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What benefit does a power supply with 1 x MOPP output to ground isolation provide?

Applied Part Medical Legislation BF Rating 60601-1

Monday, April 3, 2023

Introduction

When IEC 60601-1, the internationally recognized standard for the Basic Safety and Essential Performance of Medical Electrical Equipment, introduced the third edition it transitioned to a hazard, risk-based format. In addition to basic safety, it accounted for the fast-changing evolution of medical equipment by considering use and performance in its environment, particularly with regard to the patient and operator.

Designers and manufacturers of medical devices now conduct and document risk management in accordance with ISO 14971.

Medical devices increasingly have direct contact with the patient which can impact the risk of a patient receiving an electrical shock and/or an associated leakage current.

MOP, MOPP and MOOP

One aspect that was introduced in the updated standard is means of protection (MOP). This identifies one requirement for the operator (MOOP - Means Of Operator Protection) and one for the patient (MOPP – Means Of Patient Protection).

MOOP applies to situations where the equipment will not come into contact with the patient. For example, IVD (in-vitro diagnostic) devices which are used only in medical laboratories, such as centrifuges, are often classified as requiring MOOP only.

MOPP on the other hand recognizes that a patient might well be in a weakened condition and is more likely to be harmed than a healthy technician, nurse or doctor in the event of a failure in a safety isolation barrier.

Creepage and clearance

The IEC/EN 60601-1 standard defines the minimum creepage and or clearance of the insulation barrier. The creepage distance is measured across the surface of an insulator and the clearance measured through the air between the primary and secondary sides of a power supply for example (Figure 1).

Figure 1: Creepage and clearance

Levels of insulation

Depending on the MOOP or MOPP classification, IEC60601-1 defines different levels of isolation, insulation and creepage. See Table 1.

1 x MOOP is equivalent to basic insulation for an operator under IEC 62368-1

2 x MOOPs is equivalent to reinforced insulation, the same as in IEC 62368-1

1 x MOPP is equivalent to basic insulation for a patient under IEC 60601-1

2 x MOPPs is equivalent to reinforced insulation under IEC 60601-1

Classification	Withstand Voltage	Creepage	Insulation
1 x MOOP	1,500Vac	2.5mm	Basic
2 x MOOP	3,000Vac	5.0mm	Double/Reinforced
1 x MOPP	1,500Vac	4.0mm	Basic
2 x MOPP	4,000Vac	8.0mm	Double/Reinforced

Table 1

Note, the term “Withstand Voltage” refers to the test to verify the isolation barrier and is not a continuously applied voltage during the operation of a product.

The benefits of a power supply with full MOPPs isolation

There are a large number of medical power supplies on the market that offer 2 x MOPP between the input and output(s) and 1 x MOPP between the input and earth isolation. The output to earth isolation though is usually only 500Vac or 500Vdc.

To meet Type BF (body float) classification for products like ultrasound probes, incubators, medical lasers or patient monitoring equipment, it usually requires a 1 x MOPP isolation between the power supply output and earth.

Examining Figure 3, showing the MOPP isolation of a typical BF (body float) medical power supply, we can see that without 1 x MOPP isolation between the earth ground and the output, it is often necessary have a safety certified, 1 x MOPP isolated “Applied Part” between the output and patient.

Figure 3: Applied part between the power supply and patient

TDK-Lambda has introduced a number of modular (configurable) power supplies in the last five years with 1 x MOPP 1,500Vac isolation between output and earth. The most recent being the 1U high MU4 800W with up to 5 outputs (Figure 2).

Figure 2: 1U high MU4 800W modular power supply

Figure 4 shows the diagram for a “full” MOPP isolation power supply.

Figure 4: Power supply with “full” MOPPs isolation

In some applications a “BF ready” power supply which will simplify or eliminate the need to provide a certified Applied Part between the power supply and patient.

Summary

With a wide choice of medical power supplies on the market, it is highly recommended to partner with a power supply company that has experience with medical applications. Their knowledgeable, locally based, Field Application Engineers can offer invaluable advice and interface between the user and the design teams. This can save time, money and speed up system compliance.