, and diagnostic ultrasound equipment. It’s important to note that the specification covers not only the power supply unit itself but the equipment it’s powering too. The qualification applies to the complete system, so external power supplies and wall-mounted power supply adapters need to come under the standard's requirements.

When selecting a power supply for a medical application, it must conform to the IEC 60601 standard. First introduced in 1977, and now in its 4th edition, the standard has been accepted in Europe, the US, and Canada. Other regions of the world are either in the process of adopting the standard in its entirety or drafting legislation based around the basic tenets of IEC 60601. If your medical end-product is likely to be shipped to other parts of the world, it would be prudent to ensure compliance with local implementations of the medical safety standard.
 

Safety for medical power supplies

For a mains-operated power supply, the primary safety concern is the isolation of the patient from any lethal mains voltages. This isolation needs to be achieved from the mains voltage primary side of the power supply as well as the low voltage secondary side. If a breakdown of a critical component occurred, a lethal mains voltage might conduct across to the low voltage secondary. The focus of IEC 60601 is all about assessing the risks involved and implementing insulation barriers to provide electrical isolation. From the standard's perspective, a hazardous voltage is anything around 42.2 VAC or 60 VDC.

The 2nd edition of the standard introduced three different types of risk assessment related to sensors or pads applied to the patient, namely body (B), body floating (BF), and cardiac floating (CF). The 3rd edition introduced the concept of means of protection (MOP), for the patient - means of patient protection (MOPP) - and for the operator - means of operator protection (MOOP). The 3rd edition also stipulated that a formal risk management process, as defined by ISO 14791, was introduced to the design and compliance process to identify and document potential safety risks.

The means of protection required within the power supply follows the safety principles of IEC 60950, a standard that’s currently being superseded by IEC 62368. The MOP, MOOP, and MOPP requirements are still valid in the 4th edition standard.

The 4th edition extended the aspects of risk to include electromagnetic compatibility (EMC). This addition to the standard is investigated later in this article, taking into account that the power supply or the equipment it is operating may be susceptible to electromagnetic inference such that it malfunctions, posing a safety risk. Likewise, the equipment itself may generate levels of electromagnetic interference that causes other items of medical equipment in the vicinity to operate incorrectly.

IEC 60601 stipulates the number of safety protective barriers needed together with the type of insulation provided, explained below, and the minimum isolation voltage it is capable of withstanding. Two means of protection or 2 MOOP/MOPP is the minimum requirement for the power supply to conform to IEC 60601.

Figure 1: Classification of isolation, insulation and creepage
 

Insulation classifications

Insulation classifications include functional, protective, basic, supplementary, double, and reinforced.

Functional insulation is typically anything required for the reliable operation of the power supply rather than anything specifically for safety. For example, keeping PCB tracks of the mains voltage input sufficiently apart, or putting plastic sleeving on the leads of components raised above the PCB so they don’t touch any other parts during use.

Protective in terms of insulation refers to the use of the protective earth connection to prevent electrical shock to the operator or patient. Note that Class II power supplies, usually wall plug units, do not use the earth connection, so it cannot be a safety feature. Class I power supplies do use the protective earth connection. Leakage current from the equipment through the patient or operator is a significant factor in IEC 60601.

A single layer of insulation defines the basic insulation classification, an example of which is the main lead from the power socket into the power supply. Mains leads that have snagged under doors or those pulled around hard corners can expose live wires very quickly.

Supplementary insulation adds an extra independent layer of insulation in case the basic layer fails. Examples include the use of a plastic insulating layer above the mains terminals inside the power supply or a plastic barrier cover above all the mains voltage components.

Double insulation comprises both basic and supplementary, with each being two separate, distinct methods of insulation.

Reinforced insulation uses only a single layer of insulation to provide insulation capability equivalent to the double classification.

