MOPP vs. MOOP: how to specifiy medical power supplies

By Philip Lechner   |   December 8, 2020

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IEC/EN 62368 covers the electrical safety of mains-powered equipment such as computers, communications equipment, and audio/video systems, recently replacing the previous IEC/EN 60950 and IEC/EN 60065 standards. The standard applies to the power supply, any subsystems, and the end system.

The electrical safety of medical equipment, such as heart rate monitors, drug dispensers, and surgical lasers, is covered by IEC/EN 60601, which is now in its 4th edition. The medical safety standards are undoubtedly all-encompassing, but when selecting a power supply for an item of medical equipment, there are two factors to consider.

Firstly, a power supply that conforms to IEC/EN 62368 but is not medically approved may be suitable for some healthcare applications. Secondly, a power supply that states it is certified to medical approval standards does not always mean that the end product it is powering will conform to IEC/EN 60601.

Let's investigate the reasoning behind these factors. First, the type of medical equipment has an influence. The two categories of medical equipment are either a medical device (MD) or an in vitro diagnostic medical device (IVD). Then the level of protection, whether for the operator of the equipment, or the patient it might be connected to or monitoring, as set out by IEC/EN 60601, is a critical factor.

Medical devices are often used close to patients or might have connections to them. There are many different examples of medical devices, for example ultrasound probes, blood pressure monitors and oximeters. That contrasts with in vitro diagnostic equipment that does not usually come into contact with patients. Examples of IVD equipment include centrifuges and blood glucose analysers.

Risk management is a crucial aspect of the medical safety standard, and the responsibility is on the equipment manufacturer to decide the likelihood that a patient may come into direct contact with an item of equipment. A formal risk management process as stipulated by ISO 14791 became part of the 3rd edition of IEC/EN 60601 and continues in the current 4th edition.

Having conducted a thorough risk assessment investigation, if the manufacturer decides there is no significant risk, then the requirements of IEC/EN 62368 applies, not IEC/EN 60601. For that specific application, a power supply conforming to IEC/EN 62368 is valid.
 

Understanding means of protection (MOP)

The term means of protection (MOP) relates to preventing a patient or an equipment operator coming into contact with the lethal mains voltage of the medical device.

Safety insulation and the use of a protective earth connection are all valid MOPs. Also, the medical standard defines the minimum distances for creepage, air gap clearances, along with other protective impedances, and combinations of all of these MOPs provide compliance towards IEC/EN 60601.

Creepage is the distance measured along the surface of insulating material between two points, and clearance is the direct distance through air between them.

There is sometimes confusion between the terms protective earth and functional earth. A functional earth aids the suppression of electromagnetic inference and other unwanted noise; it has no role in safety. By contrast, a protective earth does provide a means of protecting equipment users from electrical shock.

As mentioned, the medical standard differentiates between the risk to a patient or an operator. MOP classifications indicate who is being protected and can be either a means of patient protection (MOPP) or a means of operator protection (MOOP). Figure 1 shows the different isolation, creepage and insulation criteria for each MOP.

The primary difference between 1 MOOP and 1 MOPP is the creepage distance, both being satisfied using basic insulation. The requirement to achieve 2 x MOPP is more stringent, requiring a creepage distance of 8 mm - twice that of 1 x MOPP - and capable of 4000 Vac isolation.

The dielectric test voltages are higher under IEC/EN 60601-1 than under IEC/EN 60950, as set out in Figure 1.

Figure 1: IEC/EN 60601-1 requires differing levels of isolation, insulation type, creepage, and clearance, depending on the MOP level
 

What causes leakage current?

Leakage current is a by-product of the effects of capacitive coupling across power transformers and filter capacitors used to mitigate the impact of electromagnetic inference. The IEC/EN 60601-1 medical safety standard stipulates the same leakage current limits across all classes of medical devices, from those with no direct patient contact, direct contact, and direct contact with the patient's heart. Figure 2 indicates the leakage current limits either under regular operation or when a fault condition occurs.

In the North American market, the UL 60601-1 standard applies with a slightly different interpretation of the maximum leakage current allowed, which is 0.3 mA compared to 0.5 mA.

Note when using a power supply for an in vitro device, although the appropriate safety standard of IEC/EN 62368 applies, adherence to the leakage current limits of IEC/EN 60601-1 is mandated.
 

How to select a power supply

As mentioned earlier, if the manufacturer conducts a risk assessment and demonstrates there is no significant risk associated with the patient coming into contact with the product, the MOOP requirements may be met by using a power supply that conforms to IEC/EN 62368.

The highest level of safety protection comes from using 2 x MOPP. Note that power supply manufacturers typically highlight power supplies as meeting medical approvals whether the unit has 1 MOPP or 2 MOPP. Buyers should check their requirements and the proposed power supply datasheet before placing an order.

An AC/DC power supply that provides just 1 x MOPP can still be incorporated into a design that requires 2 x MOPP (or MOOP) through the addition of an isolated DC/DC converter. Although this adds additional cost, it might become the preferred solution, particularly if more than one DC rail is required.

Initially, when medical safety standards were first introduced there were not many power supplies that met the medical criteria compared to volume industrial products. However, over the years, the medical devices market has grown significantly, and medical power supplies no longer attract the premium they previously did.

In some situations, it might be beneficial to opt for a single 2 x MOPP power supply that suits all medical products. The likelihood is that the price premium compared to an IEC/EN 62368 unit might be small, the savings for reducing inventory complexity and sourcing costs more than making up the cost of any premium.

Figure 3: IEC/EN 60601-1 power supplies from leading manufacturers
 

A practical example: powering a hospital bed

Modern hospital beds are fairly complex mechanical devices and increasingly use motors and hand controls to operate them. From an electrical safety perspective, there are two potential ways to power a bed. It could incorporate a built-in AC power supply, or an external low voltage power supply could power it.

The former approach of using a built-in AC power supply is probably most common since it keeps the number of external cables to a minimum.

Other factors involved in the decision-making process can include the power supply shape, weight, and the vulnerability of damage or fluid ingress. Also, with a metal bed frame there is a high degree of risk that the patient could come into contact with conducting parts, so a 2 x MOPP power supply is required to meet the safety requirement.

Instead of using a mains power supply, there are benefits to using an external low voltage supply. Safety concerns will be lower if fewer electronics are on the bed, plus under 24 volts annual portable appliance testing is not required. That said, an external power supply will need to be mechanically robust, have an ingress protection rating of IP4X or above, and connector cables to have an adequate strain relief.

With the potential for leakage currents, reinforced isolation is essential, and the requirement is for the use of a 2 x MOPP external power supply. Such a power supply will use a Class II protective earth, and since it is an external low voltage output unit will not require annual PAT testing, making it popular for hospital bed applications.

Noise is of particular concern for many hospital applications, so the availability of a fanless power supply is essential. A range of external convection-cooled 2 x MOPP power supplies with outputs up to 220 watts are available from a variety of reputable manufacturers.
 

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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Philip Lechner

Philip Lechner studied electronics and telecommunications in Amsterdam before beginning his career i...

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