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Figure 2: Functional diagram of an AC/DC power supply

Any form of insulation used will have a limit to how much voltage it can isolate against before breaking down. Figure 1 indicates this voltage for both the MOOP and MOPP. While the primary concepts of electrical isolation are the same between IEC 60950 and IEC 60601, the differences are the leakage current, isolation voltage, and creepage and clearance distances.
 

Creepage, clearance and leakage currents

Figure 3: Diagram showing the difference between clearance and creepage distances

The IEC 60601 specifications for creepage, clearance and leakage current criteria all need careful attention when designing an AC/DC power supply for medical applications. The distance measured between two conductive parts along a surface is the creepage. Clearance is the direct distance between two points measured through the air. Both creepage and clearance parameters depend on the material involved, the working voltage, and the degree of air pollution.

Creepage and clearance distances serve as a form of insulation. It is prudent to assume worst case conditions when undertaking high voltage test measurements, including a high degree of pollution and the maximum universal AC mains voltage of 264 VAC. When quoting the working voltage from an isolation perspective, it relates to the maximum voltage possible over an extended period. Voltage field stress occurs between any differences in potential, and typically will involve input to output, input to earth, and output to earth. In the medical context, the route to earth might be via the patient or operator.

When selecting a power supply, be careful to check that it conforms to your needs in terms of MOPP or MOOP. Figure 1 highlights the critical differences between MOPP and MOOP in terms of isolation voltage and creepage. A 1 x MOOP power supply is less demanding in terms of isolation distances and working isolation voltages so will probably be cheaper to build. When only operators will come in contact with the equipment, 2x MOOP is sufficient. When patients could come into contact, even by accident, the 2x MOPP must be applied.

Figure 4: Table of creepage distances/clearance for IEC 60601 compared to IEC 60950 (Source: MEAN WELL)

Figure 4 above illustrates the minimum creepage and clearance distances for MOOP/MOPP. A MOPP of two requires 8 mm of creepage, 5 mm of clearance, and a 4000 VAC isolation between input and output terminals of the power supply.

As mentioned earlier, the leakage current is a crucial factor. Note that the maximum permissible leakage current is 0.3 mA, a tenth of the IEC 60950/IEC 62368 requirement.
 

EMI susceptibility

For the first time, the IEC 60601 4th edition heralded the introduction of electromagnetic inference into the standard. With the increase in wireless connections, the potential for interference from any form of wireless device operating in the vicinity of the medical equipment has significantly risen. Smartphones, wearable consumer tech, RFID and other wireless-enabled products could introduce unacceptable levels of interference that disrupt the safe working of the medical equipment.

Within the standard, electromagnetic immunity is categorised according to three different use case environments; professional health care facility, home health care, and special environments. In the latter case, examples could include high power MRI equipment used for diagnostic purposes.
 

Conclusion

When researching medical power supply requirements, the selected power supply must conform with the IEC 60601 standard. Also, the type of application will determine if it needs to meet the requirements for means of operator protection (MOOP) or the more stringent means of patient protection (MOPP). The product's datasheet should indicate how the power supply conforms to the IEC 60601 specifications.

Avnet Abacus' industry leading linecard features products from the world's best power suppliers, designed and manufactured to medical safety standards for a range of applications: 

• Medical imaging: ultrasound scanners, MRI, CT, PET, X-Ray
• Surgical devices: robotics, electro surgery, laser surgery
• Medical devices: patient therapy, patient monitoring, patient transport, beds, ventilators, powered air-purifying respirator, anesthesia machines, aspiration and suction pumps, autoclaves, sterilisers, blood chemistry analysers, centrifuge and many more
• Dental equipment: oral care systems, CAD/CAM systems, digital radiography
• Wellness and beauty: laser hair removal, UV light therapy, laser therapy, ultra-sound

To view the linecard and explore solutions from our suppliers, visit our medical power solutions page. Alternatively, if you would like to discuss your power requirements in detail, get in touch with our team of technical specialists in your local language.

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